Assessment of weed/pest complexes in Southern New Mexico chile fields

              ABSTRACT
Chile peppers (Capsicum annuum) are an economically important crop in New Mexico. Pests, including weeds and insects, can form complexes with pathogens and/or nematodes, leading to low yields and poor chile pod quality. Understanding pest prob­lems within New Mexico chile cropping systems can help in the development of effec­tive, economical pest control recommendations. This study examined the relationship between weeds and other pests and pathogens within chile cropping systems in two counties in southern New Mexico. Soil, weed, and chile plant samples were collected from four fields each year in 2014 and 2015. The fields were selected based on a his­tory of southern root-knot nematodes (SRKN, Meloidogyne incognita), Phytophthora root rot, Verticillium wilt, and weeds. Leaf tissues from chile plants and adjacent weeds were sampled for viruses, while root balls from weeds and asymptomatic chile plants were tested for the presence of SRKN. Soil from the root balls was tested for soil tex­ture and organic matter. Extreme weather events in 2014 and 2015 increased levels of Phytophthora and other pathogens. Disease pressure was high in chile fields with dense weed populations, while healthier cropping systems had little to no weed competition. Viruses identified using enzyme-linked immunosorbent assay included curtoviruses, Alfalfa mosaic virus, Tobacco etch virus, and Tomato spotted wilt virus. Phytophthora root rot was found in all eight fields, while Verticillium wilt was found in a hail-damaged field. Weeds tested positive for the presence of SRKN and viruses. Nematode presence was detected in every field through visual in-field examination of chile roots for gall­ing and extraction of nematodes from soil and roots in root balls. Two fields in Dem­1
Respectively, M.S. Graduate, Department of Entomology, Plant Pathology and Weed Science
(EPPWS); Professor, EPPWS; Emeritus Professor, EPPWS; and Emeritus Professor, EPPWS,
New Mexico State University.
COLLEGE OF AGRICULTURAL, CONSUMER AND ENVIRONMENTAL SCIENCES
aces.nmsu.edu/pubs • Agricultural Experiment Station • Research Report 794
New Mexico State University
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Assessment of Weed/Pest Complexes
in Southern New Mexico Chile Fields
Sharon Martinez, Rebecca Creamer, Steve Thomas, and Jill Schroeder1Research Report 794 • Page 2
ing and one in Las Cruces had high levels of infestation with SRKN in chile and in the roots of nearby Palmer amaranth, spurred anoda, Wright’s groundcherry, and tall morningglory. The prevalence of high populations of SRKN in chile grown in medium- to heavy-textured soils under drip irrigation was not anticipated, and reinforces the need to assess fields of all soil types and textures for SRKN. Integrated pest management strategies that included effective weed control were essential for reducing overall pest problems in cropping environments with extreme weather events.
INTRODUCTION
New Mexico has a long history of producing chile pepper (Capsicum annuum) (Bosland, 2015), and its popularity has grown since Fabián García first developed the now familiar long green chile nearly 100 years ago (Bosland and Walker, 2014). Today, chile is a significant and economically impor­tant crop in New Mexico and worldwide.
Chile production is variable and has seen alternating years of decline in New Mexico (Hawkes et al., 2008). Problems directly affecting growers in New Mexico include crop pests, foreign and domestic competition, and labor costs and short­ages. The chile pest complex found in New Mexico comprises organisms that can cause wilts, rots, and stunting of the plant, and abiotic diseases that arise from lack of nutrients, flood­ing, and drought (Goldberg, 2001). Insect pests, weeds, and pathogens work in concert to create challenges for chile crop production year-round.
Chile is slow-growing and not very competitive with weeds (Lee and Schroeder, 1995). As an example, a study conducted by Schroeder (1992) showed that, if left untreated, weeds alone can reduce crop yields by 33 to 76%. The occurrence of weeds and their relationship to the presence of various pests and diseases found within chile pepper cropping systems ad­versely affects chile production and interferes with harvesting (Murray and Lee, 1999). The weeds found within and around cropping systems can become alternative hosts to pests and pathogens and can compete for nutrients, water, and sunlight (Capinera, 2005). Every year, growers need to address pest problems by using additional chemicals, hand crews, and machinery or purchasing resistant cultivars to meet consumer demands (Hall and Skaggs, 2003).
The weeds usually found in agricultural regions are ones that take advantage of disturbed soils, that is, soils that are re­peatedly plowed, disked, cultivated, or hoed (Ross and Lembi, 2009). Agricultural soils provide perfect weed seedbank condi­tions. As the farming process moves the soil, latent weed seeds are brought up from deeper layers to the surface where they can germinate, grow, and reproduce, depositing more seed into the soil “bank” (Benvenuti, 2007).
The objective of this study was to examine the occurrence of weeds and their relationship to the presence of SRKN, Phy­tophthora capsici, Verticillium dahlia, and plant viruses found within chile pepper cropping systems in southern New Mex­ico. Part of the selection process for choosing fields therefore depended on a history of weed problems. The weed complex for this work consisted of kochia, tall and ivyleaf morning­glory, Palmer amaranth, and spurred anoda. The fields in Las Cruces and Deming, New Mexico, all had histories with these specific problematic weeds.
MATERIALS AND METHODS
Field Selection
Four chile fields were assessed each year, two located near Dem­ing and two located near Las Cruces, New Mexico (Table 1). The locations were chosen to represent differing farming practices and climates. These differences included flood or drip irrigation, growing seasons, and soil textures. Since farmers use crop rota­tion as a form of pest control and to prevent soil erosion, no two fields were the same between years. The fields were specifically se­lected for this study based on their histories of SRKN and weeds. Disease histories, including Verticillium wilt and Phytophthora root rot, were noted. The fields also had past problems with the following weeds: kochia, red morningglory, ivyleaf morningglory, tall morningglory, Palmer amaranth, and spurred anoda.
Experimental Fields
Deming Fields, 2014
The two Deming locations were drip irrigated. Field prepa­ration involved shredding and cultivating the plant debris from the previous season. Fertilizers, including nitrogen, were delivered through the drip irrigation system. Weed control was addressed using mechanical cultivation early in the growing season.
Field 1-14 was a 38-acre field that had Mimbres silty clay loam soil (Neher and Buchanan, 1975). Field 1-14 was sur­rounded by desert brush on two sides, allowing for the move­ment of wildlife in and out of the field. No preplant soil treat­ments were made before planting the chile pepper. However, the grower used machine cultivation to control weeds a month after emergence of chile in mid-July. The field had previously been in corn. Root balls were collected from weeds only near gall-damaged chile plants to test for the presence of SRKN eggs within the root systems and second-stage juveniles within the soil. Chile plant tissues were collected from healthy-looking and symptomatic chile plants along with their adjacent weeds for virus testing.
Field 2-14 was a 21-acre field planted at the end of March 2014. No preplant soil treatments were made before planting. The field had previously been in cotton. The field was bordered by corn and alfalfa fields. Root balls were collected for SRKN analysis from weeds only.
Las Cruces Fields, 2014
One field was abandoned by the grower due to summer flood­ing that resulted in total loss of the chile crop. The remaining field in Las Cruces (Field 1-14) was flood irrigated. It was plowed and laser leveled before planting. Tilling and mechani­cal cultivation were used for weed control, and the field was deeply ripped in areas of water accumulation. During heavy Research Report 794 • Page 3
rains, deep pits were dug in these areas to aid in removing ex­cess water from the field. Field 1-14 was a 15-acre field that had been planted at the end of March. Soil texture for this field was found to be a sandy loam with organic matter ranging between 0.75 and 1.13%. Both weed and chile root balls were collected for SRKN analysis, along with plant tissues for virus testing.
Adverse Weather Conditions in 2015
A system of heavy hail-producing thunderstorms moved through southern New Mexico in July 2015, causing damage to more than 500 acres of chile crops in and around Deming and damage to chile crops south of Las Cruces.
Deming Fields, 2015
As in 2014, both Deming fields were drip irrigated. Field 1-15 was an 11-acre field with soil texture that ranged from clay loam to sandy clay loam to loam, and organic matter ranging from 0.77 to 1.39%. This chile field was heavily impacted by several hail-producing storm systems between July 6 and 10, causing losses of 75 to 80% of the crop’s canopy. The crop re­covered well, producing a well-closed canopy, and the grower was able to harvest the field approximately four times in Oc­tober. Chile root balls and leaf tissue were collected for SRKN and virus analysis, respectively.
Field 2-15 also received hail damage during July 6 to 10, and partially recovered. Chile root balls and plant tissue were collected for further SRKN and virus testing, respectively.
Las Cruces Fields, 2015
As in 2014, both Las Cruces fields were flood irrigated. Field 1-15 had soil texture that ranged from sandy loam to loam, and organic matter that ranged from 0.78 to 0.94%. This field sustained light hail damage and recovered quickly. Root balls and plant tissue were collected for SRKN and virus analysis, respectively.
Field 2-15 was planted with a new paprika cultivar, ‘Ra­mon Vincent Hernandez’ (RVH), in early May. This field sustained only light hail damage and recovered quickly. Root balls and plant tissue were once again collected for pathogen analysis.
Sampling Methods
Plants chosen for sampling were representative of field condi­tions at each site. Locations of each sample were recorded using an eTrex Legend Cx GPS unit (Garmin Ltd., Olathe, KS). Samples of chile plant leaf tissues expressing foliar symptoms characteristic of damage from viruses and leaf samples from adjacent weeds were collected for virus analysis. Symptoms of viral infections included mosaic patterns on leaves and pods, chlorosis, stunting, and a waxy feel to stiff leaves and stalks. The leaf samples were placed into plastic bags and stored in a cooled ice chest for transport. The samples were stored at 5°C until they were processed for enzyme-linked immunosorbent assay (ELISA) testing.
Chile plants with symptoms of plant wilting were assessed for Verticillium wilt. Whole symptomatic plants were collected and bagged and stored in a cooled ice chest for transport. Sam­ples were stored at 5°C until testing.
Plants possibly infected with SRKN showed signs of wilt­ing, stunted growth, chlorosis, and early ripening of pods. Root systems from 10 chile plants expressing foliar symptoms of SRKN damage and 10 adjacent weeds were examined for root galling, along with five healthy-looking plants per field if avail­able. Plants that showed galling were counted as SRKN-positive samples. Root systems from plants that showed aboveground symptoms of SRKN but had no visible galling were collected for further testing, as were root balls from the five plants with no foliar symptoms. The root ball was carefully removed to obtain as much of the root system as possible along with ample soil for texture and organic matter testing. The soil around
Table 1. Experimental Field Attributes
Year
Location
Field size (acres)
Location coordinatesa
Soil type
Chile
cultivar
Irrigation
Weeds at planting
Planting date
2014
Deming 1-14
38
32.147379 -107.736451
Mimbres silty clay loam
‘Big Jim’
Drip
Yes–high
Mid-May
2014
Deming 2-14
21
32.150888 -107.72100
Mimbres silty clay loam
‘Big Jim’
Drip
Yes–moderate
Late March
2014
Las Cruces 1-14
15
32.239659 -106.750620
Sandy loam
‘Big Jim’
Flood
No
Late March
2015
Deming 1-15
11
32.189502 -107.829258
Clay loam, sandy clay loam, loam
‘Big Jim’
Drip
No
Early May
2015
Deming 2-15
6
32.223009 -107.797365
‘Big Jim’
Drip
No
Early May
2015
Las Cruces 1-15
8
32.239269-106.747291
Sandy loam, loam
‘Big Jim’
Flood
No
Early May
2015
Las Cruces 2-15
3
32.438818 -106.881491
‘RVH’
Flood
No
Early May
aLatitude and longitude reported in decimal degrees.Research Report 794 • Page 4
each root ball was placed into a large plastic bag to keep the roots moist and retained for further testing. The root balls were moved to a chilled ice chest as soon as they were removed to keep the nematodes and eggs alive. The root balls were then kept at 5°C until testing.
Testing Methods
Enzyme-linked Immunosorbent Assay
Plants that exhibited mosaic patterns on leaves, mottling, chlorosis, stiff and waxy leaves, and color variations were sam­pled. Chile plants were tested for Alfalfa mosaic virus (AMV), Cucumber mosaic virus (CMV), Curtoviruses (BCTV), Potato virus Y (PVY), Tomato spotted wilt virus (TSWV), and Tobacco etch virus (TEV) using enzyme-linked immunosorbent as­say (ELISA). Indirect ELISA was carried out essentially as described in Rodriguez-Alvarado et al. (2002). Presence of viruses was assayed by using a 1:800 dilution of a specific rab­bit immunoglobulin G (IgG) for AMV, CMV, BCTV, PVY, TSWV, or TEV and a 1:2000 dilution of goat anti-rabbit IgG conjugate. Presence of a virus was indicated by a color reaction that was assessed using an Emax Microplate Reader (Molecular Devices, Sunnyvale, CA).
Verticillium dahlia Testing
Verticillium dahlia testing was conducted using modified NP-10 media with Czapek-Dox agar in accordance with Korolev et al. (2000). Plant tissue was surface sterilized with a 10% bleach solution (NaClO and deionized water). The tissue was then thinly sliced and plated onto Czapek-Dox agar (24.5 g Czapek-Dox agar into 500 ml deionized water, 0.1 g strepto­mycin sulfate, 0.1 g chloramphenicol, 0.1 g chlortetracycline hydrochloride, and 1 ml Tergitol). Plates were incubated and a slide was made of the mycelial growth and viewed with a microscope. The presence of septate hyphae along with conid­iophores was used to identify presence of the fungus.
Testing Roots and Soil for SRKN
Collection of root balls from chile plants and their adjacent weeds took place after crop harvest. The root balls were stored at 5°C until tested for the presence of SRKN. Before process­ing, 100 g of soil were retained for soil texture and organic matter testing. Two processes were used to test sampled chile and weed plants for the presence of SRKN. The roots were tested for the presence of eggs, while the soil was tested for the presence of second-stage nematode juveniles (J2). Egg extraction from roots was done using NaClO (Byrd et al., 1972). Eggs were counted using a counting slide to enumer­ate number of eggs/ml of egg suspension and extrapolated to number per gram of dry root weight. The volume of soil in each root ball was estimated using a 500-ml plastic beaker and used later for quantifying J2 counts. The root ball was placed on a sieve over a bucket and the soil was gently rinsed into the bucket using tap water. After incubating at room temperature for 48 hours, the contents of the bucket were wet sieved and nematodes were separated by centrifugal flotation (Jenkins, 1964). Juvenile SRKN were counted using a chambered counting slide and numbers were extrapolated to 100 cc
of soil based on root ball volume.
Soil Texture by Hydrometer
Soil samples reserved from the root balls were tested for soil texture and organic matter using standard operating protocols from the former New Mexico State University Soil, Water, and Agricultural Testing lab (SWAT) (Smith, 2009). Before testing, the soil was air dried at room temperature. Once dry, the soil was ground and sifted through a 2-mm mesh sieve.
A blank was prepared using 990 ml deionized water and 10 ml dispersing agent (5% sodium hexametaphosphate
in deionized water) in a 1000-ml cylinder, and the tempera­ture was recorded for use in determining soil texture. Fifty grams of the soil sample were placed into a blender cup with 10 ml of the dispersing agent and deionized water and blend­ed for 5 minutes. The suspension was decanted into a 1000-ml cylinder and deionized water was used to bring the volume of the cylinder up to 1000 ml. Using a plunger, the soil was thoroughly dispersed into the water. A hydrometer (VWR International, Radnor, PA) was used to measure the specific gravity of the soil solution where larger particles settle more quickly than smaller particles. The hydrometer was carefully inserted into the soil solution 40 seconds after agitation and specific gravity was recorded. The hydrometer was inserted again after 6 hours and a second specific gravity was recorded. A second temperature reading of the blank was retaken after one hour for use in determining soil texture using standard sedimentation curves.
Soil Organic Matter Using Wikley-Black
Acid Dichromate Digestion
Soil organic matter was measured using the Wikley-Black acid dichromate digestion method according to SWAT lab standard operating protocols (Smith, 2010). Two blanks and a quality control soil sample were used as standards for the test. The blanks, which consisted of potassium chromate and sulfuric acid only, were used as a standard. The quality con­trol soil sample—a soil of a known organic matter content of 1.36%—was used as a standard to ensure the quality of the reagents used during the process. Soil samples were digested by adding 10 ml potassium chromate and 20 ml sulfuric
acid to 1 g of the soil. The samples were set aside to digest for 30 minutes to 2 hours before titration. During digestion, the organic matter was oxidized. Deionized water was added to bring each sample up to a volume of 250 ml in the beaker. A colorizer was then added to the sample followed by the titra­tion of ferrous sulfate (FeSO4). The ferrous sulfate reduced re­sidual chromate, causing the titrated solution to change from purple to green if organic matter was present. The amount of ferrous sulfate solution used during the titration process was used to calculate the amount of oxidized organic matter. Cal­culation software developed by the NMSU SWAT lab (Smith, 2010) was used to extrapolate the amount of organic matter present in the soil sample.Research Report 794 • Page 5
RESULTS
Differences were observed in pest levels and damage between locations and years, and the fields differed in their canopy clo­sures, surrounding vegetation, and weed pressure (Table 2).
There were extreme weather events in 2014 and 2015 that influenced the cultivation of chile and the collection of chile samples. Large hail-producing storm systems moved through the Deming and Las Cruces areas in July 2015. Both fields in Deming were severely damaged by several hail events that oc­curred from July 6 through July 10. The fields were allowed to recover and were harvested in October.
Deming Field 1-14 had high weed pressure from spurred anoda, along with large clumps of Johnsongrass throughout the field and surrounding areas. There were misshapen fruit pods in the field and intermittent patches of wilted or necrotic chile, symptomatic of Phytophthora root rot. All the chile plants within the field showed signs of stunting and chlorosis, symp­toms of SRKN, and severe root galling. Root balls of weeds were collected from the northern, eastern, and western sections of the field and assessed for the presence of SRKN. High num­bers of eggs were recovered from all but one of the root balls of the weeds. Although the soil at the time of sampling was relatively dry, four of the soil samples contained SRKN J2. Very high egg numbers were detected on two of the spurred anoda plants and Palmer amaranth roots (Table 3). Stunted plants with stiffened stems and leaves indicative of beet curly top virus were found in the field. Of the 15 chile plants assayed for vi­ruses, 11 were positive for AMV and four for BCTV.
Field 2-14 in Deming had a closed canopy with lush chile plants. Cotton seedlings from the previous season’s crop were present, along with Wright’s groundcherry along the margins of the field. All representative chile plants assessed from through­out the field had thick, elongated galls from SRKN infection. Root balls from Wright’s groundcherry and spurred anoda were collected from several areas within the field (Figure 1). SRKN eggs were recovered from the roots of 11 of the 12 weeds col­lected. Soil from three of the spurred anoda and one Wright’s groundcherry root balls yielded J2 counts (Table 4). Plants symptomatic for AMV were noted. Of the 15 chile plants as­sayed for viruses, five were positive for AMV, four for CMV, and five for BCTV. The spatial distribution of the BCTV-infected chile was clustered along the western edge of the field.
Field 1-14 in Las Cruces initially had a closed canopy. Heavy monsoon rains prompted disease so that, by late Octo­ber, the crop had severe powdery mildew, bacterial leaf spot, and large patches of Phytophthora stem and pod rot (Figure 2). Some SRKN galling was found on roots among symptomatic chile plants during sampling. Root balls from chile plants and adjacent weeds were collected for further testing from a central area in the field. Nematode eggs were recovered from all roots of all 10 chile plants tested and from six of the 10 weeds tested. Nematode J2 were found in soil from seven samples where egg recovery from roots showed nematode infestation. Roots from the three chile plants labeled a, d, and h yielded high numbers of SRKN eggs and correspondingly higher numbers of J2 from root balls. The corresponding adjacent weeds also had detect­able SRKN eggs from their roots. Chile labeled f, g, and i were asymptomatic of nematode injury and yielded low numbers of SRKN eggs from roots in root balls. The roots of correspond­ing adjacent weeds yielded no eggs (Table 5). Symptoms of AMV were seen on chile plants. Of the 15 chile plants assayed for viruses, seven were positive for AMV, one for CMV, and six for BCTV.
Virus symptoms in chile, including those of curly top, were more prevalent in all three locations during 2014 than in the four locations in 2015 when weeds were sparse. Although virus symptoms were not evident in weeds, viruses were detected in some weeds tested in both years (Table 6).
Table 2. General Crop, Pest, and Pathogen Characteristics of all Fields Sampled in Deming and Las Cruces, NM, 2014–2015
Year
Location
Chile stand
Weed
abundance
Weed species
Nematode
abundance
Pathogen
presence
Sample
collection dates
2014
Deming 1-14
Sparse; approx. 7–8 acres lost to wildlife
Very high
Morningglory, spurred anoda, Palmer amaranth, kochia, oakleaf thornapple, John­songrass
High–SRKN on every chile plant
Phytophthora
October 18
October 19
2014
Deming 2-14
Closed canopy
Moderate
Oakleaf thor­napple, Wright’s groundcherry, spurred anoda, cotton
High–SRKN on every chile plant
October 25
October 26
2014
Las Cruces 1-14
Dense canopy
Very low
Moderate
Phytophthora, powdery mildew, bacterial leaf spot
October 26
October 28
October 30
2015
Deming 1-15
Closed canopy
Very low
Low
October 14
2015
Deming 2-15
Poor canopy
Moderate
Morningglory
High–SRKN on every chile plant
Verticillium
October 19
2015
Las Cruces 1-15
Closed canopy
Very low
Moderate
Phytophthora
October 28
2015
Las Cruces 2-15
Closed canopy
Very low
No galling
September 28 September 30Research Report 794 • Page 6
Figure 1. Deming Field 2-14 mapped locations of weed plants testing positive for SRKN. This field was located next to a cotton field along the western mar­gin. Cotton seedlings from the previous season’s growth were present, along with Wright’s groundcherry, within the field and margins. The southern part of the field was bordered by a dirt road. Beyond the dirt road was a small drain­age ditch with rangeland plants on the southern side of the ditch. There were silverleaf nightshade, yucca, thistle, and locoweed growing beyond the ditch. ‘Big Jim’ chile plants symptomatic for curtovirus were located among the western rows of the field nearest to the cotton crop. The northern part of the field faced open rangeland. All chile plants sampled in the field had nema­todes, and nematodes were ubiquitous throughout the field.
Figure 2. Las Cruces Field 1-14 mapped locations of chile and weed plants testing positive for SRKN, and field soil texture. This field was in a large agricultural area. It had an un­paved road around all margins. There was an onion field across the eastern side of the fields. A large cabbage field was located across the northern margin. Along the southern margin of the field was undeveloped private property. The field site was lower than surroundings areas and slopped downward toward the west. During the monsoon rains, it was heavily inun­dated with moisture, causing powdery mildew to appear throughout. Pods left on the chile plants decayed rapidly and littered the soil.Research Report 794 • Page 7
Table 3. Southern Root-knot Nematode Second-stage Juvenile (J2) and Egg Counts, Field 1-14, Deming, NM, 2014a
Plant type
J2/100 cc soil
Eggs/g dry root
Galls present
‘Big Jim’ chile
NT
NT
P (all)
*Palmer amaranth
0
6,190
ND
*Spurred anoda
0
1,000
ND
*Spurred anoda
0
1,667
ND
*Spurred anoda
70
1,000
ND
*Spurred anoda
84
2,400
ND
*Spurred anoda
3,700
44,222
ND
*Spurred anoda
130
1,857
ND
*Spurred anoda
0
235
ND
*Spurred anoda
0
16,750
ND
a Soil texture was not tested for this field due to lack of samples.
* - Asymptomatic plant; showed no aboveground symptoms and no galls on the roots.
NT - Not tested due to the presence of galls on roots.
P - Galls detected. All chile plants that were visually assessed had galls on roots. These plants were considered a positive for SRKN eggs and were not further tested.
ND - None detected; no visible galls were detected on the roots.
Table 4. Southern Root-knot Nematode Second-stage Juvenile (J2) and Egg Counts, Field 2-14, Deming, NM, 2014a
Plant type
J2/100 cc soil
Eggs/g
dry root
Galls present
‘Big Jim’ chile
NT
NT
P (all)
*Wright’s groundcherry
0
27,000
ND
*Wright’s groundcherry
0
0
ND
*Wright’s groundcherry
0
4,500
ND
*Wright’s groundcherry
0
667
ND
*Spurred anoda
29
307
ND
*Wright’s groundcherry
0
1,252
ND
*Wright’s groundcherry
0
2,000
ND
*Spurred anoda
29
307
ND
*Spurred anoda
0
200
ND
*Wright’s groundcherry
186
333
ND
*Wright’s groundcherry
0
400
ND
*Spurred anoda
100
1,428
ND
a Soil texture was not tested for this field due to lack of samples.
* - Asymptomatic plant; showed no aboveground symptoms and no galls on the roots.
NT - Not tested due to the presence of galls on roots.
P - Galls detected. All chile plants that were visually assessed had galls on roots. These plants were considered a positive for SRKN eggs and were not further tested.
ND - None detected; no visible galls were detected on the roots.
Table 5. Southern Root-knot Nematode Second-stage Juveniles (J2), Eggs Recovered from Roots, and Soil Texture, Field 1-14, Las Cruces, NM, 2014
Plant type
J2/100 cc soil
Eggs/g root DW
Soil texture
Galls
present
‘Big Jim’ chile a
480
4,584
Sandy loam
ND
*‘Big Jim’ chile b
0
517
Sandy loam
ND
*‘Big Jim’ chile c
70
275
Sandy loam
ND
‘Big Jim’ chile d
1,230
8,625
Sandy loam
ND
‘Big Jim’ chile e
40
297
Sandy loam
ND
*‘Big Jim’ chile f
0
67
Sandy loam
ND
*‘Big Jim’ chile g
0
88
Sandy loam
ND
‘Big Jim’ chile h
400
13,378
Sandy loam
ND
*‘Big Jim’ chile i
0
93
Sandy loam
ND
*‘Big Jim’ chile j
0
325
Sandy loam
ND
*Palmer amaranth a
0
181
Sandy loam
ND
*Palmer amaranth b
0
428
Sandy loam
ND
*Wright’s groundcherry c
0
769
Sandy loam
ND
*Spurred anoda d
10
46
Sandy loam
ND
*Tall morningglory e
0
0
Sandy loam
ND
*Palmer amaranth f
0
0
Sandy loam
ND
Palmer amaranth g
0
0
Sandy loam
ND
*Palmer amaranth h
0
360
Sandy loam
ND
*Spurred anoda i
0
0
Sandy loam
ND
*Tall morningglory j
70
125
Sandy loam
ND
DW - Dry weight.
Plants followed by the same letter designate a chile plant and weed collected together with shared soil.
* - Asymptomatic plant; showed no aboveground symptoms and no galls on the roots.
ND - None detected; no visible galls were detected on the roots.
Table 6. ELISA Results for Detection of Viruses from Weed Leaves Collected from Deming and Las Cruces, NM, in 2014 and 2015
Weed species
Year
Number
of plants assayed
AMV
CMV
PVY
TEV
Palmer amaranth
2014
6
4
1
ND
ND
2015
2
ND
ND
ND
1
Spurred anoda
2014
8
2
1
ND
ND
2015
5
1
1
ND
ND
Wright’s
groundcherry
2014
9
3
ND
5
3
2015
1
ND
1
ND
ND
AMV = Alfalfa mosaic virus, CMV = Cucumber mosaic virus, PVY = Potato virus Y, TEV = Tobacco etch virus.
ND - Not detected.Research Report 794 • Page 8
In 2015, Deming Field 1-15 recovered very well from the hail damage it sustained in early July. The field had a well-closed canopy, and chile was harvested prior to the October 14 sampling. Galling due to SRKN was found on chile plants only on the northern side of the field (Figure 3). Of the non-galled plants tested, four chile plants and one Palmer amaranth removed south of the area of galled chile showed the presence of SRKN. Three of the chile plants with large numbers of eggs were growing in sandy clay loam soil, while one of the four growing in clay loam contained eggs. Only four weeds were found within the field, and root balls were collected from the weeds. The root ball from the Palmer amaranth contained SRKN J2 and a high count of eggs on roots. No eggs were extracted from the spurred anoda, while only one spurred anoda root ball had a very low J2 count (Table 7). Of the 15 chile plants assayed for viruses, two were infected with CMV and one with BCTV.
Deming Field 2-15 did not recover well from the hail damage and was counted as a loss by the grower. The field was sparse with stunted and chlorotic chile plants showing SRKN galls on the roots; plants were not collected for fur­ther SRKN testing. The vascular systems of the chile plants had brown discoloration, symptomatic of Verticillium wilt. Three of the five symptomatic chile plant samples tested positive for Verticillium dahlia. Spurred anoda was observed growing throughout the field. Morningglory was found along the northwestern margins of the field. Upon examina­tion, the root systems of the morningglory were enlarged and showed extensive SRKN gall damage.
Figure 3. Deming Field 1-15 mapped loca­tions of chile and weed plants testing posi­tive for SRKN, and field soil texture. This field recovered very well from hail damage that occurred earlier in July. The field was surrounded by an unpaved road. A portion of the field had been denuded by wildlife along the northern margin and along the northwestern corner. Beyond the road, there were areas of scrub rangeland consisting of creosote, mesquite, grasses, and silverleaf nightshade along the western, northern, and eastern margins of the field. To the south was a fallow field. Three outer rows on the western and eastern margins of the field had been planted with sorghum. The surround­ing areas were kept nearly clear of weeds.
Both fields in Las Cruces in 2015 suffered light hail damage and recovered. Las Cruces Field 1-15 had a closed canopy. While the field had no weeds, Palmer amaranth and some morningglory were found growing along the margins. The western half of the field showed severe symptoms of Phytophthora root rot. Gall damage from SRKN was found in a majority of plants examined. Seven root balls were col­lected from representative chile plants in the eastern half of the field along with the root ball of an adjacent Palmer ama­ranth. Of the root samples collected, SRKN eggs were recov­ered from five chile plants grown in sandy loam and loam soil along with corresponding J2 from root balls. No SRKN eggs or J2 were recovered from the Palmer amaranth sample. The chile plant with the highest egg counts was found in sandy loam soil (Table 8). Of the 15 chile plants assayed for viruses, 10 were infected with AMV, one with TSWV, and three with BCTV.
Las Cruces Field 2-15 was nearly weed-free, except for grasses growing along the margins. The majority of the chile plants and roots appeared healthy. Of the chile plants exam­ined, only two had visible galling. Root balls of representa­tive chile plants and weeds collected from the northern edge of the field revealed the presence of SRKN J2 in all but one sample. Eggs were extracted from chile roots in all the root balls but one. Soil from many of the chile samples gave very high J2 numbers. Four weeds collected near the chile gave similar numbers of SRKN to those recovered from the near­by chile. Chile plants with the highest SRKN egg counts were found growing in sandy clay loam and sandy loam Research Report 794 • Page 9
(Table 9). Of the 15 chile plants assayed for viruses, 13 were infected with AMV, one with PVY, and two with BCTV.
DISCUSSION
This study was designed to examine the relationships between weeds and crop health. However, this study coincided with the emergence of an El Niño weather cycle in 2014–2015. This type of weather system has historically been shown to change weather patterns that in turn affect temperature, moisture, and winds, leaving crops vulnerable to the elements (Motha, 2011). Weed growth and disease trends observed during this study were strongly influenced by weather events produced by the El Niño cycle.
Weeds were of interest in this study because they can be key to many pest issues within agroecosystems. They are known to outcompete crops for nutrients, water, and sunlight (Lee and Schroeder, 1995). They are frequent hosts for diseases (Lee and Schroeder, 1995), and are known to be alternate hosts for in­sects that vector disease (Lam et al., 2009). Insect vectors often rely on these weeds for survival. When their source of food dis­appears, insect vectors often move into crops for survival.
In 2014, Deming Field 1-14 proved to be a textbook case of the consequences of severe weed pressure within a cropping system. Some factors that might have contributed to the demise of Deming Field 1-14 included dense weed populations left to grow along field margins and poor weed control measures. Al­though the grower tried to use shallow cultivation in the spring to reduce weed pressure, the weeds growing closest to emerg­ing chile plants were not removed. The chile had been heavily thinned to remove plants symptomatic for virus and wilt, leav­ing large openings throughout the field. The heavy thinning prevented the crop canopy from closing, which allowed for ample weed growth (Holt, 1995). The sparse cover and lack of shade provided an inviting environment for curly top-vectoring leafhoppers (Creamer et al., 2003).
The weed populations in sampled fields differed between 2014 and 2015. Kochia populations seen during the 2014 growing season were low compared to the 2015 growing sea­son. There was also a difference in the rate of disease found within sampled fields. The fields in 2014 had moderate rates of curly top present in chile plants, compared to 2015 when the virus was hard to find in chile. The differences between the two growing periods involved a change in weather. The growing conditions in 2014 began with a normal weather pattern. In the U.S. Southwest, this means that spring is usu­ally warm and somewhat dry. These conditions are important since they lead to the dieback of kochia, causing curly top virus-vectoring beet leafhoppers to migrate into chile crops once their food source is gone (Lam et al., 2009). In compari­son to 2014, the spring and early summer of 2015 were cooler and wetter, allowing for ample growth of kochia all season long. The kochia grew in thick, dense patches along railroad tracks, streets, and around agricultural fields, providing a de­sired habitat for the beet leafhoppers. As a result, leafhopper migration into chile fields did not occur and the disease in chile crops was rare.
Table 7. Southern Root-knot Nematode Second-stage Juvenile (J2) Numbers, Eggs Recovered, and Soil Texture, Field 1-15, Deming, NM, 2015
Plant type
J2/100 cc soil
Eggs/g root DW
Soil texture
Galls present
‘Big Jim’ chile
NT
NT
NT
P (9 plants)
*‘Big Jim’ chile a
140
9,857
Sandy clay loam
ND
*Spurred anoda a
0
0
Sandy clay loam
ND
‘Big Jim’ chile
60
53,275
Sandy clay loam
ND
‘Big Jim’ chile
5,208
27,647
Sandy clay loam
ND
*Spurred anoda
0
0
Sandy clay loam
ND
*Spurred anoda
7
0
Sandy clay loam
ND
*‘Big Jim’ chile
0
0
Clay loam
ND
*‘Big Jim’ chile
0
0
Clay loam
ND
*‘Big Jim’ chile
0
0
Clay loam
ND
*‘Big Jim’ chile
50
7,818
Clay loam
ND
*Palmer amaranth
1,260
20,757
Loam
ND
DW - Dry weight.
* - Asymptomatic plant; showed no aboveground symptoms and no galls on the roots.
Plants followed by the same letter designate a chile plant and weed collected together with shared soil. Only one chile plant/weed pair was found in this field.
NT - Not tested due to the presence of galls on roots.
P - Galls detected. Nine plants were found with galls on roots.
ND - None detected; no visible galls were detected on the roots.
Table 8. Southern Root-knot Nematode Second-stage Juvenile (J2) Numbers, Eggs Recovered, and Soil Texture, Field 1-15, Las Cruces, NM, 2015
Plant type
J2/100 cc soil
Eggs/g root DW
Soil texture
Galls present
‘Big Jim’ chile
NT
NT
NT
P (16 plants)
*‘Big Jim’ chile
0
0
Sandy clay loam
ND
*‘Big Jim’ chile
80
4,163
Sandy loam
ND
*‘Big Jim’ chile a
73
820
Sandy loam
ND
*Palmer amaranth a
0
0
Sandy loam
ND
*‘Big Jim’ chile
533
57,222
Sandy loam
ND
*‘Big Jim’ chile
50
1,454
Sandy loam
ND
‘Big Jim’ chile
890
3,847
Loam
ND
DW - Dry weight.
* - Asymptomatic plant; showed no aboveground symptoms and no galls on the roots. Plants followed by the same letter designate a chile plant and weed collected together with shared soil.
NT - Not tested due to the presence of galls on roots.
P - Galls detected. Sixteen plants were found with galls on roots.
ND - None detected; no visible galls were detected on the roots.Research Report 794 • Page 10
Although infected weeds rarely show symptoms, they are frequent hosts for viral diseases (Lee and Schroeder, 1995). ELISA results did detect virus in some weed species. However, the likelihood of detecting a positive result for curtovirus or TSWV in the weed samples can be extremely low (Lam et al., 2009). The lack of detection of these viruses in weeds could be due to the limitations of ELISA to detect some curtovirus strains, and the virus titer of curtovirus is known to be very
Table 9. Southern Root-knot Nematode Second-stage Juvenile (J2) Numbers, Eggs Recovered, and Soil Texture, Field 2-15, Las Cruces, NM, 2015Plant typeJ2/100 cc soilEggs/g root DWSoil textureGalls present‘RVH’ chileNTNTNTP (2 plants)*‘RVH’ chile3701,315ClayND*‘RVH’ chile a1332,491ClayND*Wright’s groundcherry a561,111ClayND*‘RVH’ chile b1017,894Sandy clay loamND*Spurred anoda b14020,217Sandy clay loamND*‘RVH’ chile00Sandy clay loamND*‘RVH’ chile10319,391Sandy clay loamND*‘RVH’ chile5348,387Sandy clay loamND*‘RVH’ chile13112,954Sandy clay loamND*‘RVH’ chile1,00063,275Sandy clay loamND*‘RVH’ chile1,20018,981Sandy clay loamND*‘RVH’ chile1,55632,500Sandy clay loamND*Spurred anoda75062,778Sandy clay loamND*‘RVH’ chile40175,000Sandy loamND*‘RVH’ chile1,03699,571Sandy loamND*‘RVH’ chile10072,045Sandy loamND*‘RVH’ chile3772,619Sandy loamND*‘RVH’ chile20100,000Sandy loamND*‘RVH’ chile c5213,793LoamND*Tall morningglory c3576,000LoamNDDW - Dry weight.* - Asymptomatic plant; showed no aboveground symptoms and no galls on the roots.Plants followed by the same letter designate a chile plant and weed collected together with shared soil.NT - Not tested due to the presence of galls on roots.P - Galls detected. Two plants were found with minor gall damage on roots.ND - None detected; no visible galls were detected on the roots.low in some weed hosts. It is not known if Palmer amaranth, spurred anoda, and Wright’s groundcherry are poor hosts for the two viruses.
The distribution of alfalfa mosaic virus in the chile fields was generally along the margins of the fields closest to alfalfa fields, as was expected. The distribution of curtovirus in Deming Field 2-14 was also along one margin of the field. This was unexpected since curtovirus-infected plants are usu­ally more randomly distributed within chile fields.
Several of the fields during both growing seasons had prob­lems with fungal and bacterial diseases. The damp conditions arising from the El Niño weather patterns caused heavier seasonal rains. Water accumulation from rains often causes an increase in disease pressure in chile. The results from heavy monsoons in 2014 were as expected. All fields had some level of powdery mildew and Phytophthora rot. Both diseases are commonly seen toward the end of the growing season (Gold­berg, 2001). Added monsoon activity worsens disease pressure in chile cropping systems during this critical growth period. However, the fields in Las Cruces were hit particularly hard near harvest time, causing heavy crop losses. Sampling was not conducted in one of the fields due to increased Phytoph­thora pressure. As a result of increased disease pressure, the crop was plowed under before harvest.
Hail storms in Deming during 2015 caused severe dam­age to more than 1,000 acres of farmland (Moorman, 2016). Nearly 500 acres of chile were severely damaged or destroyed. Both fields in Deming that year had been devastated by hail. The grower made the decision to leave the crops as they were in hopes of a late harvest. Surprisingly, Field 1-15 recovered com­pletely, producing a high yield of green chile. Field 2-15 did not recover very well and showed very obvious symptoms of diseas­es associated with hail damage. By the end of the growing sea­son, the field had developed a pest complex of Verticillium wilt, spurred anoda, and severe gall damage to root systems caused by SRKN. The intricacy of this pest complex was discussed by Sanogo et al. (2013). The presence of SRKN in this field may have aided in increasing the infection rate of V. dahliae. Shep­herd and Huck (1989) surmised that cracks and open wounds created by SRKN may provide an inlet for disease organisms to enter the root systems of plants. The cooler temperatures and the available water provided by drip irrigation may have cre­ated the optimal conditions that contributed to the increase of V. dahliae found in the chile plants (Goldberg, 2010). In the case of this field, the high SRKN pressure along with favorable weather conditions may have caused the high rate of disease seen in the field and bears further study.
An association between SRKN presence in chile and in weeds was present in some fields and not present in others. Spatial distribution of chile and weeds in Las Cruces Field 1-14 (Figure 2) was particularly complex and lacked solid associa­tions between nematode presence and weeds, and there were no spatial patterns of nematode presence. The field showed some chile with SRKN eggs and juveniles adjacent to Palmer amaranth with only SRKN eggs, spurred anoda without any SRKN, and tall morningglory with eggs and juveniles. It also contained chile with only SRKN eggs adjacent to tall morning­Research Report 794 • Page 11
glory and Palmer amaranth without any SRKN, Palmer ama­ranth and Wright’s groundcherry with only SRKN eggs, and spurred anoda with SRKN eggs and juveniles. In contrast, Deming Field 1-15 showed a clear gradient of presence of SRKN eggs and juveniles in chile and weeds from the north­western toward the southeastern areas of the field (Figure 3).
Southern root-knot nematodes were found in every field except Field 2-14 in Las Cruces, which was the only field com­pletely devastated by P. capsici. Heavier-textured soils that retain water are poorly suited to SRKN population development and better suited to Phytophthora root rot (Goldberg, 2001). The plant-parasitic nematode results for both of the Deming fields in 2014 were surprising in that the Mimbres silty clay loam supported large populations of SRKN. Accepted textbook theory explains that SRKN needs the larger pore size of sandier soils to thrive (Perry and Moens, 2006). However, while the Mimbres silty clay loam is described as having a finer texture (Neher and Buchanan, 1975), it still hosted sizeable popula­tions of nematodes. The high presence of SRKN could be due to the history of the fields. Previously, the fields would have been heavily tilled, mechanically leveled, and flood irrigated before the grower switched to drip irrigation. These practices could account for the distribution of SRKN throughout the fields. That particular grower did not treat the soil with a ne­maticide pre-plant, which could have helped to decrease SRKN populations (Noling, 1999).
Southern root-knot nematodes were detected in almost every sample from Las Cruces Field 2-15. The field was plant­ed with the new paprika variety ‘RVH’. The plants looked very healthy and produced a good yield. Surprisingly, galls were not evident on the roots. The lack of visible symptoms could be due to SRKN tolerance carried along by the selective breeding process used to develop the paprika cultivar. The absence of root galling by SRKN in an otherwise susceptible host was first reported in cotton, where researchers deter­mined that gall formation was regulated by different genes than those responsible for host plant resistance (Shepherd, 1979). This emphasizes that growers cannot always detect nematode presence by the appearance of their fields. Without soil or plant testing, nematode populations can remain unde­tected. The soil in this field was a combination of clay, sandy clay loam, sandy loam, and loam. The presence of SRKN in this field within this soil profile was also unexpected due to soil texture preferences previously discussed, and is similar to results observed from chile in heavier-textured soils grown using drip irrigation in Deming.
This work revealed that SRKN can be found at damaging population levels in finer-textured soils in southern New Mex­ico. Some research has shown that the presence of clay could aid in the migration to roots by infective J2 (Prot and Van Gundy, 1981). Whether the presence of nematodes in the clay soils is due, in part, to the presence of root exudates or debris in this instance is unclear. However, J2 and SRKN eggs were extracted from clay and silty soils and chile roots growing in such soils. Use of drip irrigation may have facilitated SRKN populations in such soils, as was seen in both Deming fields in 2014. Before those fields were converted to drip, they were laser leveled and flood irrigated, likely distributing the nema­todes throughout both fields.
The weather had a profound effect on weed populations and viruses in chile fields. Originally, the chile growing season of 2015 was forecast to be a heavy curly top year. However, with the onset of the El Niño conditions, key weeds that usually play a role in leafhopper migration were positively affected. The longer cooler temperatures and wet­ter soil conditions allowed London rocket, a winter alternate host for the beet leafhopper, to linger well into spring. Ko­chia, a spring weed and preferred host for the beet leafhop­per, also benefitted from the weather and grew profusely. The presence of both weeds at the same time provided abundant food for the leafhoppers and also for aphids. The insects did not need to migrate into crops for food, and viral diseases were rare within chile crops.
The extreme weather during the 2014–2015 growing sea­sons also demonstrated the continued importance of using integrated pest management within chile cropping systems despite such adverse conditions. The growers affected by hail during this study relied on existing integrated pest manage­ment protocols. The measures they took included debris removal from their fields, aggressive weed control using chemical herbicides along the margins of their fields, hand removal of weeds within their fields, and aggressive scout­ing for insects. The most aggressive weed policies resulted in well-closed canopies and bountiful harvests. Less diligent weed control increases the field’s weed seedbank, which can trigger the need for heavier crop thinning early in the sea­son and interfere with canopy closure, as observed at some locations in this study. Uncontrolled weeds also frequently provided reservoirs for SRKN and some viruses. Except for fungal pathogens, the degree of weed control provides a vis­ible measure of successful integrated pest management for most pests and pathogens in chile.
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Rebecca Creamer is a Professor and plant pathologist in NMSU’s Department of Ento­mology, Plant Pathology, and Weed Science. She teaches courses in plant pathology, plant virology, and integrated pest management. Her research program focuses on curly top virus and its insect vector, weed hosts, and disease epidemi­ology and management, and she leads a multi-state project that studies curly top and its impact in the western USA.
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June 2019 Las Cruces, NM