On-Farm Investigations of Stale Seedbeds with Biofumigation for Improved Management of Weeds and Soil-Borne Diseases in Chile Pepper

Results for hypothesis 1: a mustard cover crop can be grown after a fallow season false seedbed.  The fallow-season false seedbed successfully reduced seedbanks of weed species that hinder chile pepper production at the farm near Las Uvas.  Most notably, the false seedbed initiated by irrigation on September 17 caused considerable emergence of black nightshade (Solanum americanum) — a summer annual species that is the primary weed management challenge at this study site.  Species-specific plant densities at 22 days after irrigation were as follows (mean ± standard error): black nightshade, 395 ± 31 plants m^-2; yellow nutsedge (Cyperus esculentus), 9 ± 2 plants m^-2; common lambsquarters (Chenopodium album), 2 ± 1 plants m^-2; spurred anoda (Anoda cristata), 1 ± 1 plants m^-2; morningglory species (Ipomoea spp.), 1 ± 1 plants m^-2; pigweed species (Amaranthus spp.), 1 ± 1 plants m^-2; yellow woodsorrel (Oxalis stricta), 1 ± 1 plants m^-2.  Emerged weeds were terminated prior to flowering.  Thus, emerged weeds represent reductions in weed pressure for chile pepper grown in 2019, especially for resident weed species with annual lifecycles (black nightshade, common lambsquarters, spurred anoda, pigweed and morningglory species, yellow woodsorrel).  The farmer was pleased with the initial results of the fallow-season false seedbed and plans to use the tactic again in the future. 

Although the fallow season false seedbed reduced the number of weed seeds in soil, it did not support the mustard cover crop that was seeded after the fallow season stale seedbed.  This is known from comparisons of mustard cover crop establishment among study sites.  In early December 2018, the mustard cover crop was robust at Deming (106 ± s.e. 5 plants m^-2, 85 ± s.e. 1.6 % cover) and Leyendecker (241 ± s.e. 14 plants m^-2, 89 ± s.e. 2.1 % cover), moderately robust at Columbus (85 ± s.e. 14 plants m^-2, 50 ± s.e. 6.7 %) and weak at Las Uvas (85 ± s.e. 14 plants m^-2, < 5% cover).  Low biomass for the mustard cover crop at Las Uvas was caused by a frost shortly after seeding.  This frost was not entirely unexpected considering the typical time for first frost at Las Uvas (Table 1).  Although disappointing, the Las Uvas results indicate the need to seed a mustard cover crop well before the expected first frost.  Because mustard cover crop seeding at Las Uvas was delayed by the false seedbed operation, Las Uvas results suggest that false seedbeds implement in late-summer do not provide sufficient time for mustard cover crop seeding. 

Results for hypothesis 2:  mustard cover crops suppress weeds and disease in chile pepper.

Site-to-site differences in mustard cover crop vigor in December persisted to spring.  For three sites (Columbus, Deming and Leyendecker), mustard cover crop aboveground biomass at termination ranged from 463 to 597 g m^-2 (Table 1), which was comparable to published reports of mustard biomass in other regions of the United States (Weed Science 53:695-701; Agronomy Journal 108:151–161; Agronomy Journal 107:1235–1249), but less than maximum levels of mustard cover crop biomass reported in the SARE publication “Managing Cover Crops Profitably, 3^rd edition” (approximately 900 g m^-2).  At Columbus, mustard cover crop biomass at termination was greater than the biomass for the site-specific alternative (barley).  At Deming, mustard cover crop biomass was equivalent to the biomass of the site-specific alternative (mustard with wheat). 

In general, mustard cover crops suppressed winter weeds (Table 1) and contained sinigrin — the primary pesticidal compound in the particular mustard species — at concentrations consistent with previous reports (Table 2).  At sites where the mustard cover crop established (Columbus, Deming, Leyendecker), incorporated mustard residues reduced the number of Palmer amaranth seeds in soil (Figure 2).  Palmer amaranth seeds that persisted in mustard residue were less germinable than seeds retrieved from soil without the mustard residues, suggesting that mustard residues induced secondary dormancy in Palmer amaranth seeds. 

At Columbus, the effects of mustard cover crops on weeds and disease in chile pepper was not determined because all chile pepper plants were intentionally terminated shortly after emergence.  Chile pepper plants were terminated because the farmer collaborator determined that the stand was not sufficient for strong yield, which was consistent with statewide reports of relatively few acres of chile in excellent condition (Figure 3).  The farmer collaborator did not attribute poor chile pepper performance to a specific cover crop, as demonstrated by the termination of chile pepper across the entire field.  Following chile pepper termination, the field at Columbus was seeded in cotton that was managed conventionally.  For this cotton crop, early season weeds and soil-borne diseases were not affected by cover crop treatment. 

At Deming, cover crop treatment did not affect the following variables in the chile pepper growing season: chile pepper stand density at 3 weeks after emergence; weed densities at 3, 6, and 9 weeks after chile pepper emergence; hoe times at 3, 6, and 9 weeks after chile pepper emergence; disease incidence at 13, 15, and 16 weeks after chile pepper emergence; and chile pepper fruit yield collected at 19 weeks after chile pepper emergence.  Similarly, weed densities, hoe times, or disease incidence were not affected by cover crop treatment at Las Uvas. 

Although the mustard cover crop did not affect the response variables considered in this study, the mustard cover crop may still have influenced chile pepper production at Deming.  The farmer collaborator in Deming indicated that the mustard cover crop likely harbored insects that were damaging to the chile crop.  Most notably, the farmer collaborator suspects that beet leafhoppers (Circulifer tenellus) in the mustard cover crop persisted after cover crop termination and transmitted a curtovirus to cause curly top disease in chile.  Such an occurrence would be consistent with previous research indicating that Brassicaceae plants in the U.S. Southwest are hosts for both beat leafhoppers and the beet curly top virus (Southwestern Entomologist 28:117-182; Plant Disease 89:480-486).  In our study, we were not able to detect mustard cover crop-induced curly top in chile because the viral disease, which is transmitted by leaf hoppers, was prevalent across the entire field and not confined to mustard plots.  However, additional data collected supports the farmer’s hypothesis.  Specifically, at 16 weeks after chile pepper emergence in the mustard cover crop field, chile pepper stand density (8.6 plants 10-m row^-1) was lower and more variable (coefficient of variation, 53.6%) than chile pepper stand density in a nearby field that did not follow a mustard cover crop (16 plants 10-m row^-1; coefficient of variation, 13.4%).  Further, the farmer collaborator indicated that the chile pepper yield in the mustard cover crop field was 40% lower than nearby chile pepper fields without the mustard cover crop.

Initial economic analyses indicated that the cost of growing and terminating the mustard cover crop ranged from $104 to $158 acre^-1.  This was considerably more expensive than cover crop production costs presented in the SARE Technical Bulletin “Cover Crop Cover Crop Economics: Opportunities to Improve Your Bottom Line in Row Crops.”  In this study, the maximum cost for cover crop production occurred at Columbus where the cost for mustard cover crop termination was $121 acre^-1.  The high cost for mustard termination at Columbus was caused by the sequence of tillage operations needed to terminate the cover crop on raised beds.  At Deming, where the mustard cover crop was not grown on raised beds, the cost for cover crop termination and subsequent soil preparation for chile pepper was $51 acre^-1.  These results reveal the difficulty in terminating the mustard cover crop on raised beds. 

At Leyendecker, the weed community was comprised of Palmer amaranth (Amaranthus palmeri), Wrights groundcherry (Physalis acutafolia), spurred anoda (Anoda cristata), junglerice (Echinochloa colona) and large crabgrass (Digitaria sanguinalis). The mustard cover crop incorporated into soil reduced broadleaf and grass weed densities, as well as hoe times at 3 weeks after chile seeding (Figure 4). The weed suppressive effect of the mustard cover crop was diminished by 6 weeks after chile pepper seeding. Disease incidence data are not reported because there was no occurrence of soil-borne disease in this field.

In the greenhouse, soil with mustard cover crop residues produced larger chile pepper plants than soil without mustard residues (Table 3).  This is consistent with a previous study that determined that relative growth rates of onion (Allium cepa) and celery (Apium graveolens) were greater in soils with yellow mustard cover crop residues as compared with soils without mustard cover crop residues (J. Sustain. Agric. 34, 2–14). Chile pepper plants grown in soil without mustard residues exhibited symptoms of wilting diseases (Table 3).  This wilting contributed to reductions in chile plant height for chile pepper plants grown in bare ground soils. Wilting of some plants in bare ground soils was likely caused by a soil-borne pathogen; however, laboratory cultures of chile plants with wilt symptoms did not show Verticillium dahliae pathogen.

Results for hypothesis 3: mustard seed meal (MSM) added to soil before or after chile pepper seeding suppresses weeds and soil-borne diseases in chile pepper.

MSM applications before chile pepper seeding. 

MSM amendments to the soil reduced weed density and hoe times early in the chile growing season (Figure 5). Weeds at the study site included Palmer amaranth, Wrights groundcherry, spurred anoda, and ground spurge, as well as several grasses that were unidentifiable to the species level at the seedling stage.  MSM at 4400 kg ha^-1 reduced overall weed seedling emergence by 71% and hand-hoeing time by 35% compared to the untreated control on May 30th. MSM at 4400 kg ha^-1 reduced hand-hoeing time on June 19, but MSM effects were not significant on July 18th. There were also no significant differences in yield across treatments which is generally consistent with the literature (Weed Technology 9:669-675).

MSM applications after chile pepper seeding. 

Greenhouse experiment.  Results from the first greenhouse experiment indicated that chile plants from the 2-leaf to 6-leaf stages were severely injured by MSM applications to the soil surface.  Ninety-two percent of the 2-to-6-leaf stage plants were terminated by MSM at 4400 kg ha^-1.  MSM at 2200 kg ha^-1 killed 67% of chile plants from the 2-leaf to 6-leaf stages.  Severe crop injury from MSM without incorporation was consistent with previous research that determined the biofumigation properties of MSM were attributed to volatile compounds that are toxic to many plant species. 

Results from the second greenhouse study indicated that MSM injury on chile was reduced or eliminated by incorporating MSM in soil.  Specifically, greenhouse study results indicated that MSM at 4400 kg ha^-1 on the soil surface caused lasting reductions in chile plant photosynthesis; but, photosynthetic rates recovered from initial injury when MSM at 4400 kg ha^-1 was buried (Figure 6).  For MSM at 2200 kg ha^-1, surface applications initially reduced photosynthesis, whereas buried applications generally did not affect photosynthetic rates in chile plants.  Reductions in photosynthesis caused by MSM on the soil surface resulted in diminished plants at 14 days after application (Figure 7).  Together, the greenhouse experiments indicated that burying MSM protects chile plants from damage that can occur from post-emergence applications of MSM.  We applied this knowledge to our field study where we evaluated the potential for crop injury and yield loss from post-emergence applications of MSM.

Field experiment.  Post-emergence applications of MSM provided varying degrees of weed control, with weed control generally higher for MSM at 4400 kg ha^-1 (Table 4) compared to MSM at 2200 kg ha^-1 (Table 5).  For both rates and at all sites, post-emergence applications of MSM did not cause visual injury and did not reduce chile pepper yield (Table 4, Table 5).  These results are promising because (1) this represents one of the first reports of safe, effective application of MSM after crop emergence, (2) if not controlled, weeds that emerge during the middle phases of the chile pepper season severely reduce both yield and harvest efficiency, and (3) farmer collaborators on this project specifically asked for strategies to apply MSM after chile pepper emergence.