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Applied soybean disease and insect management research – 2021
Submitted by Bruce Potter, University of Minnesota Extension IPM Specialist, May 31, 2022
Management-resistant weeds, insects, mites, plant pathogens, and nematodes have been selected through the reliance on single management tactics for crop pests and the ‘insurance’ application of pesticides and host plant resistance genes. Additionally, new soybean pest problems continue to emerge, and old pests become unexpected problems under favorable environmental conditions.
Together, the overall goal of the following studies on insects and plant disease is to improve understanding of how plant disease and insects affect yield, profitability, and management practices of Minnesota soybean growers by examining 1) The efficacy and potential economic benefits of pesticide applications and 2) Short and long-term changes in the insects and pathogens affecting Minnesota soybean production. This project is composed of three small plot research and survey objectives.
Objective I. Evaluate insecticide and fungicide efficacy in an ongoing and systematic way
I a) Foliar fungicide studies
To help determine the yield and economic benefits of insurance applications of foliar fungicides applied to rotated soybeans, studies have been conducted since 2016 at three University of Minnesota Research & Outreach Centers (ROCs) located across southern Minnesota. These conventionally tilled study sites were not selected based on the expectation of a particular disease (e.g., Sclerotinia white mold, frogeye leaf spot). In 2021, two foliar fungicides (Miravis® Neo and Delaro® 325) were applied to three soybean varieties (1.5, 2.0, 2.3 RM) at the R3 growth stage. The fungicides were compared to an untreated control with respect to their effects on soybean disease and yield.
An early-season drought affected all three sites but moderated at different times during the season. Higher yielding sites had an earlier onset of rain. Stem and foliar disease pressures were at the lowest levels since we initiated these multi-site studies in 2016. While varieties varied in yield, no significant yield differences (p= 0.10) among the untreated check and the two fungicides were observed.
Before 2021, one, or both, fungicides had a significant ( p=0.10) positive yield response in 10 of 13 studies (76.7%). When the results from 2021’s drier early and midseason weather are combined with previous years, only (56%) of the studies had a significant yield response.
These data show that foliar fungicides can help maintain soybean yield in some southern Minnesota environments and provide economic benefit if used selectively. However, consistent, profitable yield responses are unlikely to be obtained when, as in this study, applications are not targeted to specific environments and diseases. They do not provide evidence that insurance applications of foliar fungicides will compensate for bad weather or poor agronomic decisions. The 2021 data do not counter the hypothesis that grain moisture and harvestability influence yield responses to foliar fungicides when disease pressure is low.
Foliar fungicides can be targeted toward known or expected diseases based on field history, weather, or symptoms with some expectation of yield protection. However, in many cases, fungicides are applied before the type and severity of the disease are known. Analyses correlating yields of these longer-term studies with factors such as planting date, seasonal rainfall seasonal temperatures may provide clues to help growers increase the probability of positive economic returns for their fungicide applications.
I b) Insecticide efficacy
Throughout Minnesota, extremely low 2021 soybean aphid (Aphis glycines) populations prevented a planned insecticide efficacy study. However, two-spotted spider mites (Tetranychus urticae) flourished in the same warm, dry, often windy weather and drought-stressed soybeans. A planned insecticide efficacy study was shifted to a two-spotted spider mite population at the University of Minnesota SWROC near Lamberton, MN.
The performance of eleven foliar insecticides and acaricides was compared to an untreated check. These treatments included compounds labeled for two-spotted spider mite in soybean, an insecticide that was not expected to control mites (sulfoxaflor), and varying rates of the pyrethroid bifenthrin alone and in combination with other insecticides. Applications were made to a very high population mite population where it was likely that some soybean yield loss had already occurred.
Meaningful data analyses for this study were complicated and limited by the near complete collapse of the mite population within eight days after the application of the pesticides. The most likely cause for the rapid decline of mites at the study site and in nearby commercial soybean fields, is an entomopathogenic fungal disease, possibly Neozygites.
Numerically, the results appeared to suggest some efficacy of dimethoate, the need to maintain mite-labeled rates of bifenthrin, and the need to apply narrow-spectrum miticides early in an outbreak.
This study site experienced reduced chlorpyrifos field efficacy with confirmed resistance during 2012. The 2021 data provide no evidence that chlorpyrifos resistance has persisted at this location. This mirrors observations from most commercial fields where chlorpyrifos was applied. As a result, there was no apparent advantage to combining bifenthrin with chlorpyrifos. However, due to recently revoked crop tolerances, chlorpyrifos is no longer a treatment option for soybean pests.
Ongoing insecticide efficacy studies for soybean aphids have been conducted in Southwest Minnesota since 2003. MNSR&PC funding has provided support to ensure the studies continued. These long-term studies have monitored annual changes in aphid populations, the long-term efficacy of insecticides, and the relative performance of new compounds. Sometimes, as in this “failed” efficacy study, they can provide a glimpse into the benefits that biological control, normally working unrecognized in the background, can provide.
Objective II. Monitor soybean pests and pathogens both short and long-term.
This objective supports objectives I a, I b, and objective III b might also be considered part of this objective. Long-term pest information is useful in understanding changes in pest populations. The long-term fungicide studies include disease susceptible varieties. These types of information could be part of, and benefit from, broader public-private cooperative efforts on crop pest information management.
Methods.
Plots at the fungicide study sites were visually rated for the presence of diseases and insect pests during the early vegetative (ca. V3) stage, at the time of fungicide application (R3), and late in the season (ca. R6). Stem diseases were evaluated by destructively sampled stems from plots bordering the fungicide study during the late season rating. Diseases and pests were not at levels that justified more intensive sampling.
Results.
While they are not positioned in areas where soybean losses from any pathogens or insect pests are expected, the plots in these studies continue to reflect general local disease and insect pressure in local areas. Disease incidence was very low overall, particularly at the very dry SW MN location and none reached levels expected to reduce soybean yields. Arthropod pest populations at these study sites did not reach economic threatening levels. Unusual for Southern Minnesota. Unusual for Minnesota,
As typical, Japanese beetles (Popillia japonica) and their defoliation were only observed at the RROC site in SE MN. However, increased bean leaf beetle (Cerotoma trifurcata) populations were observed at all three sites and were observed at the SWROC for the first time since these studies were initiated in 2016. Soybean aphid populations remained below economic levels in 2021. Two-spotted spider mites (Tetranychus urticae) took advantage of the same weather and were present at all sites. Sub-economic spider mite defoliation injury to soybean foliage was visible at the RROC and SWROC sites but mite populations in some nearby fields required treatment.
Objective III. Examine the distribution and potential host range of the soybean gall midge (SGM)
in Minnesota.
III a) SGM hosts
The SGM is a new pest of soybeans in the Midwest. In soybean, SGM larvae typically infest near the base of stems where the injury caused by their feeding can cause plants to lodge, or wilt and die. The SGM can cause near-total yield loss on field borders and up to 35% whole-field yield loss. Management of this insect has proven difficult because susceptible soybean plants are exposed to multiple and extended adult flight periods. Although an injury to the stem may provide an attractive site for SGM to lay eggs, naturally occurring fissures are produced near the base of soybean stems as they expand during the V2, and later growth stages also provide egg-laying sites.
It is not known whether SGM is native to North America or an introduced pest and little is known about the host range of this new crop pest. In addition to soybean, the SGM has been found to infest sweet clover and, much less frequently, alfalfa. There is a single anecdotal report from bean (Tiger Eye, a cultivar believed to have originated in South America) in NE (Dr. Tom Hunt, UNL, pers. comm.).
Mobile sentinel plants
The following in-field methods were selected for this pilot study, in part, because SGM has not yet successfully been maintained as a laboratory colony. Additionally, the known MN infestations with consistently high population densities are located in commercial soybean seed production fields, limiting what could be seeded, and where herbicide applications could injure some species.
Seedlings of fifteen varieties/cultivars of nine annual legume species were greenhouse-grown in potting mix within 4-inch square injected molded pots. When overwintered and 1st generation adults were active, the containers were placed within the border rows of a Rock County Minnesota field with a history of yield-limiting SGM infestations. The plants were left in the field to be infested for 7-10 days, then returned to the greenhouse for 4-5 days to allow larvae to develop. The stems were evaluated for injury symptoms and examined for SGM injury symptoms and larvae. During this 2021 pilot study, SGM injury symptoms and larvae were found only within a low percentage of soybean stems.
Other observations
Commercial dry bean fields in Cottonwood, Kandiyohi, Lac Qui Parle, Renville, Stevens, and Swift Counties that were encountered during surveys for SGM in soybean were examined. No SGM larvae or signs of infestation were found in dry bean fields in these counties with histories of SGM infestations.
In Minnesota, SGM larvae have been observed in Alfalfa (Rock Co.) and sweet clover (Kandiyohi, Lac Qui Parle, Rock, Yellow Medicine Cos.) but only when nearby soybeans have also been infested. No other legume hosts were found in August 2021 observations of native prairie legumes in WC and SW MN.
This project supplements other work on this insect, focusing on the possibility that dry bean or annual legumes can be infested and potentially suffer yield loss from SGM. Secondarily, any annual legume crops or native prairie legumes found infested may provide clues to the geographic origin of the SGM.
III b) Continued survey for changes in SGM distribution
MSR&PC funding for this project supplemented survey funding from a North Central Soybean Research Program project on soybean gall midge, particularly in dry bean production areas of WC MN and in prairie habitats (see Objective III a).
The detection of SGM at very low population densities is most effective in late R5 to early R6 stage soybeans, near the end of the growing season. This limits the time for growers, their advisors, or planned surveys to find initial infestations.
Despite the lower SGM infestation levels in 2021, thirteen new counties were confirmed by the SW MN IPM crew, bringing the total to 29 Minnesota counties.
Plants with SGM injury symptoms were found in soybean fields in two additional counties in Central and West Central Minnesota counties. Larvae were not found in these symptomatic plants so these counties could not be confirmed. It is probable that the SGM is even more widely distributed in Minnesota but at very low levels. Also, the distribution of SGM-infested counties may reflect sampling frequency rather than an expansion north and east out of extreme SW MN.
In addition to locating new areas with SGM infestations, larvae from each county where soybean gall midge was observed were collected and preserved in ethanol and submitted to the Koch lab for future work on parasitism.
Delimiting the range and prevalence of this insect could provide clues to the stability of SGM populations and whether its range is static or expanding. If ongoing, and particularly if new survey tools such as pheromones or weather-dependent predictive models can be developed, SGM surveys might help determine whether growers within a geographic area need to begin aggressive SGM management