7/21/2021 | Volume 24 issue 11
This newsletter and the advice herein are free. You usually get what you pay for.
A self-inflicted wounds issue.
Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather.
Some thoughts on foliar fungicides
I do not give a yes or no answer for fungicides for several reasons including:
- A lack of information on the field history, disease susceptibility of the hybrid or variety, disease pressure in the field, or the economic capabilities or risk aversion of the grower.
- Just like seed, fertilizer, pesticides, land rent, etc., to apply or not apply fungicide is a biological and economic based management decision that is usually best based on a field-by-field basis.
- Previous experience of the grower with benefits, or lack thereof, for foliar fungicide applications.
With the support of MN soybean and corn checkoff funding, the SW MN IPM project and others have been conducting small plot soybean fungicides studies at multiple locations since 2016 and corn fungicides since 2017. Two fungicides and three soybean and corn varieties/hybrids were evaluated, and these changed each year. These studies were designed to detect yield responses, and not compare efficacy among fungicide products.
Soybeans - Here are what the data say for twelve southern MN site-years during 2016-2019:
- One or more fungicides yielded numerically more in 11 of 12 site-years.
- Across all southern MN sites, the average bushel increase was 3.4 bushels/acre, and the maximum increase was 6.2 bushels numerically.
- These increases, however, were only statistically significant in 9 of 12 site-years.
- A single site at Lamberton in 2020 was responsive, with an average numerical increase of 6 bushels.
- Additionally, 3 of 4 west central and northwest MN site-years were numerically non-responsive during 2017-2019. The Northwest MN location used a narrow row spacing, and these two northern locations tended to have lower overall yields.
Corn - Here are what the data say for 10 rotated corn southern MN site-years during 2017-2020:
- One or more fungicides yielded numerically more in 1 of 10 site-years.
- Across all southern MN site-years, the average bushel increase was 3.7 bushels/acre, and the maximum increase for any fungicide was 18.5 bushels. Some responses were negative.
- These differences, however, were statistically significant in only 1 of 10 site-years.
- Additionally, a west central MN site had an average response of 0.9 bushels, and northwest MN had a negative yield response in 2018-2019. Again, the northwest site used a narrow row spacing at these two northern sites; these locations tended to have lower overall yields.
- Two southern MN continuous corn sites did not have a significant yield response in 2020.
Some additional points to consider with foliar fungicides for corn and soybeans
- These studies were not directed at soybean white mold management or other specific diseases; nor were yield-threating disease levels observed.
- Foliar fungicides do not control all diseases.
- Maturity delays and higher moisture is often observed with fungicide applications.
- In absence of disease pressure these yield differences may be due to moisture and harvestability difference, not actual physiological yield.
- We do not have any replicated data under the moisture stress conditions as we are experiencing in much of Minnesota this year.
- Foliar disease levels are currently very low in many areas.
- Plant health is whatever marketing decides it is.
- Crops cannot escape drought with a fungicide application
- Applying a fungicide will not cause it to rain. Washing vehicles or planning an outdoor event are more likely to produce rain.
- Mixing an insecticide in with your fungicide can create more problems by eliminating beneficial insects and selecting for resistant pests.
- To improve odds of an economic return in soybean: Choose fields with higher yield potential, particularly where fungal diseases are observed,
- To improve odds of an economic return in corn: Select fields where controllable disease is observed, the hybrid is highly susceptible to a controllable disease or stalk rots (delayed maturity).
Corn rootworm beetles
There are recommendations for adult corn rootworm control being made. There may be some cases where silk clipping is threatening pollination, but it sounds like many of these recommendations are being made in absence of that threat. One drawback of an IPM approach to managing silk clipping is that you need to walk past the outside of the field to figure out if you have a silk clipping problem.
Beetle bombing fields with relatively low beetle numbers always provides an economic benefit to someone but seldom to the grower. It risks flaring populations of spider mites and perhaps cereal aphids which could provide a secondary positive or negative economic effect, depending on your point of view.
Rotating out of corn for a year is still the best fix a western corn rootworm problem field and can help reduce pressure for extended diapause northern corn rootworm.
A possible justification for beetle bombing in addition to silk clipping is to reduce egg laying for those fields that refuse to rotate out of corn. In many fields and areas of Minnesota, BT-RW traits no longer provide adequate control of rootworm larvae. Killing female rootworms with a foliar insecticide application, if done timely, can reduce egg-laying and next year’s beetle populations. It might allow a Bt-RW hybrid plus a good at-plant insecticide to maintain yield with minimum lodging. Unfortunately, multiple sprays are often needed to reduce high beetle populations.
Foliar insecticide applications select for insecticide-resistant beetles but do nothing to reduce selection for Bt-RW resistance. Foliar insecticide control of low to moderate beetle populations provides little benefit to the grower. Finally, during a hot, dry year, spider mites in corn are a thing.
Populations are generally low but winged soybean aphids have started to move out of small isolated “hot spots”. This year’s aphid population will depend on how widespread these hot spots are and the weather. Larger canopies can protect aphids from high temperatures, but drought-stressed plants will trigger movement from the field. Soybean aphids cannot compete with spider mite defoliation in hot, dry weather.
Two spotted spider mites (TSSM) are at treatable levels in some of the drier areas of Minnesota. At the SWROC it is hard to find a soybean plant without a few mites, often on upper leaves. They are present in some of the droughty corn here also. Note the potential to create a mite problem with an “insurance” insecticide tank mix.
Some points to consider about spider mites:
- During most years, beneficial fungi keep spider mites under control.
- Spider mites flourish in hot weather due to reduced biological control and changes in host quality of moisture stressed plants. Reduced root function from compaction/ nematodes/disease can create moisture stress in the plant.
- A single, or even several rain events will not stop a spider mite infestation. Strangely, a shot of moisture can cause mite populations that are present to rapidly increase- possibly due to changes in host quality.
- Other than bifenthrin, most synthetic pyrethroids (e.g., Warrior, Baythroid, Asana) make spider mite populations worse. Chlorpyrifos and bifenthrin do not control mite eggs.
- Insecticide efficacy is often reduced in hot dry weather.
- At least one case of lack of performance from chlorpyrifos (e.g., Lorsban) has been reported from SW MN this year. Many western Minnesota TSSM populations are resistant to chlorpyrifos and cross resistance to dimethoate is likely. Bifenthrin resistant populations have been reported from other areas of the country.
- Do not use chlorpyrifos alone. A mix with bifenthrin may still be effective if rate of both compounds is kept at the spider mite rate.
- Agri-Mek SC and Zeal are miticides labeled on soybeans, and there are additional options on corn. Mixing some of these miticides with pyrethroids for broad spectrum control may reduce their efficacy in some cases.
- Early application will likely mean re-treatment will be needed. Retreatment with the same insecticide is not advisable and, in some cases, prohibited on the label.
- Edge treatments can be effective, but they need very careful scouting to ensure that mites from the field interior do not reinfest the predator free field borders.
Meetings and other topics of interest
- July 28 Advancing Ag webinar: Edge of field practices for achieving water quality goals.
- August 10 Pest management field day. SWROC.
- August 17 Cover crops field day. SWROC