SW MN IPM STUFF 2017 Issue 4

05/31/2017

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2017-04

This newsletter and the advice herein are free. You usually get what you pay for.

Crop Weather

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 https://swroc.cfans.umn.edu/weather.

While most corn stands are good, there are some issues. I have talked with several consultants about uneven emergence. Dry soil (yes, there were areas that the upper inch or so dried out) delayed germination of some seeds while others in warmer, moister soil germinated and emerged sooner. In some cases, corn stands range from 2-leaf to beginning to spike; putting the late emerging plants at a disadvantage.

The common denominators are shallow planting depth, aggressive tillage, variable soil textures and sometimes just bad luck. The effect on yield, if any, will depend on the percentage of plants emerging late. Very shallow planted corn will be at risk for poor root development later, particularly if the soil dries.

This is going to make scouting difficult for any stand reducing insects or pathogens more tedious. Fortunately, accuracy of today’s planters makes it easy to tell where a missing plant should have been.

Soybean emergence also looks good in most cases. There have been reports of pre-emerge herbicide injury from group 14 herbicides. These symptoms may have been caused by environmental conditions rather than herbicide or disease in some fields.

Alfalfa

Cutting of first crop is underway. Watch for alfalfa weevils in second cutting. In SW MN, populations range from adults to 2nd-instar larvae. The cool spring will stretch larva activity further into second cutting. We have not yet found potato leafhoppers at the SWROC but your fields might be different. Second and third cuttings usually experience the highest populations of these migrants.

upper canopy stripe rust infection in winter wheat

Figure 1. Pocket of severe upper canopy stripe rust in 2017 winter wheat.

Cereal Grains

Encouraged by rainy conditions and cool weather, wheat stripe rust caused by the fungus Puccinia striiformis became severe last week in the more susceptible varieties in U of M winter wheat variety trials located at the SWROC. The winter wheat varieties ranged from boot stage to beginning flowering.

The rust inoculum is now present in SW (and probably other areas) Minnesota. I have not seen stripe rust in 2017 spring wheat as of yet but will continue to look as the season progresses.

stripe rust on winter wheat

Figure 2. Stripe rust on winter wheat. Note the light orange spore color and linear arrangement of fruiting structures on the leaf blades.

Stripe rust is well controlled with foliar fungicides. This does not mean that your spring wheat crop needs an early fungicide application in advance of that needed to protect upper leaves and help minimize Fusarium head blight. Many varieties are less susceptible than the winter wheat varieties that are currently affected. Continue to include rusts in your southern MN spring wheat scouting activities.

Warm temperatures minimize the impact of this fungus and spring wheat may benefit once weather warms up.

powdery mildew on winter wheat

Figure 3. Powdery mildew on winter wheat. Black fruiting structures are formed during the sexual stage of this fungus.

Another disease favored by this cool, wet spring weather is powdery mildew, caused by the fungus Blumeria graminis f. sp. tritici and it is now easily found in the lower canopy leaves and leaf sheaths of many of the winter wheat varieties.

Wheat powdery mildew does not persist in warm, dry conditions and is intolerant of the ultraviolet light that occurs in the recently lacking sunshine. Although capable of reducing yield, the white fuzzy mycelia of this fungus disease is seldom seen at any significant level of Minnesota wheat. You probably have seen a related form in shady areas of your bluegrass lawn.

There are other genera in this order of fungi that infect plant hosts other than grasses.

symptoms of barley yellow dwarf on winter wheat

Figure 4. Symptoms of barley yellow dwarf on winter wheat. The virus creates symptoms more red than yellow on oats and is also known by the common name “oat red leaf”.

Remember the aphids I mentioned earlier this spring? There are some small infection centers showing symptoms of barley yellow dwarf virus starting to show up in winter wheat. This virus is transmitted by several species of aphids that feed on wheat and other small grains. Unlike rusts, powdery mildew and many other foliar fungal diseases of wheat, there is no control for barley yellow dwarf once infection occurs.

Things that go bump in the night

The black light trap at the SWROC has been catching true armyworm moths for the past few weeks - consistently but at low numbers. So far the levels have been low. Watch corn for leaf feeding by small larvae in fields where winter rye covers, especially those terminated late, were planted.

I have not heard reports of black cutworm larval feeding in Minnesota. These insects may also have an affinity for cover crop plantings. We are looking at insect populations in cover crop research at several sites.

Soybean aphid watch 2017

Weather delays during soybean planting have created a wide range in soybean emergence in many areas. At the latitude and local environment of Lamberton, MN buckthorn has pollinated and berries have just started to form on the female plants (there are male and female buckthorn). Research in Ontario has shown soybean aphid eggs on buckthorn hatch as buds open. Historically, I usually find aphids on the largest soybeans about the time the buckthorn berries begin to swell. I am curious to see how consistent this relationship actually is.

early buckthorn berry development

Figure 5. Early buckthorn berry development.

The earliest planted fields, without insecticide seed treatments and near buckthorn are where aphids are often found first. It’s still very early in the season but refer to soybean aphid scouting for information on scouting aphids.

I am trying to fit together a couple of pieces to the puzzle of early season soybean colonization by aphids. Your information might help me do that. I would appreciate hearing from folks across MN or elsewhere when you find the first soybean aphid of the season and what stage of development your local buckthorn is at.

The following buckthorn staging based on berry development might work as good as anything: 1) Flowering, berry set, 2) 1/8” berry, 3) full size berry (~ ¼ inch), 4) berry turning dark, and 5) berry ripe.

The buckhorn will be easier to find than the first aphid colonies on soybean. Finding aphids on buckthorn in the spring is a task for those who enjoy ticks and frustration.

-Bruce Potter