SW MN IPM Stuff 2020 Issue 14

Volume 23 number 14   8/4/2020

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Greetings,

For the most part, the crop in SW (and much of Minnesota) looks pretty good. It could always be better but so far this one looks pretty good.

Crop Weather

Minnesota map showing precipitation as a percentage of normalFigure 1. Where it’s wet and where it’s not. Source: MN DNR

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.

Much of Minnesota currently has experienced growing season weather that Goldilocks would approve of. However there are areas that have seen weather that has been too wet (sometimes including hard water) or too dry (Figure 1). Adequate soil moisture levels will be needed to maintain large corn and soybean canopies.

Corn

For the most part, pollination has been good. There has been some minor silk clipping from red-headed flea beetles and where populations were very high, corn rootworm beetles. Even with the latter pollination went OK in most cases.

In the fields I have been in, fungal leaf disease has been relatively rare with very few northern corn leaf blight lesions and only an occasional sighting of very low levels of gray leaf spot in continuous corn. Even common rust has been more rare than typical in SW MN. Foliar Physoderma symptoms (brown spot) are present in some fields but this disease also is less prevalent than previous years. Bacterial leaf streak is probably the most commonly seen foliar disease of corn, at least in those areas of southern Minnesota that have seen abundant rainfall.

Life is not a box of chocolates in all cornfields, however. Corn rootworms are having a very good year. Most issues continue to be longer-term continuous corn. In addition to corn without Bt-RW traits, there has been unexpected damage to multiple Bt-RW pyramids from several seed companies. Western corn rootworm is implicated in most of these cases but northern corn rootworm have also caused their damage. Some of the better performing soil insecticides have been helping reduce damage. If you have planted a rootworm hybrid and have rootworm damage, it is important to contact your seed company. Irrespective of lodging, rootworm yield impacts, can be greater under droughty conditions.

Watch for spider mite populations developing in drier areas. Stalk rots might be an issue if high yield potential meets moisture, or other late-season stress. Cereal aphid populations might start to increase on stressed corn. Both species can be favored by earlier fungicide/insecticide applications.

Soybeans

Soybean field with light colored plants on the end rowsFigure 2. Two-spotted spider mite damage on the edge of a west central Minnesota field.

Most early-planted soybeans are late R4 with some short-season beans in early R5.

Bacterial blight is prevalent in areas where heavy rains have occurred. Warm, wet weather has been favorable for Rhizoctonia.

In the drier areas of Minnesota, more fields with economic threshold populations of two-spotted spider mites are showing up. Off-colored canopies are symptoms of mite damage on the edges of affected fields and can often be seen from the road (Figure 2). Initial symptoms of spider mite symptoms are small yellow spots (stipples) on the upper leaf surface and the presence of mites and webbing on the undersides of leaves. Parts of west central Minnesota eastward have seen less rainfall and more spider mites (Figure 1). Spider mite reproduction will be slowed but not stopped by the stretch of cooler weather in the forecast.

There have been chlopryrifos (Lorsban, etc.) applications to control spider mites. I have not heard of any resistance issues. I have talked to a few agriculturalists that have treated field borders for spider mite infestations. These fields need to be watched as field interiors typically have some infestations where damage symptoms develop later.

green cloverworm moth on a leafFigure 3. Green cloverworm adult on soybean. Most individuals are darker than this. Note the large eyes and projecting mouthparts.

We are currently conducting a survey of soybean fields in Minnesota for soybean gall midge. So far, infested fields have been found in areas similar to last year. At this point, Minnesota infestations have not become more widespread or severe. If you observe this insect, please let me know. We should see another generation appear on soybeans soon. The white mold gall midge, a similar appearing non-pest species, has been observed on white mold infested soybeans in Stearns County this year.

You may be seeing the dark, skittish green cloverworm moths (Figure 3) in soybean fields. Unlike most moths, green cloverworms can be active during the day. Often, the larvae experience high levels of mortality from disease and predators. 

three Japanese beetles on leavesFigure 4. Japanese beetles.

Although the populations are not large, Japanese beetle (Figure 4) has been seen a bit farther west than usual this year. In southern Minnesota, Highway 169 has been the western front, more or less. Some crop consultants have observed low beetle numbers in soybeans near Glencoe, Waconia, and Mankato. Others may also be having their first Japanese beetle experience in their area.

Soybean Aphid Watch 2020

So far, 2020 soybean aphid populations have been slow to develop. For the most part, aphid populations have been low, extremely low in some parts of southwest Minnesota. Some economic threshold population (250 aphids/plant) fields have been treated in central and east central Minnesota and perhaps elsewhere. These locations are similar to those described as areas to watch in an earlier issue of IPM stuff. Hot, windy weather and open canopies earlier in the season probably slowed infestations in many areas as did heavy rain events.  

In those areas where soybean aphid populations are higher, they tend to be highly variable from field to field. Populations are now starting to build at the SWROC with one SCN study currently much above threshold (Figure 5). Here too, the fields and plots vary greatly in soybean aphids populations.

Soybeans infested with aphidsFigure 5. Aphids in an SCN study at the SWROC. Many, if not most of the nymphs on these plants will develop wings and leave the plant. There is a lot of stress on these late-planted soybean plants: lower potassium soil tests, root disease and SCN in a long-term continuous soybean rotation.

Yes, it’s that time of year again. In parts of Minnesota, some soybean fields are seeing winged aphids leaving some fields and arriving in others. I am not aware of any heavily infested areas in Minnesota that will can supply a large number of winged aphids to others.

As late R4 (full pod) stage plants start to cease leaf production at the top of the main stems, aphid populations often rapidly increase and produced winged adults. Large scale movement of aphids between fields and into new areas often occurs. This happens about the same time each year and coincides with the Farmfest ag show and the Sturgis Motorcycle Rally.

With most fields planted early and similar in growth stage, I am curious how this year’s dispersal event will play out. Typically, these migrants seek late-planted or the longer maturity season soybeans to colonize.

It is possible that they will colonize some these older stage soybeans uniformly and be spread out and we won’t see large populations develop until later (late R5-6). Aphid populations often increase more slowly part of the R5 (beginning seed) stage but can increase rapidly during late R5 and early R6 stages. Do at least some targeted scouting through R5 to help find potential economic infestations.

Winged aphidFigure 6. Part of the 2020 Sturgis dispersal event?

Within two weeks, we will know what the aphids decided. Although Farmfest 2020 was cancelled, Sturgis is on (Figure 6).

Some agriculturalists chose to apply an insecticide with a fungicide applications a couple weeks ago. It just might be worth paying extra attention to those.

Alfalfa

Potato leafhoppers are still active. As August wanes and winds begin to shift to the north, populations usually start to decline, perhaps due to adults moving south. 

Seeing lots of yellow butterflies out there? The sulfur butterflies and their caterpillars are seldom big yield robbers in MN alfalfa (or soybean).

Happy trails,

Bruce