SW MN IPM Stuff 2016 Issue 11

Volume 19, Issue 11 - July 15, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-11

This newsletter and the advice herein are free. You ususally 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 http://swroc.cfans.umn.edu/weather

We have received 1284 degree-days (base 50°F) and 10.63 inches of rainfall May 1 -July 14. An additional 105 degree-days (base 50°F) accumulated from April 11-30. Above average degree-day and precipitation accumulations continue.

Crop growth has been good the past couple weeks, and most of the drier crop producing areas of western Minnesota have now received welcome rain. As was common this spring, some areas of SW MN have once again received a bit more rain than they needed at the time. This has been an unusually bad year for hail to the east of the SWROC.

As typical, thinned stands from hail and flooding will produce an environment for late-season waterhemp emergence. This particularly obnoxious weed has started to poke above the soybean canopy. Fortunately, the corn crop is now tall enough to hide these waterhemp breeding grounds until you remember them when combining. Those of you with giant ragweed escapes in corn won't have to wait that long.

We did manage to find a pocket of probable glyphosate resistant common ragweed at the SWROC. It's a nice complement to the glyphosate resistant waterhemp in the same area.

There have been several fields in southern Minnesota that have identified populations of PPO herbicide (e.g. Flexstar) resistant waterhemp. If you suspect a problem, the disease clinic at University of Illinois can test your waterhemp weed sample. See instructions for testing common waterhemp for resistance to group 14 herbicides for more information.

Corn

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Mature corn borer larva.

The most advanced corn in the area has tasseled and is now beginning to silk. Most fields are looking better and have regained a dark green color. There are many fields that are very uneven in height from early water damage. This will have consequences for future corn rootworm and weed control.

We did pick up a large true armyworm flight in the SWROC black light trap on July 11th. This is probably too late to have any impact on crops.

European corn borer moth captures remain low at the SWROC. Any 1st generation larvae that survived should have pupated. We do not see many of these since Bt corn was widely adopted.

I was wrong about not having much corn rootworm damage in southwest Minnesota this year. There is significant damage in research trials placed in areas with damage and high western corn rootworm beetle populations in 2015.

Always be sure of your target There are low levels of northern corn leaf blight in some fields. Northern corn leaf blight and gray leaf spot are two fungal diseases that can sometimes benefit from fungicide applications. Corn following corn planted to susceptible hybrids (I am not sure why anyone would want to do that) is most likely to see a response. Fields with no or low levels of disease will not benefit.

Not all the leaf spots on corn are controlled by fungicide, or are even plant disease for that matter. The following examples will not be controlled by fungicides. Not even if you close your eyes, click your heels together and repeat..."ROI...ROI...ROI"

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The corn biotch leaf miner fly produce small uniform scars when laying eggs.

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Corn biotch leaf miner larva feeding produces a distinctive leaf mine damage.

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Feeding damage by western corn rootworm beetles. The beetles will feed on leaves if silks are not yet available.

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Goss's blight and wilt early leaf symptoms. While similar in size and shape to northern corn leaf blight note the "freckles" of this bacterial disease. Fungicides will not control this disease and there are no chemical controls. Expect to see this disease on susceptible varieties in areas that received hail.

Northern corn leaf blight and gray leaf spot photos can be found on the internet.

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Most soybeans are at R2 with some at R3. Many with 30-inch rows have nearly closed canopies. There are some issues. Uneven soybean development between fields and within fields because of planting date will make tracking soybean aphids more difficult this summer.

There are a number of reasons for soybeans to suddenly turn yellow during reproductive stages. Iron deficiency is not the only reason for yellow leaves in higher pH rims. Nitrogen deficiency because of poor nodule function, several pathogens, and soybean cyst nematode (SCN) injury are also common causes, and multiple problems often occur together. Fields where upper leaves occasionally turn yellow and recover make me suspect the SCN and/or Phomopsis and Diaporthae spp. pathogens.

The SCN and soybean aphid both can increase potassium deficiency symptoms on soils testing low for that nutrient. I suppose you should thank these pests for helping you find inadequacies in your soil fertility program.

Soybean Aphids

Populations are building, but very slowly in most areas. If you have not done so, you might want to find a field or two to help gauge aphid population increases.

There are several species of aphid predators and parasitic wasps feeding on the aphids in most fields. They work cheap... so why not let them help you. The first indication that soybean aphid may be starting to outperform natural enemies in your field is an increasing percentage of plants infested with aphid. Another indication of a potential problem is numerous small aphid hot spots developing within the field. These hotspots often produce numerous winged aphids.

Remember, regardless of the soybean price, you are safe in waiting to apply a treatment until 80% of the plants have aphids, aphid populations average 250/plant, and aphid populations are increasing.

Soybean defoliators

Remember to assess defoliation through the canopy and in several parts of the field. Rate defoliation on top, middle, and lower green leaves. Insect species all prefer different parts of the canopy and soybeans can fill pods from leaves at nodes above and below those that are damaged. Use 20% "whole plant" defoliation as an action or insecticide treatment threshold. Yes, the soybean plants will look horrible with that much damage but you will feel worse about it than them.

Recent insect observations in soybean fields. Stuff like this usually hits Twitter first.

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Oblique banded leaf roller caterpillar. This moth species has a whide host range but I have never seen large populations on soybean.

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Cocoons of parasitic brachonid wasps. The victim was likely an alfalfa caterpillar that was devoured by the larvae.

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Caterpillar of the white-lined sphinx moth. One of the plants that the caterpillars feed on is purslane. My garden is in desperate need of a significant infestation. These are large caterpillars and more than a meal for many birds. The moths are common around tubular flowers such as petunias. Like many sphinx or Hawk moths, they resemble hummingbirds in flight.

Happy trails,

Bruce Potter