SW MN IPM Stuff 2019 Issue 13

8/16/2019

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

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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 SWROC Weather

As of August 12, we remain about three days behind on growing season degree-days (1717 degree-days compared to a 1795 degree-day long-term average from May 1). 

Unlike some areas to the east and the north we continue to miss heavy rains, winds and tornados. At 14.15 inches of precipitation since May 1, we are now 1.65 inches above long-term average but we did pick up a welcome 0.12 inches last night. Of course, on your farm, your mileage may vary. 

Field corn ranges from blister to milk stage.   

Depending on planting date and maturity group, most soybeans are R4-R5 (full pod to beginning seed stage) stage.

Crop Pests

Corn

European Corn Borer (ECB)

ECB degree-day development models can be found at VegEdge Degree-day Models & Forecasts. Scouting for 2nd generation ECB should be winding down in southern and west central MN.

ECB populations appear to remain relatively low but I’ve had a few calls on economic threshold populations of corn borer in SE and WC MN (Thanks Luke!). A history of numerous fields of corn without Bt traits increases the odds of finding ECB in an area.

If you have fields without above ground Bt traits that you would be willing to let us examine for fall survey please contact me at bpotter@umn.edu. We will be looking for ECB infestation levels statewide and collecting larvae to check for parasitism and percent single generation (univoltine) biotype.

Corn disease stuff

I’ve been getting some photos of circular lesions on corn leaves that resemble both holcus leaf spot and eyespot. Many have a yellow halo around the lesion but are larger than expected for eyespot. We will have samples incoming to the University of Minnesota Plant Disease Clinic (https://pdc.umn.edu/) and could have an ID soon. The Plant Disease Clinic is a good tool for identifying the cause of unfamiliar disease symptoms. In addition to ECB, we will be looking for disease during the fall corn borer survey.

Soybean

Soybean aphid watch

Soybean aphid populations continued to increase over the past week. However, there are many fields and areas with very low populations where aphids ares still concentrated on edges or pockets in the field.

I have been noticing that some winged aphids are leaving SWROC soybeans, particularly where new leaves are no longer produced. While some of these aphids are redistributing within the same field, others are moving longer distances. The aphids know whether they are coming or going and you can too. Winged aphids associated with nymphs with wing pads indicate the colony will leave the plant and perhaps the field. Winged aphids with small nymphs are an indication they are immigrants.

Aphid populations will be increasingly concentrated near pods and then on mid - lower canopy leaves in R5 stage soybeans. Check lower in the canopy when scouting. Early-planted, short-season beans will be safe from yield loss first, perhaps as soon as next week.

This year, because of the large number of late-planted soybean acres, we may need to scout later in the season than usual. On the other hand, aphids could begin to move to buckthorn anytime from now until mid-September. Dense, moist canopies and cool temperatures can help induce aphid population collapses from entomopathogenic (insect-killing) fungi.

soybean leaves with brown dead spotsFigure 1. Frogeye leaf spot symptoms

Soybean disease stuff

If you see Frogeye leaf spot symptoms (Figure 1) in your fields this year let Dean Malvick (dmalvick@umn.edu) or me know. I have seen little of this disease in the drier, western part of Minnesota this year but have heard of symptoms in areas where rainfall has been more frequent the past few weeks.

This disease can vary widely by variety and local environment. Of particular interest are frogeye symptoms in fields where fungicides were applied. This fungal disease has shown resistance to strobilurin fungicides in some parts of the country. Frogeye leaf spot is among the diseases we will be rating as part of MN Soybean checkoff funded fungicide studies across MN.

Minnesota map of counties with several counties in west central and southwest MN shaded inFigure 2. Minnesota counties where soybean gall midge larvae have been detected in 2019.

Soybean gall midge

Sharp eyed observers have added a few more counties (Figure 2) where soybean gall midge larvae (Figure 3) were observed in 2019. This week, larvae were found in plants in Southwest Minnesota (Murray County (MDA)) and also in West Central MN (Chippewa and Bigstone Counties). Most of these detected infestations have relatively few infested plants and may not produce yield loss. However, the number of fields where the midge can be found in Rock County and the Yellow Medicine, Lac Qui Parle, Chippewa and Bigstone County area causes some concern for the future.

The actual distribution of this insect in Minnesota is probably wider currently documented because: 1) In many areas, this spring’s late planting may have left adult flies with few soybean plants to colonize and 2) It is very easy to miss scattered infested plants with symptoms (dead or wilting, brittle stems breaking near the ground) within the canopy, particularly if you have not seen the symptoms of moderately infested plants (dark discolored epidermis at the base of the plant and occasionally near the junction of a branch and the main stem) in person.

soybean stem infested with orange soybean gall midge larvaeFigure 3. Soybean gall midge larvae can be found below the epidermis of the lower stem of soybean. Second generation larva can occasionally be found inside the stem pith or higher on the plant near branches.

Determining the extent of infestations of this insect is one of the first steps in management. Please let me know if you suspect this insect in your fields!

Again this year, there are also reports of small, orange midge larvae infesting soybean plants with white mold. It is important to document these locations as well but these may turn out to be a different species.

I’m trying to set up a field day in West Central Minnesota in the near future. The purpose of this brief meeting will be to show symptoms discuss this potential impact of this insect. Stay tuned!

two tan banded leafroller mothsFigure 4. Oblique banded leafroller moths.

Things that go bump in the night

In addition to seeing them in soybean fields, we have been picking up green cloverworm moths in the SWROC light trap. If you hear complaints about medium size, dark moths in large numbers; these may be the suspects.

Also present in soybean fields and light trap captures are the small oblique banded leafroller adults. They seem more abundant here than most years. The forewing shape of these moths is distinctive. The larvae roll leaves together to form a sheltered feeding area. This insect has a wide host range and can be a pest of apples. In soybean, it is a very minor pest. 

banded leafroller larvae on soybean leafFigure 5. Oblique banded leafroller larva.

Happy trails,

Bruce