West central Michigan vegetable regional report – August 31, 2016

We are in the home stretch, but growers should still look out for certain insects and diseases.

A yellow asparagus cladophyll dying with two purple spot lesions. All photos: Ben Werling, MSU Extension.

A yellow asparagus cladophyll dying with two purple spot lesions. All photos: Ben Werling, MSU Extension.

Purple spot symptoms are increasing in Oceana County asparagus fields as the season goes on. Cladyophylls in some fields are dying and falling to the ground. Michigan State University Extension research shows that asparagus plants do not finish moving carbohydrates and nutrients to the crown until mid-October. Major defoliation prior to this time can reduce nutrient reserves to fuel next year’s harvest.

If you are considering making your last fungicide application soon, think about ending the season with a protectant like mancozeb or chlorothalonil, but also tebuconazole. Tebuconazole will compliment protection against purple spot, provided by protectants, with approximately two weeks of protection against rust to close out the season (where this disease is of concern).

For carrots, there has been a very steady accumulation of disease severity values this year. Last year, major accumulation didn’t happen until Labor Day. This means tops may have significant foliar disease if fungicides have not been applied on a disease severity value-based schedule.

Cucurbit virus symptoms detected earlier this month in yellow zucchini were confirmed as having watermelon mosaic virus. This virus can cause green discoloration of yellow fruited squashes. This virus is carried from weedy hosts to cucurbits by aphids. There are some varieties of yellow zucchini with some level of resistance to watermelon mosaic virus.

Yellow zucchini with watermelon mosaic virus

Yellow zucchini from a plant infected with watermelon mosaic virus, showing green discoloration.

Cucurbit downy mildew pressure has ramped up in trial plots in southwest Michigan, suggesting recent weather has been favorable for this disease.

If your Jack O’ Lanterns are ripe, consider harvesting them if the crop canopy is no longer providing protection. Pumpkins can get sunscald when canopies succumb too quickly to powdery mildew.

For onions, recent research by MSU Extension has shown that bacterial leaf blight can be reduced by a good thrips control program. Two years of research showed that, even with copper, uncontrolled thrips populations led to more severe bacterial leaf blight. On the other hand, good thrips control reduced incidence of this disease.

For potatoes and tomatoes, forecast risk of late blight development is relatively low for the next five days at the Big Rapids, Muskegon, Grand Rapids and Holland Enviro-weather stations.

Increased captures of corn earworm in sweet corn continued early this week at an Oceana and Ottawa County farm. Captures show why having your own trap can be helpful, as they were higher in the Oceana versus Ottawa location early this week. Having trouble keeping “the worms” out? One of the most important keys to corn earworm control is good coverage of silks. You can spray all you want, but if you are not protecting growing silks, control will be compromised.

Graph of corn earworm captures

Corn earworm catches remained elevated in monitored locations early this week.

In the past, growers with issues have detected poor coverage using water-sensitive paper placed on the silks. If coverage is not adequate, adjustments can be made and coverage can be re-tested (e.g., gallonage of water, drop nozzles, etc.). “Confirm coverage with water-sensitive paper” by Sprayers101 is a nice resource including videos that show how to use water-sensitive paper.

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