West central Michigan vegetable regional report – August 3, 2016

Home gardeners can help our pickle growers by keeping an eye out for cucurbit downy mildew.

Disease severity values (DSV) calculated from Oceana Tomcast weather sensors.

Disease severity values (DSV) calculated from Oceana Tomcast weather sensors.

Asparagus growers should be aware that disease severity value (DSV) accumulation ramped up over the week ending last Wednesday, July 27. Foliar disease may continue to develop in asparagus despite dry weather, so fungicide covers will be important (see graph). Chlorothalonil or mancozeb can be applied every time 15 DSVs accumulate to protect against purple spot.

DSVs can be calculated from in-field weather sensors to time fungicide applications for asparagus purple spot, as seen in the graph. DSV accumulations ramp up when foliage remains wet and air temperatures are warm. DSV accumulations accelerated starting July 16 at the Oceana Tomcast sensors shown in the graph due to high dew points and overnight temperatures. Foliar disease will likely start to develop now too.

Carrot growers should be aware that DSV accumulation ramped up over the week ending last Wednesday, July 27. Foliar disease may continue to develop despite dry weather, so fungicide covers will be important. Chlorothalonil alternated with a strobilurin such as azoxystrobin can provide protection against foliar disease. Chlorothalonil currently remains the backbone of foliar disease control in carrots.

Cucurbit growers and home gardeners should continue to be on the lookout for downy mildew. Please report occurrences to .(JavaScript must be enabled to view this email address) along with a picture of the upper and lower part of a leaf. Michigan State University Extension plant pathologist Mary Hausbeck recommends a program of Zampro alternated with Zing! on a weekly basis for west Michigan cucumber and melon growers. This pathogen is likely already present in west Michigan.

Are you a gardener? Please keep an eye on your cucumbers, as your report could help a commercial grower save their crop! It will be kept anonymous. Note: powdery mildew creates a powdered sugar look on plants – this is not downy mildew. In contrast, downy mildew creates blocky lesions on leaves that look “dirty” underneath when it sporulates under moist conditions (e.g., dewy mornings). If you suspect your plants have downy mildew, please take a picture of the top and bottom of the leaf and send it via email to .(JavaScript must be enabled to view this email address).

Downy mildew vs powdery mildew

A cucurbit leaf with downy mildew (left) and powdery mildew (right). Powdery mildew looks like powdered sugar. This pathogen is common this time of year and is not necessary to report. Downy mildew produces blocky or “angular” yellow areas on leaves. If you turn over a leaf with downy, you may see dark “mold” if it’s sporulating. Please report potential cases of downy mildew to .(JavaScript must be enabled to view this email address). Photo: David B. Langston, University of Georgia, Bugwood.org.

For onions, Stemphyllium is causing problems in some areas. Luna Tranquility is now labelled for onions and provides good control of this disease.

Risk of late blight development for tomatoes and potatoes was high for four out of the five days, Sunday, July 31 through today, based on the weather forecast at the Holland, Grand Rapids, Kalamazoo and Muskegon Enviro-weather stations.

For sweet corn, there is a risk of scattered corn earworm flights into sweet corn for the middle of this week. Western bean cutworm activity has heightened this year, and moth activity has recently peaked. Consider scouting your pre-tassel corn for egg masses. Growers who normally do not apply insecticides this time of year can be caught off guard by this damaging pest. A long-lasting pyrethroid (e.g., bifenthrin) applied as soon as silks start to emerge can help protect against these western bean cutworms. If you want to target it specifically (e.g., outside of a normal earworm spray), you can estimate when caterpillars will hatch based on egg color and make an application around that time. “Managing western bean cutworm in field corn” by MSU entomologist Christina DiFonzo is an excellent fact sheet.

These caterpillars initially feed on the tassel, but move down the stalk once the tassel pokes out. During their journey to the ear they can be killed if they contact a pyrethroid with good residual activity. Published thresholds for western bean cutworm vary: Purdue University Extension recommends treating when 4 percent of scouted plants have egg masses or larvae, while Cornell University recommends a lower threshold of 1 percent. To scout, examine the top surface of the top four leaves on 20 plants at five locations. This is where females tend to lay eggs.

Western bean cutworm egg mass.

A western bean cutworm egg mass on west Michigan sweet corn two weeks ago in mid-July 2016. Females lay eggs on pre-tassel corn on the top surface of the upper four leaves. Photo: Ben Werling, MSU Extension.

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