Southwest Michigan vegetable update – Aug. 1, 2018

They’re back! Say hello to squash bugs and mildews.

Early morning fog
Photo 1. Early morning conditions suitable for powdery mildew spread and development in cucurbits. All photos by Ron Goldy, MSU Extension.


High temperatures ranged from 76 to 81 degrees Fahrenheit and lows from 55 to 62 F. There was around 0.20 of an inch of rain across the area. The 50 F degree-day units are at 1,772 for 2018 compared to 1,767 for 2017 and 1,719 for the five-year average. We are pretty much having a normal year temperature-wise.

Crop reports

Cucurbit downy mildew is now present throughout the area, so a tighter spray schedule with more effective products is advised. Morning dew conditions will continue to be conducive for powdery mildew spread. Morning fog in low areas is a good indicator (Photo 1).

Squash bug egg masses (Photo 2) are easily found in most fields. If left uncontrolled, they will develop into large enough numbers that they will damage fruit and they vector diseases. They are best and easiest to control when nymphs are small. Virus incidence has had a slight increase since last week.

squash bug egg mass

Photo 2. Squash bug egg mass on a zucchini leaf.

I often receive calls about the white areas along the leaf ribs in cucurbits (Photo 3). These are not a problem and are genetic. Some varieties have the white areas while others do not.

zucchini leaves

Photo 3. Leaves of two zucchini with a total green leaf (left) compared to a variety that has white areas next to the leaf veins (right). Both conditions are normal.

Sweet corn harvest continues. Corn earworm is not a serious concern at this time, possibly because many field corn plantings are also silking. Fresh silk is attractive to females for egglaying. As soon as field corn silk dries down, females will start looking for fresh silk and sweet corn plantings will come under greater pressure.

The biggest problem in corn at this time appears to be vertebrate damage—birds, raccoons and deer. Raccoons are big and strong enough to pull the ear from the stalk, so look for ears that are pulled down. Raccoons can be deterred with electric fencing and are most active near woodlots.

Birds (crows, starlings and others) sit on the ears and peck through the husk, so their damage is often limited to the upper side of the ear. Birds are hard to control. In my experience, many sight and sound devices work for a time, but birds eventually get used to them. One of the best defenses is a good stand and a high plant population. Together, these can deter some birds from flying into the middle of the planting since it is hard for them to take off amongst all the leaves. This tends to limit damage to outside rows.

Deer are browsers and walk through the planting, usually feeding on fresh silk. You will see where silk is removed from the ear tips and tracks.

Incidence of bacterial diseases in tomatoes and peppers has increased. Since infection generally starts with infected seed, early infections can occur in one variety and not another, or in one transplant tray and not others. So early infections can have definite breaks between infected and non-infected areas. Later in the season, this will disappear as rain and wind spread the disease.

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