Southeast Michigan vegetable update – July 8, 2020

A brief break from the heat is on the horizon, but the pests and plant stress may linger.

Powdery mildew on the underside of leaves
Powdery mildew first appears in discrete areas, usually on the underside of older leaves. Photo by Marissa Schuh, MSU Extension.


The heat should break Thursday into Friday, though medium and long range forecasts are fairly confident that we will be back in the 90s by the middle of next week. There have been some very isolated precipitation in our area. There is a chance of some rainfall as the next weather front moves into our area.

The table below shows rainfall totals for the Michigan State University Enviroweather stations in southeast Michigan, as well as degree-days calculated using the Baskerville-Emin Method. Degree-day average for Commerce and Hudson is over five years, while Deerfield is over four years. Rainfall is in inches. For a refresher on degree-days and how to get this information in your area, see “Accessing growing degree days with Enviro-weather” from MSU Extension.

Rainfall and degree day totals as of July 8, 2020


Degree days (base 42)

Degree days (base 50)

5-year degree day average (base 50)

Rainfall since April 1





9.07 (+0.0)





8.52 (+0.02)





9.76 (+0.0)

General notes

Hot temperatures will impact flowering crops. High nighttime temperatures will mean some blossom drop in tomatoes, eggplant and peppers, especially those in hoop houses. Hot temperature so early in the day will likely limit bee activity, so we may see some misshapen zucchini and cucumbers and some poor pollination in squash and pumpkin.

Black plastic has also been reaching high temperatures and causing plant damage. Thermal imaging has shown Michigan black plastic reaching temperatures of up to 130 degrees Fahrenheit. Farms across the state are reporting lesions and meltdown in young plants that are touching the plastic. There can also be air trapped under the plastic, especially if the material isn’t tightly stretched—that can become quite hot and escape from under the plastic around the plants, creating a chimney of hot air.

Crop reports

Cabbage is being harvested. Watch preharvest intervals when selecting products for caterpillar control. The online Midwest Vegetable Production Guide page on cabbage caterpillars can be used to quickly compare preharvest intervals of different products if sprays are needed.

As wheat is drying down and being harvested in this hot, borderline drought-like weather, thrips pressure may be high. When treating thrips in cabbage, make sure to rotate chemistries. If pressure if high, Radiant tank-mixed with a non-ionic surfactant is a good choice. Research at Ohio State University has investigated how the ordering of the thrips products can improve control. They also found that lower rates of Exirel provide similar control to high rates when placed thoughtfully into a spray program.

Garlic harvest is approaching. Dry soil at harvest will help garlic wrappers come out clean—an upside of this weather?

In potatoes¸ potato leafhopper populations are high statewide. Look for yellowed, distorted leaves. If you give afflicted plants a shake, leafhoppers should fall out if they are present.

In sweet corn, I am catching low numbers of corn earworms. My trap in Lenawee County is averaging 1.14 moths a night, and my Monroe County trap is averaging 0.57 moths a night. If you had issues with birds last year, remember that the key window for keeping birds out of corn is before they realize the ears are there. This means deploying deterrents early. There are many tools available to scare birds away, though birds are smart, so multiple strategies may be needed.

As vine crops start to set fruit and flower, it is time to scout weekly for powdery mildew. I have not yet detected it in our area, but this disease does not necessarily need water to spread, so the hot, dry weather won’t slow this disease down once it’s established.

When managing powdery mildew, product selection is important. Powdery mildew is a pathogen that is constantly evolving new resistances, so make sure the products you are using are still effective against powdery mildew. Make sure to rotate by FRAC codes to prevent new resistances from occurring. Products that have shown good efficacy in Ohio trials include:

  • Aprovia Top (FRAC 3 plus 7)
  • Inspire Super (FRAC 3 plus 9)
  • Luna Experience (FRACE 3 plus 7)
  • Microthiol Disperss (FRAC M2)
  • Procure (FRAC 3)
  • Quintec (FRAC 13)
  • Rally (FRAC 3)
  • Vivando (FRAC U8)

I know many like to rely on Quintec, but Cornell University recommends using Quintec sparingly (e.g., less than the four times permitted by the label) as there is some level of resistance out there. Fungicides in Groups 1 and 11 may not be effective. Fungicides that are affected include Cabrio, Flint, Quadris, Satori, Sovran and Topsin.

If you want a deep dive into product considerations, dig into Cornell’s 2020 powdery mildew recommendations. There is a lot of good insights here, though note they may have different populations of powdery mildew there than we have here.

Cornell has released a rundown of organic products available. Trials in New York have found sulfur products (e.g., Micro Thiol Disperss) to be effective, while Michigan trials have shown resistant/tolerant varieties work well with Serenade (Bacillus subtilits) and Trilogy (Neem oil). If using sulfur, do not apply this close to the application of oils. This is because the oils can allow the sulfur to move into the plant and injure it.

I have not caught any squash vine borer adults in the last two weeks in my traps in Lenawee and Washtenaw counties.

I am getting pictures of moldy first fruit of zucchini and yellow squash. It is pretty common for the first flush of female flowers to not be pollinated particularly well; it seems to take bees a little while to find the plants. If this is the cause of the issue, you should stop seeing these issues as bees find the plants and it cools down and bees become more active.

Reach out

Please contact me at or 517-264-5309 with questions, concerns or to schedule a field visit. I have the ability to visit fields on a limited basis.

Great Lakes Vegetable Producer’s Network

The Great Lakes Vegetable Producer's Network is a live weekly roundtable discussion during the growing-season for commercial vegetable producers in the Great Lakes and Midwest region. It is broadcast live via Zoom at 12:30 p.m. ET/11:30 a.m. CT every Wednesday from the first week of May to the first week of September. Listen live or later. If you have a pressing vegetable production issue that you would like discussed, simply email it, along with your phone number, to

Next week will be “Irrigation setups for vegetables.”

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