East Michigan vegetable update – June 26, 2019

Conditions for diseases are perfect, but the pathogen must be present to cause harm.

Drawstring damage and yellowing
Drawstring damage and yellowing from active swede midge larvae feeding. Photo by Ben Phillips, MSU Extension.


The next week will warm up with daily highs in the low 80s, but with a chance of rain and cloud cover for six out of ten days and humidity hovering around 85-90% overnight. These are fantastic disease conditions. The most destructive above-ground and spreadable pathogens for vegetable growers are downy mildew in cucurbits, and late blight in tomatoes/potatoes. At this point, the pathogens themselves have not been reported in the state or surrounding states. But, the stage is poised for rapid disease spread. Be at the ready with preventative and effective fungicide programs.

One benefit of the wet weather is the routine washouts of potential thrips outbreaks, and also the unique biocontrol fungus (Entomophthora muscae) that infects seed and root maggot flies and reduces the second and third generation numbers. In cool cloudy conditions, the fungus compels them to climb to the tops of the leaves, a symptom known as summit disease, before sporulating and killing the fly. The spores then blow in the wind and attach to more flies. These are easy to spot as motionless flies perched on the tips of crop leaves. In warm sunny conditions, the flies can fight the infection by sitting in the sun, a symptom known as behavioral fever. They can’t regulate their own temperatures, so they expose themselves to hot temperatures deliberately to cook out the pathogen in their bodies.

You can find more detailed weather information for your area by visiting the MSU Enviroweather station closest to you:

Here is a table summarizing European corn borer activity, based on Growing Degree Day models. I used Lapeer as an example and removed the seed and root maggot data because those pests are not much of a threat going forward.


European Corn Borer emergence – Base 50 F

Current degree days (Lapeer)


Overwintering generation start to emerge and lay eggs

450 (occurred June 11)

Peak flight and egg laying of overwintering generation

700 (predicted to occur June 29)

Peak flight and egg laying of 1st generation


Peak flight and egg laying of 2nd generation



Swede midge trapping
Swede midge trapping utilizes the scent of female midges to attract males. Insecticide applications must be applied during these flights to protect plants from the maggot that hatches within about a week after mating occurs. Adults live for up to five days for mating and eggs take about three days to hatch. Effectiveness of row covers in emergence areas can be limited, because emergence and mating occurs on and around plants. Covers are most important on seedlings hardening off outside, as these plants serve as a vector to infest new fields. Photo by Ben Phillips, MSU Extension.

In cole crops, Swede midge damage has increased in the two weeks since the large trap catches started in Sanilac County. At this point, it is mostly “drawstring” deformation in the leaves expanding from the middle. Scarring on the inner leaf stem was not yet evident. Larvae and adults were easy to find on symptomatic plants. We witnessed larvae jumping to the soil to pupate.

Some broccoli and cauliflower plantings have buttoned and are bolting. According to H.C. Wein’s book, “The Physiology of Vegetable Crops”, cauliflower can bolt once they have developed a critical number of leaves and then experience an accumulation of time with “relative cold” temperatures between 50 F and 70 F. Broccoli can form heads in “relative cold” of 80 F, and so are more adapted to mid-summer production than cauliflower. Temperatures that are cooler or warmer than this will delay head formation. Typically, we see our warm/hot day time temperatures favoring leaf growth, and our “relative cold” night time temperatures banking hours to trigger head formation. It is extremely variety dependent with these crops, and so growers must choose their varieties and planting schedules accordingly.

I think some of these growers were seeing a lack of consistently warm temperatures to hold off head formation. Instead, the crops experienced perfect head forming conditions and formed one as soon as they reached their critical leaf number and were still relatively small plants. Once head formation begins, warmer temperature actually pushes it along faster, which causes them to bolt and flower more quickly.

Transplanted sweet onions from out of state are bolting at a higher percentage than in other years. According to the MSU Bulletin E1307, “Temperatures must be below 50 degrees F for several days to induce bolting. The effect is cumulative – more bolting occurs at lower temperatures and with a longer period of cool temperatures. Some cultivars are more susceptible to cold temperatures than others. Onions grown from sets and transplants are very susceptible to bolting because they can be induced to bolt before planting.” This means, storage prior to planting should be managed to prevent those temperatures to minimize bolting. Onions need about five leaves before they can bolt, and sometimes larger transplants can have that many, or will soon develop them in the field while conditions are still cool. H.C. Wein’s book, “The Physiology of Vegetable Crops” reports that once onions reach their critical size or leaf number, they can bolt when exposed to an accumulation of about 20 days at 50 F.

Bacterial and Stemphylium leaf blights are favorable in these conditions. Growers can read up on Stemphylium controls here: https://www.canr.msu.edu/news/protect-onions-against-stemphylium-leaf-blight

Garlic scapes were at markets last week. What a neat byproduct of the routine production of garlic.

Sweet corn over the V6 stage will be susceptible to the overwintering generation of European corn borer, which is poised to begin its peak egg-laying phase this week. Some questions about grass weed control came up. Laudis and Shieldex are two good products for most grasses. Shieldex has a shorter plant-back window for other vegetable crops.

Melons and watermelons, zucchini and cucumbers are all under the gun of bacterial diseases right now. I have seen angular leaf spot on the oldest leaves of multiple plantings. Downy mildew spore counts can help give us a heads up that an infection is upon us. So far, we have only detected hops downy mildew, which is a similar but different pathogen of hops. Spore counts from around the state are available.

Cercospora leaf spot and Alternaria leaf spot have both been showing up in sugar beets and red beets. Check out “Leaf spot detections in Michigan sugarbeet fields” by Dan Bublitz and Jaime Wilbur for more information.

Asparagus picking is slowing down and lay-by plans are next. I am not sure how the term “lay-by” came to be. It is similar to strawberry renovation in its purpose and its timing.

Strawberry season has been extended from the relatively cooler cloudier days. Berry size has gone down. Renovation plans are next, and I don’t know how that term was created either, but it is a pretty drastic process.

Get togethers

Young Growers Exchange Bus Tour. This upcoming tour is designed to allow the “next generation” of vegetable growers to network with each other while visiting West Michigan farms and is open to all growers in the Midwest. The tour will take place over two half days on July 17-18. Day One will focus on soil health, showcasing Morgan’s Compost and a major potato grower who is part of a soil health initiative. Day Two will include another major potato grower plus muck vegetable production. There has been a lot of grower interest, so early registrations in the grower category are filling up fast. We are also offering a single-day option to allow for additional participants to caravan and leave on their own. Please consider registering via: https://events.anr.msu.edu/vegbustrip/

It is never too early to make accommodations to attend the Great Lakes Fruit, Vegetable and Greenhouse Growers EXPO, December 10-12, 2019, in Grand Rapids, Michigan. Hotel blocks are open and tend to go fast. The combination of grower-focused, research-backed presentations and an exhibit hall featuring a diverse set of vendors make it a can’t-miss event.

Please contact me at phill406@msu.edu or 616-901-7513 with questions, concerns, or to schedule a farm visit. You can also send plant materials to MSU Diagnostic Services.

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