West central Michigan vegetable update – July 24, 2019

Onion thrips population may be on the rise after last week’s warm weather. It’s time to balance pest control objectives with pollination in bee-dependent crops.

Thrips on onion plant
Thrips populations increased this week after our hot weather. Make sure to scout your fields and consider applying Radiant if populations are high. Photo by Ben Werling, MSU Extension.


Over the past week, we saw very high dewpoints and temperatures. Mean temperatures were 3-6 degrees Fahrenheit above normal. Precipitation was very variable; most fell Friday night, July 19, into Saturday, but totals ranged from less than 0.5 inch in the north to over 6 inches in some spots. Mason, Lake and Manistee counties had some very high totals.

In the short term, there is a high-pressure, Canadian-origin air mass over the state. The vast majority of Michigan will be dry today with seasonable temperatures. The next chance for precipitation will be overnight Friday, July 26, into Saturday for northern areas, and Sunday/Monday to the south. The forecast calls for 0.25-0.5 inch of precipitation for the next week for most of the Lower Peninsula.

In the medium term, outlooks call for above-normal temperatures and precipitation for July 29 – Aug. 6. Michigan State University agricultural meteorologist Jeff Andresen feels this is a bit aggressive, and we may see closer to normal temperatures and precipitation.


Asparagus growers are focusing on cover sprays to protect fern from purple spot and rust. Purple spot will likely become more noticeable over the next week given last week’s high overnight temperatures.

Common asparagus beetle larvae were active in four fields we visited last Friday. Larvae were large and there were few adults, suggesting we are at the end of a generation (with larvae going into pupation). Japanese beetle adults were also active and tarnished plant bugs were present. If you have a field like this that is in bloom and need to make a pesticide application, consider applying permethrin in the evening. Permethrin has activity against asparagus beetle larvae, Japanese beetle and tarnished plant bug, but growers report it is not as effective against asparagus beetle adults. Sevin XLR Plus could be included at your next fern spray after bloom; at that time, adult beetles may be more prevalent.

Carrots had potential Alternaria on old leaves at one site I visited this week; given our recent weather, it makes sense to be on top of a fungicide program now.

Celery pest complexes have been dominated by caterpillars this early summer. Scouts report Variegated cutworm captures were high this week in Ottawa County, and caterpillars have been seen in the field. Celery leaftier has been flying and caterpillars have been present, but to date worms have not been detected in the heart. Aphids have not been detected in significant numbers to date, and aster leafhopper populations are very low. Anthracnose has been present at some level, but overall scouts report that disease pressure is not high.

Cole crop caterpillars continue to be present. Cabbage looper is present in addition to diamondback moth at one location MSU Extension interns are scouting. If you are an organic grower, spinosad is a good option if you have significant cabbage looper present, as Bt is reportedely less effective against loopers compared to the other caterpillar pests.

Cucurbit downy mildew has not been reported in Michigan yet or in the Midwest, but spores were captured back in June. After that initial capture in Allegan County on June 28, no spores were captured through July 15.

Powdery mildew was present at one hoophouse I visited, and pumpkin and squash growers have been protecting their crops with regular sprays. If you have not started already, it is time to protect cucurbits from powdery mildew with products that target this pathogen.

Insect pests were active this week, but growers will need to balance protecting pollinators with pest problems. Squash bug adults, eggs and nymphs were present in one pumpkin field I have been checking in Mason County. The threshold for treatment is one egg mass per plant; in this field there were about 0.5 egg masses per plant, so it was below threshold. Squash vine borer was present at both an Oceana County and Mason County field, but I did not see any damage. Striped cucumber beetles were present at one site in blossoms, but numbers were well below threshold. Both squash bees and honey bees were present. In this situation, it makes sense to “wait and see” if pesticide applications are necessary based on continued scouting.

If an insecticide application is warranted, consider applying a pyrethroid in the evening. At this time, the current days’ blossoms will be closed and tomorrow’s will not have opened. Pyrethroids are toxic to bees, but have short residual compared to some other pesticides, which may reduce exposure.

Note, we may see some pollination issues in cucurbits given the very hot weather last week.

For copper application, to limit the potential for injury make sure to apply it when it will dry rapidly. High humidity increases drying time and potential injury. If you have struggled with angular leaf spot, which has caused fruit lesions for some spaghetti squash growers, now is a good time to assess if further copper applications are needed. If you do not see foliar symptoms of angular leaf spot, you can consider discontinuing it. Not all fields will have the pathogen.

Onion thrips populations were above threshold for the first time at one Kent County field I have been scouting, not surprising given last week’s very hot weather. Treatment with Radiant (one-day preharvest interval) makes sense if populations are high (over two thrips per leaf). If populations are moderate (one to two thrips per leaf), Exirel could be an option (one-day preharvest interval).

Note, do not use Exirel if you have already used Minecto Pro. In this case, follow up with Radiant if treatment is needed. If you apply Radiant this week and/or next week, you could finish the season with Lannate (seven-day preharvest interval). Tank-mixing Lannate with Warrior (14-day preharvest interval) may improve its efficacy.

Minor purple blotch and botrytis have been present in two fields I have been checking, but Stemphylium remains the main foe. Below is a potential spray program for Stemphylium:

  • Week one: Luna Tranquility at 22 ounces per acre plus Bravo
  • Week two: Tilt EC at 8 ounces per acre plus Bravo
  • Week three: Luna Tranqulity at 16 ounces per acre plus Mancozeb
  • Week four: Tilt EC at 4 ounces per acre plus Bravo
  • Week five: Luna Tranquility at 16 ounces per acre plus Mancozeb
  • Week six: Tilt at 4 ounces per acre plus Bravo

Luna Tranqulity, Mancozeb and Bravo all have a seven-day preharvest interval, while Tilt has a 14-day preharvest interval, so the above can be adjusted based on harvest timing. Growers I have talked with are on either “week three or four” of this spray program. The above program adjusts rates to stay within season totals based on the label and includes mancozeb with Luna to limit the number of Bravo applications.

For potatoes, there have not been any reports of late blight in Michgian. It has been confirmed in Wisconsin potatoes in Wood County. The genotype was US-23, which is Ridomil-sensitive. Spore traps in Ontario have also detected late blight spores, but the disease has not been detected in the field. The risk in Michigan has remained moderate, but given the above it is worth intensifying scouting. A chlorothalonil-based program is recommended until the pathogen is detected in the state. In Montcalm County, apothecia of white mold and initial infection have been reported and weather has been very favorable.

For peppers and tomatoes, night temperatures above 75 F can cause flower abortion, leading to reduced fruit set. We may see some “skipped” sets after last week’s weather.

Sweet corn maturity in early plantings has been reportedly variable with some ears ready and some not. Corn earworm captures averaged five per night at an Oceana County location I am monitoring. At this trap catch, it makes sense to protect silking corn on a five-day spray schedule. Corn earworm sprays typically clean up this pest if they are made on a timely schedule.

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