West central Michigan vegetable regional report – June 19, 2013

Insect and disease activity could pick up as warm, humid weather arrives and winds shift to the southwest.

General update

Over the next week, temperatures and humidity are expected to increase, which should increase reproduction of some insect pests. In addition, winds may shift to the southwest, which could bring migratory pests up from southern areas. For example, given the weather forecast, there is a low risk of corn earworm colonizing southwest Michigan over the next week (see www.insectforecast.com).

Crop update

Major asparagus growers in Oceana County indicated they expect major asparagus harvest to cease the middle of next week. However, temperatures are expected to climb above 80 degrees Fahrenheit this weekend, which could lead to a decline in spear tip quality and an end to harvest. Once temperatures climb, the tips of asparagus spears are more likely to “open,” which lowers quality.

Importantly, the forecast is calling for humid, warm weather next week with potential chances for rain. This represents weather that could favor disease, making it important to start protectant fungicide programs in fields where fern has developed. The last major in-season opportunity for herbicide application will occur soon; this “lay-by” application is made immediately post-harvest, after spears are picked. Applying after a picking reduces the risk of herbicide injury – as opposed to applying when spears are emerged or have developed into fern – as there is less chance of materials contacting living tissue.

There have not been reports of high numbers of aster leafhoppers in carrots as of this Monday, June 17. However, I did capture higher numbers in one Oceana County location last week, so timely scouting remains important, especially since warmer weather could increase local reproduction and shifting winds could bring aster leafhoppers up from southern areas.

A local celery scout reported that variegated cutworm adults were captured in slightly higher numbers this week in both Decatur, Mich., and Hudsonville, Mich. Aster leafhopper captures continued to be low early this week in both these areas.

In cucurbits, striped cucumber beetle adults are active in southeast Michigan, reportedly causing damage to zucchinis and other cucurbits. I would expect activity to pick up in more northern areas soon, including west central Michigan, if it has not already. Scout field edges two to three times per week for this pest; numbers can increase rapidly early in the season as overwintered adults move from field boundaries into crops.

FarMore FI-400 and Cruiser treated seed both contain thiomethoxam, which should provide two to three weeks of control after planting, as will drenches made at planting with Platinum (a.i. thiomethoxam) or Admire (a.i. imidacloprid). An alternative, and complimentary approach once systemics have worn off, is to apply foliar insecticides using thresholds. A table of thresholds is below. To determine if a field is over threshold, count beetles on two plants at 30 locations scattered throughout a field. Note, the thresholds below are general as some varieties of each crop are more susceptible to bacterial wilt, the disease vectored by striped cucumber beetles, than others. A good summary of varietal susceptibility can be found at Cornell University’s Vegetable MD Online.

Thresholds for use in decision-making for striped cucumber beetle control in cucurbits.

Crop

Threshold

Cucumbers

1 beetle/plant

Pumpkins with less than three true leaves

1 beetle/plant

Muskmelon

1 beetle/plant

Pumpkins with more than three true leaves

5 beetles/plant

Zucchini

5 beetles/plant

Winter squash

5 beetles/plant

Finally, soil temperatures have increased the past week. This means the likely time for concerns about seed corn maggot damage on mineral soils has passed, unless you have recently applied manure or incorporated a cover crop within the last three weeks.

Onion thrips are now present at below-threshold levels in most onion fields. Recent rains likely kept numbers from increasing rapidly. As weather warms, populations will likely increase. Early in the season, a threshold of one thrip per leaf can be used to decide if an application of insecticide is necessary. Thrip counts were generally below this level in onion fields Michigan State University Extension has conducted counts in as of yesterday (June 18).

Maggot damage, likely due to onion maggot feeding, has also been detected in both Grant, Mich., and Byron Center, Mich., at low levels. Seed corn maggot larvae, another species that is a “cousin” to onion maggots, generally attack germinating seeds and seedlings, resulting in early-season stand reduction, while onion maggots will feed on larger seedlings and developing bulbs. Seed treatmentrs—including FarMore FI-500 and Sepresto, can provide control of both species. There is also an organic version of the FarMore treatment.

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An onion maggot larva feeding on the basal part of an onion
plant. Leaves of affected plants may droop and then die. Onions
become more resistant to damage with age.
Photo credit: Ben Werling, MSU Extension

Finally, a low percentage of onions were showing symptoms of onion smut in both Grant, Mich., and Hudsonville, Mich., yesterday (June 18). Plants with onion smut contain blackened, spore-producing material inside leaves; leaves often die. Infected plants may remain stunted during the season. Controls for onion smut are preventive and need to be applied at planting. One method is to use seed treated with a smut-controlling fungicide plus a drench of a material with the a.i. mancozeb.

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An onion seeding with onion smut. The darkened areas in the
leaf contain smut. Holding a leaf up to the light can help you
detect this disease. Photo credit: Ben Werling, MSU Extension

In potatoes, small larvae of Colorado potato beetles have been detected in west central Michigan. Keep an eye on populations. In general, small larvae are much easier to kill with foliar insecticides than large larvae and adults. There are some stage-specific materials, including Rimon (a.i. Novaluron), which the label says should be applied when the majority of the population is at egg hatch or are small larvae.

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