West central Michigan vegetable regional report – June 12, 2013

Weather to date could favor early season disease problems. Other crops are progressing.

The wet weather we have had for the past month has been favorable for early development of diseases, which makes it key to be on the lookout for early symptoms. Michigan State University plant pathologist Mary Hausbeck cautioned that early detection and prompt initiation of protective fungicide applications will be key. Fungicides are, in general, more effective when applied early; after a disease has developed and spread, it is hard to control and they will be less cost-effective.

Asparagus harvest has continued this past week. West central Michigan growers expect to continue harvesting until June 25. Rust aeciospores have greatly increased in the past week on older plants that were left unharvested at one location in Oceana County, suggesting conditions have been favorable for this disease. I have yet to detect rust aeciospore lesions in a 1- to 2-year-old field. The first TomCast sensor deployed by Michigan State University Extension was placed in a 1-year-old asparagus field yesterday (June 11); growers are beginning to apply protective fungicides to their young fields.

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Weather data collectors can be placed in
asparagus fields to help target protectant
fungicide sprays during weather that favors
disease.

Carrots were in the 4 leaf stage yesterday (June 11) at one Oceana County location. The carrot crop is progressing with wheat and barley windbreaks either dead or dying.

Celery. A local scout reported that variegated cutworm adults were captured in traps this past week in west central and southwest Michigan. Keep an eye on your fields over the next weeks for larvae. Aster leafhopper captures were slightly higher towards the end of last week, but captures so far this week have been relatively low.

Cucurbits. Winter squash fields in Ottawa and Oceana counties that I visited last week generally had recently emerged or had one true leaf. Growers have started to sow zucchini over the past one to two weeks.

Striped cucumber beetles are active and could colonize fields. Check field edges – where beetles tend to congregate first – two to three times per week. Beetles are not always present in cucurbits, but can rapidly increase over the course of a single day as overwintering adults emerge, so scouting is key at this point in the season. Beetles can damage plants directly by feeding and by transmitting bacterial wilt, which is as or more important than the feeding damage. Muskmelons and cucumbers are relatively more susceptible to bacterial wilt, pumpkins are susceptible when young but become tolerant later, and older pumpkins, watermelons and squashes (with the exception of a few winter squash varieties) are less susceptible.

To scout, count beetles on two plants at 30 locations throughout a field and calculate the average number per plant. Thresholds are lower for cucurbits susceptible to bacterial wilt and lower for plant stages and varieties that are less susceptible. For muskmelons, cucumbers and pumpkins with less than three true leaves, a threshold of one beetle per plant can be used. For watermelon, squash and older pumpkins, a threshold of five beetles per plant can be used.

If you used at-plant drenches of a neonicotinoid or planted FarMore FI-400 treated seed, this should provide early control. If not, there are multiple foliar insecticides that can be applied using the above thresholds. These are also important for control after early-season systemic insecticides have worn off.

The onion growing season to date has been conducive to disease, so be on the lookout for symptoms earlier than you might expect. There have been recent reports of increased onion thrips numbers in some fields; recent warm weather has likely facilitated population growth. A threshold of one thrips per leaf can be used to determine if treatment is necessary early in the season when using products such as Movento. For Radiant, which should be reserved for later in the season, a higher threshold of three thrips per leaf can be used.

To scout for thrips, check at least 10 plants in five locations scattered throughout a field. Count the number of leaves on each plant and the number of thrips. Then, divide the average number of thrips by the average number of leaves. For example, if you find a total of 100 thrips on 50 plants, there is an average of 100 thrips ÷ 50 plants = 2 thrips per plant. If you found a total of 150 leaves on 50 plants, then there is an average of 150 ÷ 50 = 3 leaves per plant. Then, divide these two averages. In this example, there is an average of 2 thrips ÷ 3 leaves = 0.67 thrips per leaf, which is under threshold. Heavy rains could knock back thrips populations in areas where they have been increasing, so make sure to re-check fields afterwards.

Onion maggot damage could also be showing up now. Maggot-attacked plants wilt and fall flat. To confirm onion maggots are the cause, pull up the onion bulbs and look for maggots up to one-third of an inch at the base. Treatments for onion maggot are preventive, so there is little that can be done at this point in the season. Sprays directed at the adults are ineffective.

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