West central Michigan vegetable regional report – July 20, 2016

Updates on asparagus, cucurbit, onion and sweet corn pest management.

Asparagus purple spot lesions on an asparagus stem last week. All photos: Ben Werling, MSU Extension.

Asparagus purple spot lesions on an asparagus stem last week. All photos: Ben Werling, MSU Extension.

Asparagus purple spot lesions were visible on asparagus stems late last week in Oceana County. Rainfall is not necessary for infection if dews are forming in the morning. Common asparagus beetle adults were active in one location I visited and Japanese beetle adults were common and starting to defoliate fern in some asparagus fields late last week. Standard products used for these pests include carbaryl and permethrin, which for most growers have been effective. Recently, consultants have been working with some growers to try Assail (active ingredient acetamiprid), which is labelled for asparagus beetle and Japanese beetle, in addition to asparagus aphid and tarnished plant bug. Radiant is another product that is labelled for common asparagus beetle, and has proven to be highly effective for beetle control in other crops.

Cucurbit growers should be aware that downy mildew has been detected in three east Michigan counties. Downy mildew spore trap totals have shown a recent uptick in Muskegon County. Once numbers are consistently high, this is a signal that good programs need to be in place for melons and cucumbers.

Angular leaf spot has been diagnosed from problem fields for multiple cucurbit growers this year. Of special concern are circular lesions that have developed in 2015 and 2016 on spaghetti squash fruit. Copper is the only proven bactericide; Oxidate is unlikely to be helpful as it is only a surface sterilizer. Unfortunately, for bacterial diseases such as this, copper needs to be applied early (once true leaves emerge) and often (weekly) to be most effective. Growers have been left scratching their heads as this pathogen is favored by wet conditions. However, any water splash (e.g., from irrigation) could move this bacterium from leaves to fruit and between plants, which initially can become infected when bacteria are on the seed.

Circular lesions on spaghetti squash.

Circular lesions on spaghetti squash fruit from a plant infected by the angular leaf spot pathogen.

Got wilting vine crops? There could be a number of culprits. First, squash vine borer larvae can cause wilted runners. However, you should find an entrance hole, excrement and potentially a caterpillar if they are the culprit. Plus, only the runner being fed on, not the whole plant, may wilt. If your whole plants are wilting and not perking up when watered, they may have bacterial wilt. This bacterium is carried by cucumber beetles. Muskmelons and cucumbers are most susceptible, but other cucurbits can be affected with high beetle populations. A nice video on how to check for bacterial wilt is available on YouTube. Melons can also suffer from Fusarium wilt, a disease that can cause discoloration of the vascular tissue.

For onions, Michigan State University Extension suggests now is a good time to consider application of Radiant for thrips control. Some growers have reported to date they have only applied Movento. However, with high temperatures coming, populations could ramp up. This makes it a good time to consider Radiant, which is our best material for knocking back high populations. Bacterial leaf blight continues to be a problem in some fields this year.

For sweet corn, weather could create the possibility of additional corn earworm flights over the next week. In eastern Michigan, growers have noticed earworm in corn despite low trap catches. This makes sense, as past research at Purdue University has shown that any trap catch justifies treatment when field corn is not silking. If you are trapping for corn earworm, please make sure to put your traps on the windward (typically west) edge of your field away from hedgerows and buildings. Traps will not catch as many moths if wind is not carrying pheromone into silking sweet corn. This could lead to misleadingly low trap captures. For example, traps placed on the leeward (typically eastern) side of a field or near woods will not catch as many moths. If you are taking time to count corn earworm moths, do the most you can to be confident in your results.

Note, when field corn silks, it can attract corn earworm away from your sweet corn. Research at Purdue University has shown growers using wire Hartstack traps can use a threshold of 10 moths per night if nearby field corn is silking. When field corn is not silking, any trap catch justifies treatment.

According to Michigan Late Blight Risk Monitoring, risk of late blight development for tomatoes and potatoes is forecast to be at its highest point yet this year, with weather producing a high risk for four to five out of the next five days (July 20-24) in west central and southwest Michigan.

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