West central Michigan vegetable update – May 10, 2017

Freezing temperatures were not kind to asparagus, but harvest will start soon, especially as warmer temperatures arrive.

Cabbage white butterflies lay bullet-shaped eggs singly on brassica leaves. Eggs hatch into imported cabbage worm. Photo by Whitney Cranshaw, Colorado State University, Bugwood.org.

Cabbage white butterflies lay bullet-shaped eggs singly on brassica leaves. Eggs hatch into imported cabbage worm. Photo by Whitney Cranshaw, Colorado State University, Bugwood.org.

Temperatures in the 20s froze off early asparagus pickings in west central Michigan this week. Warmer weather is forecast to arrive early next week and will help speed arrival of widespread harvest. It is unlikely we will see another major freeze event.

Cabbage white butterflies are active in leafy cole crops. They lay bullet-shaped eggs on leaves that are easy to detect via scouting. Research at Cornell University has shown that treating cabbage transplants with Coragen (active ingredient chlorantraniliprole) can provide very good early-season caterpillar control. Early-season control could help reduce the size of later populations.

For early-season control, Michigan State University Extension suggests using narrow-spectrum insecticides such as Coragen or Bt products. There are parasitic wasps that help keep caterpillars like diamondback moth in check, and avoiding early-season sprays of pyrethroids can help conserve these, reducing chances of later outbreaks. The University of Kentucky Cooperative Extension Service has a nice factsheet on use of Bt products, “Growers’ Guide to Bt.”

For root crops, beets and turnips have emerged from April/May plantings, but have been slow-growing at some locations due to cool weather. Cultivation for weed control was ongoing in turnips this week.

Potato freeze damage was visible on emerged leaves in other parts of the state. Experienced growers often use their hillers to throw soil on emerged potatoes that are about 1-2 inches in height to protect them from frost.

Botrytis in high-tunnel tomatoes can be an issue due to extended leaf wetness when dew forms as temperatures cool at nighttime and venting is limited. A general rule of thumb is to keep relative humidity below 85 percent. Above this humidity, dew readily forms when temperatures cool overnight. A small amount of heating—raising the temperature by only 1 or 2 degrees—can help reduce relative humidity and limit dew formation.

Sensors to monitor relative humidity are relatively cheap and can be used to determine if heating might be beneficial (i.e., if humidity is over 85 percent) and monitor the effectiveness of measures you take to reduce it. Promoting air movement and venting can also help, but can be difficult in cool weather. For fungicides, mancozeb (five-day pre-harvest interval, active ingredient mancozeb) and Catamaran (zero-day pre-harvest interval, active ingredient chlorothalonil and potassium phosphite) can be used preventively for tomato production in enclosures.

Experience suggests early June is a good time to start planting sweet potatoes, when soil temperatures reach about 65 degrees Fahrenheit. Do not put transplants in the cooler to delay planting after arrival. Sweet potatoes are tropical plants and do not tolerate cold well. Planting sweet potatoes in cool soils not only results in slow growth, but can also cause sweet potatoes to take on a more rounded shape compared to the typical “sweet potato torpedo.”

Young sweet corn stands damaged by recent freezing temperatures should recover. The growing point of corn should be below ground in any field-planted sweet corn, which means emerged leaves can die but the plant will regrow. 

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