Southwest Michigan vegetable update – Aug. 9, 2017
We still need rain in southwest Michigan.
Temperatures were generally slightly below normal with highs from 67 degrees Fahrenheit to 85 F and lows from 51 F to 63 F. We continue to fall behind 2016 in growing degree-day (GDD) units. We are approximately 275 GDD behind 2016 and 73 GDD behind the five-year base 50 average. We received less than an inch across the area for the week. Most unirrigated crops are showing signs of stress.
The big story this week is a significant cool down that began last Friday, Aug. 4. Conditions were highly conducive for spread of cucurbit downy mildew and the appearance of gray wall in tomatoes.
Downy mildew spreads via wind-borne spores, especially on cloudy, wet days like we had Aug. 4. We have been detecting spores in the Southwest Michigan Research and Extension Center trap since July 17. Therefore, spores are in the area and I would expect unprotected plants to start showing symptoms this week. It will most likely show up first in homeowner and organic producer plantings or abandoned commercial fields.
To reduce inoculum levels, Michigan State University Extension advises removing or spraying to kill cucurbit fields immediately after harvest is complete. Quick removal of fields can also help reduce other diseases such as Phytophthora.
Tomato gray wall is a poorly understood physiological condition where the tomato fruit shows a yellowish-gray stripping down the side (see photo). The stripes stay hard and yellow and never ripen to red. When the fruit is cut around the equator, darkened vascular bundles can also be seen in the yellow areas. Typically, gray wall happens after a period of high sunshine and good growing temperatures followed by cool, cloudy and wet weather—just like we had Aug. 4. It may show up at the next harvest or two. In some years, it can affect as much as 75 percent of the fruit.
Significant squash bug populations are present in some pumpkin and squash fields. These should be controlled since squash bugs cause damage and also transmit diseases. It is best and easiest to control them when they are young. The female will lay a cluster of 30 to 50 copper-colored eggs that will hatch into nymphs that feed on a wide range of cucurbit crops.
Bacterial disease in tomatoes have increased. Symptoms are most readily observed in the earlier planted fields that experienced the rainy periods during May, June and July.