Southwest Michigan vegetable regional report – August 5, 2015

Cool nights can lead to purpling in some crops.

Zippering scar on tomato. Zippers occur on the exterior of fruit when anthers and petals adhere to young fruit.

Zippering scar on tomato. Zippers occur on the exterior of fruit when anthers and petals adhere to young fruit.


Warmer conditions continued for most of the period with highs from 79 to 93 degrees Fahrenheit and lows from 56 to 68 F. Rainfall for the area ranged from 0.4 to over 1.5 inches. We are near 1,775 growing degree days (GDD) base 50 compared to 1,615 for 2014.

A switch to low humidity and cooler night temperatures will potentially bring out purple pigments in some crops, especially in peppers and cauliflower. These plants always have the potential of developing these pigments, but generally this does not happen until September. However, with recent night temperatures in the 50s, purple color could potentially develop this week. A more extensive Michigan State University Extension article on purpling can be found at “Green bell pepper showing purple striping due to cool temperatures in September.”

Crop reports

MSU Extension continues to emphasize the importance of controlling downy mildew on cucumber, cantaloupe and watermelon.

Another weather-related issue that has been observed in tomatoes is a fair amount of zippering. Zippers are scars that occur on the exterior of the fruit (see photo) and are due to the anthers and petals adhering to the young fruit. This happens during periods of cool temperatures when the fruit is not quickly developing and able to shed the anthers and petals. Weather early in the season was certainly conducive for slow fruit development. One benefit of cooler weather is that tomato fruit size will be larger. Warmer temperatures will cause the fruit to ripen at a smaller size. Cool temperatures will allow the fruit to size more before it ripens.

Few virus symptoms have been observed in any vegetable crop. This is probably due to a lack of aphid vectors. However, aphid populations have been increasing over the past few weeks, so expect to see more virus symptoms showing up later this month. Little can be done to stop virus spread other than to plant tolerant varieties. 

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