West central Michigan small fruit update – June 6, 2017

Late spring frosts effect are showing up in berry crops, especially in blueberries where a heavy June drop of non-pollinated fruits has begun.

Elliott-variety blueberry bushes affected by spring frosts. Photo by Carlos Garcia-Salazar.
Elliott-variety blueberry bushes affected by spring frosts. Photo by Carlos Garcia-Salazar.

West central Michigan is passing through a dry weather period that is already lasting for more than 10 days. Only a few rain showers have occurred with no significant accumulation. So far, the total precipitation accumulated since Jan. 1 remains between 12.9 and 15.7 inches of rain.

The extended weather forecast indicates only a 35 percent probability of rain for the next seven days. Lack of rain at this time could be a problem for crops like strawberries and blueberries that are in the green fruit stage. This is the time when water is critical to produce a good crop. To compensate for the lack of rain, growers need to irrigate to maintain a healthy crop.

Weather-wise, daily average temperatures in the area have remained low at about 65 degrees Fahrenheit. The average daily minimum temperature during the past seven days has been 51 F and a daily maximum temperature of 81 F. Those temperatures allowed for a growing degree-day accumulation (base 50 F) of approximately 518 GDD.

Strawberry growth stages go from thimble-size fruit to pre-harvest, with day neutral strawberries starting the harvest this week and June-bearing strawberries to follow. There are some problems showing up that include a light crop for some varieties affected by the spring frosts of May 8 and 9 and plant diseases like black root rot.

In blueberries, the situation is changing day by day. Blueberry fields that were not showing any traces of frost damage days after the spring frost of May 8 and 9—because they were in an early stage of flower bud development—are now presenting substantial symptoms of spring frost injury. This damage is characterized by a heavy “June drop” of non-pollinized fruits and bushes with a heavy load of fruit but no leaves (see photo).  

Blueberry fields affected by spring frosts with plants looking like the one in the picture will have to make an important decision on how to proceed for the rest of the season. If plants have no leaves and they are loaded with fruit, the plant will try to grow the fruit at the expense of the rest of the plant. Since fruits are fed by nutrients accumulated on shoots and there are no new leaves to replenish those shoots, at the end of the season shoots will be depleted and die.

A possible alternative is to prune all shoots with abundant fruit but with no leaves and allow a new growth to develop from the crown during the rest of the season. Another option is to establish a special nutritional program that may help the plant to grow new shoots and compensate for the lack of foliage. At the end, those affected shoots will have to be removed from the plants to allow for new growth.

There is not an easy solution to this problem and growers need to make the best decision based on their own knowledge and experience. Michigan State University horticulture specialists may be called to assist growers to make an informed decision about how to proceed in these situations. For assistance or question, contact your local MSU Extension county office. You will directed to the right person to answer your questions or concerns.

Regarding insect pests in blueberries, cherry fruitworm and cranberry fruitworm are flying in the area. The cherry fruitworm started declining, but in the past week the cranberry fruitworm had its first peak of overwintering adults. Note that the two species will not always be present at the same time. Over the year, we have observed that in some places only one species is the predominant problem.

It is not uncommon to observe only one species affecting one particular field or farm. Adjust your spray program for fruitworm control accordingly. By this time, the honey bees are out of the fields and the second application of insecticide against fruitworms should proceed with a broad spectrum insecticide like Assail, Asana LX, Lannate or Imidan. Those insecticides are very effective against secondary pests, like blueberry aphids that are responsible for the transmission of shoe string virus, and stem blueberry gall wasp that, at this time, may be present in the field.

For more recommended products, please see MSU Extension bulletin E0154, “2017 Fruit Management Guide.”

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