Northwest Michigan fruit update – May 11, 2021

Weather continues to be cool and as a result, crop development remains slow and bee activity is minimal.

Weather report

The weather continues to be cold for the second week of May, and temperatures feel much more like mid-March than May 11. These cool conditions have slowed crop development, and as Michigan State University state climatologist Jeff Andresen suggested we are living in animated suspension. In addition to cool daytime highs, we have continued to have the threat of a frost/freeze event with overnight temperatures dropping below freezing. We have had cold overnight temperatures on May 8, 9 and 10, and we will have two more nights of cold until Friday morning, May 14.

We have lost our surplus of growing degree days (GDD), and now northwest Michigan is behind normal for GDD accumulations for base 50. The weather is predicted to be cool and dry for remainder of the week, and again, Friday morning is likely the last chance of a frost/freeze event. Today and into Wednesday, May 12, the minimum temperatures will dip down into the low 30s. Temperatures will gradually warm up by the weekend, conditions will be dry, and the next significant potential rainfall will be May 18.

The northwest region did not receive much rainfall during the last week. We had a trace amount recorded May 7 with 0.08 inches at the Northwest Michigan Horticulture Research Center Enviroweather station. Conditions will continue to be dry into the middle of May. The medium range forecast is predicting conditions to change and become warmer and wetter during the latter half of May compared to the beginning of the month.

View Andresen’s weekly weather report.

Crop report

There continues to be lots of calls about the cold weather’s continued impacts on bud development—from both the media and growers. We are still assessing damage from the mid-April frost/freeze events, but the more recent continued cold temperatures have really impacted pollination, and the verdict is still out on the crop size for both sweet and tart cherries with this cold weather. We have heard reports of limited damage in sweet cherry and more damage in tart cherry in certain areas of the region.

We checked apples at the Northwest Michigan Horticulture Research Center again yesterday, and both Gala and Honeycrisp are still green. We can detect some damage to king and lateral bloom, but good buds are still dominant. Most of the sweet cherries at the Northwest Michigan Horticulture Research Center are at some page of early to mid-petal fall.

Again, we have concerns about pollination as sweet cherries have been in bloom for the past week with cold temperatures and very little honey bee activity. Tart cherries are just starting to bloom at the Northwest Michigan Horticulture Research Center, and we are hoping the weather will warm this weekend to promote good bee activity and pollen tube growth/fertilization.

Pest report

Since green tip (April 5) in apples, there have been three major apple scab infection periods in the region during wet weather: April 8, April 28 and May 2. The May 2-4 wetting event was wet for 48 hours and resulted in a major infection period at the Northwest Michigan Horticulture Research Center. However, overall, the cold and dry weather has really kept all tree fruit diseases to a minimum.

Do not be complacent about disease control, particularly if the weather does become warmer and wet as predicted for the latter half of May. Rain is currently predicted at the start of the following week, and warmer conditions are also predicted into the coming weekend—the combination of weather events will likely result in an infection period for scab early next week. The About the Model tab on Enviroweather provides a table that describes the approximate wetting period required for primary apple scab infection at various air temperatures. This table is a quick reference for growers that are interested in calculating the risk or chance of predicted weather to result in an apple scab infection period.

In addition to Enviroweather, we are modeling the apple scab model through RIMpro. These models are available for various fruit growing areas including one for Romeo, Michigan; the Southwest Michigan Research and Extension Center in Benton Harbor, Michigan; four locations on the Ridge; one in Hart, Michigan; and three locations in northwest Michigan. For the northwest region, the model output links for Benzonia, East Leland and Williamsburg are below. Each of these outputs is linked to the Enviroweather stations in those locations so the outputs and forecasts should be consistent across both services. The outputs change as forecasts change, so be sure to check the outputs frequently to best assist management decisions.

Be cautious about using the Williamsburg data as this Enviroweather station did not report adequate rainfall during last weekend’s rain event. The RIMpro model is showing less potential for apple scab infection than it should be due to the sensor missing the actual rainfall amounts. The moisture sensor has been fixed, and the rainfall estimates have been added to the station for model outputs, but the potential for infection is still lower and less conservative than the true amount of rainfall in that region that would have been used to predict apple scab infection.

The cherry leaf spot model has also been updated on the Enviroweather website based on recent data that shows that bract leaves can become infected with the pathogen once they have expanded. The old model was not initiated until after bloom, but now the model begins when bract leaves are open. This change in the model has been challenging with these prolonged cold conditions, and it has been hard to pull the trigger to begin managing for cherry leaf spot. However, the wet weather on May 2-4 resulted in a cherry leaf spot infection. Again, however, this disease is very slow growing in these cold conditions. The cherry leaf spot model can help guide growers in making management decisions about leaf spot in different weather conditions.

Additionally, the northwest region of Michigan has had more rainfall than other parts of the state that are particularly dry (almost drought-like in some areas) and disease development is at a minimum. Be aware about the variability of rainfall in the region, which will impact how growers should manage for disease. For example, the East Leland Enviroweather station has reported far more rainfall than any other station, and these growers need to be cognizant of disease development with this additional rainfall.

Tart cherry bloom has just begun over the weekend at the Northwest Michigan Horticulture Research Center, and we estimate we are at 25% bloom in Montmorency. Other warmer areas in the region saw tart cherry bloom start late last week. This spring’s weather is a concern for European brown rot if conditions continue to be cold and are wet during bloom, which they are predicted to be cold for the next two to three days. We have not observed serious European brown rot challenges in recent years, but in the past, this disease has caused significant losses in Balaton orchards (which are more susceptible than Montmorency), and in slow drying Montmorency blocks. Indar is still the material of choice for European brown rot, and this fungicide is still effective against the European brown rot pathogen.

Cold temperatures have continued to result in little insect activity overall. The only insect we saw in the Northwest Michigan Horticulture Research Center trap line this week was spotted tentiform leaf minor, and we caught 80 in our traps compared to 186 last week. Many growers have minimized insecticide applications during these continued cold conditions. However, as soon as it warms, be on the ball for managing insect pests, particularly pests like plum curculio that are adults at this time. As soon as the temperatures warm, females will quickly become active and start to lay their eggs.

Additionally, this predicted warm up will likely overlap with shuck split in sweet cherries, and those small emerging fruit will be at risk of curculio oviposition. As soon as temperatures warm and fruits are coming out of the shuck, be prepared to take action against this pest, especially where the beetles have been problematic in the past.

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