Northwest Michigan fruit update – May 12, 2020

The arctic air mass continues to cover Michigan, and the northwest region had over six nights of frost/freeze risk. We will assess potential damage after tonight, which is predicted to be the last cold night this week.

Inversion freeze event on the Williamsburg tower on May 12, 2020
Figure 1. Inversion freeze event on the Williamsburg tower on May 12, 2020.

Weather report

The cold weather continues to hang around this spring, and it is hard to believe we have had snow and below freezing temperatures into the middle part of May. Last week, we were 10 to 14 degrees Fahrenheit colder than normal. For the past six nights, growers have been concerned about the frost/freeze conditions, and it looks like there is one more night where below freezing temperatures are predicted. Last Thursday into Friday morning, May 7 and 8, was expected to be cold, and temperatures dipped just below freezing at most Michigan State University Enviroweather stations across the northwest. We had the same situation on Friday night into Saturday, May 8 and 9, but temperatures at some stations were lower than the previous night, and Enviroweather stations recorded lows into the mid to high 20s.

Overnight cold continued into Saturday and Sunday, May 9 and 10, but temperatures did not dip below freezing. Sunday into Monday, May 10 and 11, were also cold, but most Enviroweather stations reported temperatures above freezing for most of the night; a few stations reported a few hours of just below freezing for part of the night.

Unfortunately, Monday into Tuesday, May 11 and 12, temperatures were back down to well below freezing, and some Enviroweather stations reported temperatures into the mid-20s F. This last freeze event was a classic inversion, and wind machines would have been helpful in reducing potential damage under this scenario (Figure 1). We have one more night where below freezing temperatures are a concern; after 12-13 May, the weather is expected to warm somewhat dramatically, and rain is in the forecast.

Growing degree days (GDDs) through May 11, 2020

Year

2020

2019

2018

2017

2016

2015

30 Yr. Avg.

GDD42

144

175

227

310

313

197

329.6

GDD50

34

62

105

125

128

156

148.6

The cold weather has been coupled with windy and dry conditions. Dry conditions can help reduce damage if there is little water in the fruit buds. However, dry conditions can also contribute to cold overnight temperatures as dew points are lower when the relative humidity is low. We did have precipitation in the form of snow, but the snowfall was variable around the region and disappeared fairly quickly. After tonight’s cold temperatures, the weather is predicted to be warmer and wetter. Rain is predicted to move into the region on Wednesday night into Thursday, May 14, and substantial rainfall is in the forecast. More southerly parts of the state may even see some thunderstorms pop up.

We continued to be considerably behind our average growing degree (GDD) accumulations this season. Thus far, we have accumulated 144 GDD base 42 and 34 GDD base 50. Our 25-plus-year averages are 329.6 GDD base 42 and 148.6 GDD base 50. These accumulations at this time during the season are even behind last year, which also started out cool: 175 GDD base 42 and 62 GDD base 50. The GDDs will accumulate faster as we receive warmer conditions in the week ahead.

Crop report

With the cold conditions, crop development has been incredibly slow. We are at very similar growth stages compared with last week’s observations. We are at early tight cluster in apple. We are in late green tip in Montmorency and bud burst in Balaton. All sweet cherries are at early tight cluster.

The main concern on most growers’ minds is if these multiple cold nights have caused damage. We collected buds and cut them at the Northwest Michigan Horticulture and Research Center and we did find some damage, but the levels indicate we can still have a good crop. However, those assessments were done before last night’s cold weather. We have also heard variable reports from growers and crop consultants about the variable levels of damage in the different blocks. After tomorrow night, we will be able to provide growers with a better assessment of damage caused by the cold weather over this past week.

Pest report

Warmer temperatures and wet weather are ahead, and we anticipate pest and disease activity will increase in these conditions.

Growers are planning to cover apples before rains that are forecast for Wednesday evening into Friday morning. Rimpro is calling for medium to high infection risk in these upcoming rains and spore release numbers are projected to be higher than previously in the season. The numbers of spores released will generally continue to increase as the season progresses to peak spore release which typically occurs around pink. Conditions have not been favorable for apple powdery mildew development recently, but warmer weather ahead will be more favorable for mildew. Rain is also predicted early next week and growers may need to recover between rain events depending the amount of precipitation received and new growth of leaves.

European red mite eggs and San Jose scale are still present and healthy. European red mite egg hatch has not begun.

Small leafroller larvae with black or brown head capsules have been found on ornamental crab apples, but we have not observed them in our trap line at the station. Keep an eye out for tiny larvae feeding on new leaves and flower buds.

Spotted tentiform leafminer activity has just begun. We found one adult in a trap this week. Across the state, spotted tentiform leafminers have not reached damaging levels above threshold in the recent past, and this pest is considered a more minor pest as a result.

No black stem borers have been reported in traps. Other look-alike beetles have been found in traps.

In terms of cherry diseases, when cherries reach white bud or popcorn growers will need to assess whether conditions will favor American or European brown rot. There is not much to report in terms of cherry pests. American plum borer traps are set but we have not detected this pest. We also have not trapped green fruit worm moths that can also be captured in the American plum borer traps.

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