MSU Extension

The Sunday night into Monday morning (May 12-13) temperatures dipped down into the freezing range, but most of the northwest Michigan Enviro-weather stations did not report many hours below 32 degrees Fahrenheit. The two exceptions were the Bear Lake and Benzonia Enviro-weather stations. Bear Lake was the coldest site here in the north, and the lowest temperature dipped down to 23.9 F between 3 and 4 a.m. This station also sustained these mid-20s temperatures for nine hours throughout the evening, but this Enviro-weather station is in a notoriously low spot, which may not accurately reflect the fruit sites around this area.

Benzonia’s temperatures were not as cold as Bear Lake’s, but this station did record 10 hours below the freezing mark during this time period. We did observe some damage in apples in a low site on Monday, and another grower did sustain some damage in apples closer to the lake. Many growers in this area obtained Promalin and applied this product in hopes to minimize the impacts of the frost.

Compared with last week’s nice, summer-like weather, a cold front moved through the region and brought cool and wet conditions to the area over the weekend. Daytime highs were in the low 40s, and on Sunday we did not even hit the 40 F mark; these temperatures were a sharp departure from the 70s weather we experienced earlier last week. Bees were flying up until Thursday afternoon (May 9), but with the cold temperatures that began on Friday (May 10), there was little bee activity in the orchards.

Temperatures are predicted to rise this week and there is less than a 50 percent chance of rain across the region; as tart cherries are just coming into bloom, we hope temperatures warm to promote good bee activity for pollination. We received some rainfall over the weekend and 0.61 inches of rain fell at the Northwest Michigan Horticultural Research Center (NWMHRC) on Friday and less than 0.25 inches was recorded on both Saturday and Sunday (May 11-12). So far this season we have accumulated 298 GDD base 42 and 159 GDD base 50.

Apples. Growers are moving back into the orchard to cover up for apple scab following the wet conditions over the weekend. Growers should also be including a spray for powdery mildew in this next application. As mentioned above, growers that had cold temperatures and some damage in apples are applying Promalin. This PGR has shown to increase fruit set if the material is applied at 1 pt per acre within 24 hours of the frost event. Promalin is in short supply in the northwest, and many growers have obtained it from other areas of the state.

Cherries. Unfortunately, this past weekend was good for development of two important pathogens in cherries: bacterial canker in both sweets and tarts and European brown rot in tarts, particularly Balaton. Both of these pathogens favor cold and wet conditions, which were not in short supply over the weekend. As there is no control for canker, we will have to wait and see if the weekend’s conditions caused any infections. However, most growers with Balatons looked at the forecast and applied Indar for European brown rot control prior to the weekend’s wetting events.

Growers should also begin their cherry leaf spot control program as there is adequate leaf area present in both Montmorency and Balaton. The cherry leaf spot pathogen can infect in stomata of open leaves, and in most orchards, at least some of the bract leaves are fully open on the tree. As we saw in 2012, protecting these bract leaves is important in achieving season-long control of cherry leaf spot. This early coverage is similar to our thinking in covering for apple scab at green tip: start early to control this disease, even with the small amount of tissue available for infection and the cherry leaf spot spore load is typically less early in the season. Any infection that occurs early will result in severe consequences later in the season. This is because lesions initiated early on bract leaves will be producing secondary spores (conidia) at a petal fall timing when we typically see the highest spore concentration from overwintering leaves. Thus, early infections can be devastating because they compound the spore load in an orchard.

Although there is not a lot of green tissue present in orchards early in the season, cherry leaf spot spores can find that tissue. For any spores released at this timing, their primary function is to land on that susceptible tissue (open bract leaves) and infect. Please see the Michigan State University Extension article “Cherry leaf spot: get an early start on protection before the fungus gets started on infection” for more on cherry leaf spot control.