Northwest Michigan fruit update – May 4, 2021

Tart cherry bloom is beginning, and cooler weather is forecast to return.

Weather report

The weather continues to be windy and cool, but we did see some warmer temperatures over the weekend. With these cool conditions, tree development has been incredibly slow, and growers are also unsure of when to spray for diseases as they too are developing at a very slow rate. Conditions will continue to be windy for the next few days and significantly cooler. There is even some potential rain and snow or a rain/snow mix in the coming days, but overall precipitation will be light. Conditions are predicted to be cooler than normal for the next two weeks.

There is a chance of scattered frost later this week, but with the clouds expected, there is a small chance of freezing conditions in fruit growing areas. The predicted winds in the forecast will also keep the freezing temperatures at bay. The highest potential for a frost/freeze event will be on Friday night into Saturday morning (April 7 and 8). The last freeze event was on May 1, but most of the fruit growing regions did not get well below freezing. That evening into the next morning was a typical inversion freeze event, and many growers ran their frost fans overnight.

April 2021 is one of the driest Aprils on record for the central and southern parts of the state. Soil conditions are quite dry for this time in the season. According to the U.S. Drought Monitor, most of the state is in the moderate drought category; although we did receive rain in particular locations in some parts of northwest Michigan, total rainfall across the region is quite variable. For example, the East Leland Enviroweather station recorded 1.5 inches for four days starting April 25. This weather station also recorded 1.4 inches of rainfall yesterday, May 3. In contrast, the Benzonia Enviroweather station recorded 0.28 inches on April 25 and 0.68 inches of rain yesterday. Northwest Michigan has fared much better for precipitation compared to the central and southern portions of Michigan.

View Michigan State University state climatologist Jeff Andresen’s report.

Crop report

The big issue on growers’ minds is pollination. Conditions are not favorable for honey bee activity, particularly if daytime highs only get into the mid-40s degrees Fahrenheit and if the cool conditions are accompanied by winds. In the next two weeks, the forecast is calling for cool and windy conditions to remain in the region, which will be a challenge to get open blossoms pollinated.

Sweet cherries are in various stages of bloom across the region, and bee activity in the sweet cherries at the Northwest Michigan Horticulture Research Center was minimal yesterday. Tart cherries are also starting to open. There are varying reports of tart cherry damage from the earlier freeze events in April, but this cold weather during bloom certainly will not contribute to increasing the size of this crop.

These conditions are also not good for Promalin applications. In past years, we have added Promalin to latex paint and painted sweet cherries to increase branching. This technique is effective, only if conditions are warm. We have not seen increased branching when Promalin is applied at temperatures below 65-70 F. We have a new block of high density Pearl series sweet cherries where increased branching would be welcomed, but temperatures are not conducive for Promalin applications.

Pest report

Predicting rainfall and when potential scab infections could occur to time protective sprays accordingly has been a challenge lately. As noted previously, slow development has also been a concern for timing sprays and has also raised concerns of having sufficient options to get through the primary scab season. Cool weather ahead will continue to delay development, but recent rainfall over the weekend would have washed off fungicides and recovering susceptible tissue will be necessary. The current forecast is looking dry this week, which is a double edge sword when it comes to scab; no or very little rainfall reduces infection risk, but rain is needed for primary scab spore dispersal.

Looking at the scab models on Enviroweather and RIMpro, the region is in the midst of a heavy infection period. Please note there is an error with the Williamsburg Tower output; the station is not properly recording rainfall which will impact the disease model outputs for both Enviroweather and RIMpro. We are working to get this situation resolved as soon as possible.

As we are in or approach pink in apples, the time of the seasons and recent warmer temperatures have contributed to higher spore dispersal numbers during rains. Some growers have noted that fewer spores were released during recent evening wetting events which is a typical occurrence with apple scab ascospores; daytime rain showers typically trigger greater spore releases compared with evening rain events.

Sweet cherries are blooming and at petal fall in more advanced orchards in the region. Temperature in the upper 60s and low 70s during bloom are not as optimal as the upper 70s for American brown rot, but this fungus will grow at these temperatures at a slower rate. Many growers protected blossoms and this was a good strategy considering that some areas have received extended periods of rain or damp weather over the course of bloom.

Unfortunately, these cool, wet conditions have been good for bacterial canker development and current options for management are limited mainly to cultural control measures like optimizing pruning timing to avoid wet weather during the early part of spring. Recent years’ springtime weather has favored the growth of this pathogen in region’s orchards. If this spring and early summer continue to bring weather that favor canker, we may see the water-soaked spots on green fruit that we have in years past that indicate canker infected fruit. Canker infected green fruit can host American brown rot. It is still very early to know if this will be a challenge, but something to keep in mind and be on the lookout for moving forward.

Tart cherries are starting to bloom and recent cool, wet, foggy weather have been good conditions for European brown rot. Cool weather is expected to continue into the coming week, but the forecast is also calling for drier weather. As noted in last week’s report, in favorable conditions, European brown rot can infect flowers that bloom even if the pistils are dead. Additionally, there are a few options for management, including Indar which is still effective for this disease. Check processor restrictions for materials before making applications. For more information on European brown rot management for 2021, please refer to last week’s report.

Warmer weekend temperatures helped tart cherry buds push our more green tissue. Rainy weather that began on Sunday initiated a cherry leaf spot infection for the region, the first or one of the first (depending on location and whether bract leaves were present previously) infection periods of the season. Foliage will continue to develop albeit at a slower rate in the forecasted weather. Continue to monitor the forecast to time protective fungicide applications for leaf spot accordingly.

With warmer temperatures over the weekend, we expected to see more insect activity in our trapline. However, insect activity overall continues to be slow at the Northwest Michigan Horticulture Research Center.

In apples, with fast development up to tight cluster quickly changing to delayed development, it seems like planning for a pink timing spray has been on growers’ minds for weeks.

We are still finding small leaf roller larvae rolling up in spur leaves in unmanaged trees, but their feeding activity has been reduced in cold weather.

Small aphid colonies are also forming on new leaves in flower clusters.

No oriental fruit moths were trapped, but numbers of this pest are generally low for the research station.

We did not find black stem borer in our traps. We received reports that black stem borers were active over the weekend, but in low numbers. Cool conditions will likely continue to delay their emergence.

For spotted tentiform leafminer, a more minor pest in apples recently, trap numbers jumped up this week.

We have not observed plum curculio at this time, and as with other insects, cooler conditions will likely continue to delay emergence. This pest has been a challenge for some growers to manage early in the season in recent years, in part because of fluctuating temperatures that hasten and delay fruit development and contribute to difficulty predicting the beetles’ emergence. Be cautious with plum curculio this season, particularly in hot spot areas of orchards. In the coming weeks, if the region receives warmer temperatures and fruit are coming out of the shuck, swift management action may be needed to prevent damage.

No American plum borer were found this week, but according to degree day accumulations we should start to see this pest in traps soon.

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