Northwest Michigan fruit update – May 21, 2019

Pollination and disease management have been on grower’s minds with the continued cool and wet conditions.

Weather report

Weather conditions continue to frustrate growers and locals alike. Again this week, spring seems to be far away, and the cold and wet conditions continue across the region. These conditions have been challenging to make disease control decisions with the cooler temperatures and to find a good window with no rain and wind to get into the orchards to make applications. We have accumulated 262.6 growing degree days (GDD) base 42 and 98.6 GDD base 50. These accumulations are well behind our averages, and in some cases we are two weeks behind our normals; our average accumulations for the past 25-plus years are 447.4 GDD base 42 and 211.6 GDD base 50. Michigan State University agricultural meteorologist Jeff Andresen expects this cool and wet weather to continue in northern Michigan for the remainder of May and into June.

We had rain over the past weekend, and we accumulated about 0.84-inch of rain on Saturday, Sunday and Monday, May 18-20. The rain cleared off on Monday, May 20, and the sun came out, but temperatures did not warm much. There is more rain predicted for tomorrow, Wednesday, May 22. There is a slight chance of rain on Friday in northern Michigan, but rain is likely to the southern part of the state. The next chance of rain is likely Tuesday, May 28, and we might see a dry Memorial Day weekend (we can only hope!).

There were concerns about frost/freeze events last night and into the morning, May 20-21). The Elk Rapids Enviroweather station recorded below freezing temperatures at 4-5 a.m. this morning. Some growers have reported cold temperatures and some frost in low pockets but no major or widespread reports of frost. However, there were frost fans running in some blocks this morning. Overall, the overnight temperatures were not as cold as some forecasts were predicting.

Crop report

These crop reports almost seem like a broken record over the past few weeks. With the cool temperatures, tree development has been very slow. We have had various amounts of open bloom in sweet cherries for the last two weeks, and there are many questions on the success of pollination, both with the cool temperatures limiting honey bee flight and the likely slow pollen tube growth that would result in successful fertilization.

We have had lots of questions about honey bee activity with the cool and wet conditions. Rain and cold weather impact bees and keep them in the hive, and the warmer the temperatures, the better conditions for bee activity. Honey bee flight muscles need to reach 68 degrees Fahrenheit for them to fly. However, bees can keep temperatures warmer in the hive than the ambient outside temperatures. Additionally, bees can warm their muscles and fly in temperatures as low 50 F. However, sun will impact honey bee activity, so if the air temperature is around 50 F with sun hitting the hive, the bee activity will be higher than 50 degrees and windy and rain. Honey bees will also fly in rainy weather but not in heavy rains. Many times, bees can get caught in heavy rains and will wait out the rain and dry off before returning to the hive. Honey bees will forage for up to three miles and they prefer to forage in similar crops. Most pollination will occur in the first mile from the hive, but they can forage up to three miles.

We also seem to have a bumper crop of dandelions this season. Growers have questioned whether these dandelions provide benefit to the hive or if they compete with cherry bloom. Researchers have found that the colony as a whole does better with various species of pollen. Colony hive strength is key to successful pollination, and a stressed or weak colony will stay in the hive and concentrate on brood rearing rather than foraging; healthy hives will work outside the hive more actively and will provide more pollination services. Cherry resources are high reward, and honey bees will readily choose cherries over dandelions. Hence, dandelions will likely contribute to the health of the hive rather than directly compete with cherry blooms. However, another concern is pesticide applications and exposure with more dandelions in an orchard.

Lastly, with these cold temperatures, beekeepers have been feeding bees with some type of sugar water (high fructose corn syrup or sucrose water). This supplemental feeding is necessary when temperatures are in the 30s and 40s because the bees do not leave the hive to forage but still need food. This sugar water provides resources for bees if no natural food is available (or if temperatures are too cold for bees to fly to forage for them). This sugar water supplement does not impact bee foraging or pollination services.

Pest report

With sweet cherries at or approaching full bloom, tart cherries with the first open blossoms and apples in late tight cluster, disease protection has been at the forefront of management programs over the last week. Although temperatures have been cool, extended wetting periods have been a concern for American brown rot in sweet cherries, European brown rot in tart cherries, cherry leaf spot and apple scab.

Cool temperatures have extended bloom in sweet cherries and growers have been challenged to make American brown rot management strategies. The traditional management program for this disease has been to make the first application at popcorn followed by an additional application seven days later or at full bloom. Following the long wetting period over the weekend and coming into potentially wet weather tomorrow, May 22, many growers have re-covered sweet cherries for American brown rot. Prior to potentially wet weather over the weekend, blossoms may need to be protected depending on sweet cherry bloom development.

Cold, wet weather has also been a concern for European brown rot for Balaton and Montmorency tart cherries. As a reminder, this disease infects tart cherries during bloom and the variety Balaton is especially susceptible. In slow drying conditions, Montmorency can become infected; we observed widespread European brown rot in both Balaton and Montmorency in 2013. The suggested management strategy for European brown rot is similar to American brown rot—first fungicide application at popcorn followed by an additional application seven days later or full bloom. Temperatures are forecasted to warm up in the coming days, and these warmer conditions will be suboptimal for European brown rot.

Growers have also been challenged with deciding when to begin cherry leaf spot programs. Bract leaves are present and expanded, and the first true leaves are showing and are just starting to expand at this time. Spring temperatures have been cool, which could have slowed the development of overwintering cherry leaf spot inoculum. However, there was still a risk of getting infected early during the recent extended wet weather over May 18-20, if leaves were exposed, open and susceptible. At this time, starting cherry leaf spot programs early (i.e., when bract leaves are expanded) is not a waste, as there is a risk of these small leaves to be infected and the pathogen is already present in the tree and can easily spread to other leaves in the canopy.

You do not want to battle this disease for the remainder of the season, particularly in a year when we want to keep input costs to a minimum. Keep in mind that fungicides should not be relied on for post-infection activity. However, if you are concerned there has been a leaf spot infection on open leaves, Syllit is the best material to add to the tank to provide some kick-back, if necessary.

We are about four weeks into primary apple scab with green tip dates ranging from April 22-25 across the region. Since this timeframe, many areas have had three significant scab infection events, with the most recent event spanning May 18-20. According to the apple scab model on Enviroweather, disease symptoms from infection periods in early May could be showing up this week. As the season has progressed, we are approaching the point during primary apple scab season where many spores will be discharged in upcoming rain events (see RIMpro for spore discharge predictions). Warmer temperatures will help scab spores reach maturity, meaning that there is a greater potential for larger spore discharges in upcoming rains as well as faster disease progress than we have observed in past infection periods.

We had our first detection of black stem borer (one beetle) in our trap at the Northwest Michigan Horticulture Research Center this week. While emergence was not detected at exactly 100 GDD base 50, it was a more precise indicator than forsythia bloom, which has been blooming for at least two weeks. The first step for black stem borer integrated pest management is to keep young trees healthy and stress-free. The beetles are attracted to and attack trees stressed due to drought, flooding, winter injury, etc. In orchards with an existing infestation or a history of damage, treatment may be needed to kill the emerging beetles. However, treatments should be targeted for peak emergence to maximize efficacy.

For most insects, cool rainy weather has continued to impact activity. We observed an uptick in spotted tentiform leafminer activity in our trapline, and a few small aphids have been observed on unsprayed crab apples in the area. Degree day accumulations suggest oriental fruit moth could be flying, but we have not detected this pest at the station this season and very few are detected in our traps each year. For the week ahead, be on the lookout for small larvae in flower buds.

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