Northwest Michigan fruit update – May 26, 2020

Tart cherries are in full bloom and apple bloom is starting. Growers are finding time between forecasted rains to protect blossoms and leaves.

Weather report

We hope everyone had a nice Memorial Day weekend. We had quite a mixed bag of weather over the past week, and there was no doubt that Michigan State University agricultural meteorologist Jeff Andresen was correct in saying that we were going to have some warmer and wetter weather since last Tuesday, May 19. The weather seemed to have jumped right to summer this past week, and we had daytime temperatures that reached into the mid- to high 70s, and yesterday, May 25, the Northwest Michigan Horticulture Research Center Enviroweather station recorded a daytime high of 83.5 degrees Fahrenheit. These temperatures make our recent concerns over spring frosts seem like a distant memory. Even our nighttime temperatures seem more like summer where we only dipped down into the 60s over the past week.

These warm conditions really increased our growing degree day (GDD) accumulations; we accumulated 350 GDD base 42 and 151 base 50. We have jumped ahead of 2019, which also started out cool. At this time last year, we had accumulated 336 GDD base 42 and 138 GDD base 50. However, we are still behind our 25-plus-year average: 520.7 GDD base 42 and 253.2 GDD base 50.

In addition to warm conditions, we also had some rainfall over the weekend. We were dry this past week until Sunday, May 24, and we had a bit of rain that day and into Monday. At the Northwest Michigan Horticulture Research Center, we accumulated only 0.21 inches of rainfall on Sunday and a trace amount on Monday. However, humidity levels have been high across the region, and we even had quite a bit of fog on Sunday and into Monday in Leelanau County.

Weather is predicted to be warm for the next three days and then cool slightly as we head into the weekend. Growers will get some relief as the chances of rain will drop at that time too.

Review Andresen’s weather report for May 26, 2020.

Crop report

The warm weather really accelerated crop development. Sweet cherries moved through full bloom and some varieties started dropping petals with the warm weather. Many tart cherries across the region are in full bloom. In apples, we move through pink quickly in many blocks, and we observed open blossoms on Monday, May 25.

Pest report

There was a lot of activity in orchards over this holiday weekend. Early varieties like Ida red started blooming and king blooms opened on Honeycrisp over the weekend. Warm conditions and bloom brought concerns for fire blight, particularly with the erratic predictions for rain yesterday and throughout the week ahead. Because of the rapidly changing forecasts, EIP (Epiphytic Infection Potential) values on the fire blight models also changed quickly. Many growers were proactive and covered for fire blight ahead of the scattered rain on Monday, May 25, which was a good decision. We have also had heavy dews that can wash bacteria down into the floral nectaries on open flowers.

With orchards at or approaching king bloom petal fall this week, growers will be thinking about starting plant growth regulator (PGR) programs, for example, Apogee/Kudos. Because temperatures will be warm and there is a chance for rain over the next few days, EIPs are high, in the 200-300s for some areas, you should be managing for fire blight. Remember that the threshold warranting treatment with a bactericide is an EIP of 70-plus. Bactericides like kasugamycin and streptomycin should provide up to two days of protection from fire blight. Furthermore, resistance to streptomycin has been found throughout the region and kasugamycin is a safer choice to better ensure effective fire blight control. Cooler conditions toward the end of the week will slow the growth of fire blight bacteria.

Covering up fast growing leaf and shoots was also a concern for growers. We are in the midst of a long infection period for primary apple scab. Rains on May 24 set the disease clock and high relative humidity (i.e., 90% or more) has kept the clock rolling between rain events. RIMpro is showing two spore discharge events that followed the two most recent rain events on May 24 and 26. With a chance for rain over the next few days, the model is calling for additional spores to be released during each event and spores are maturity quickly in warmer weather. Primary scab is ongoing, but approximately three-quarters of the spores have discharged at this time.

Growers also covered for cherry leaf spot as there was a good amount of growth and these leaves were susceptible to infection. Similar to apple scab, the cherry leaf spot model has also been running since the May 24 rain event and is reporting a heavy infection for areas that have remained wet and humid. We have received questions about the model output as it seemed that there was a sufficient drying period between the rains. High relative humidity plays a role in the development of this disease and this model. We are investigating the data fed into the model more closely at this time. However, whether the event was one or two events, an infection was likely due to favorable conditions for this disease; use preventive measures to protect fast growing tissues.

American brown rot was a concern during sweet cherry bloom as warm, wet and humid conditions favored the disease and sporulating mummies were found in orchards with brown rot last season. Fortunately, bloom has moved quickly in sweet cherries and as bloom ends the risk for brown rot infection will be minimized.

As anticipated, insect activity picked up on warm and sunny days, which were good conditions for bees and pollination. We also observed an uptick in natural enemies like lady bugs and syrphid flies in orchards. Pest activity in our trap like also increased.

European red mite eggs have stared hatching, and nymphs are visible on leaves.

Black stem borer activity began on Thursday, May 21, and a good flush of emergence occurred over the weekend. We found a total of 31 beetles in our traps and have reports of high numbers in traps throughout the region. If orchards challenged with black stem borer are blooming, wait until after bloom to treat for this pest.

Oriental fruit moth is active in the region. We had reports late last week of high numbers in hot spot areas in Leelanau County, and these orchards have set biofix. This week, we found the first detection of oriental fruit moth in our trap line with an average of one moth per trap at the station. However, catches of this pest are typically low in our trap line. Two consecutive catches of this pest are needed to set biofix.

Spotted tentiform leaf miner numbers increased to 267 moths per trap, but populations are well below a treatment threshold.

We have not found aphids on terminals at the research station, but they could show up soon.

In cherries, American plum borer started flying; we found an average of one moth per trap. Borer management targets the immature stage and we are not yet in the window for treating this pest.

Traps for codling moth, San Jose scale, lesser peach tree borers and plum curculio will be set this week.

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