Northwest Michigan fruit update – May 14, 2019
Trees are developing faster than degree days are accumulating. We are anticipating sweet cherry bloom later this week.
The weather continues to be cool here in the north. It almost seems that spring will never arrive or at least stick around for more than one day at a time. Temperatures were in the mid-50s last week with lots of cloud cover. The sun did come out over the weekend and yesterday, May 13, and the daytime high hit over 60 degrees Fahrenheit. Today is also expected to be pleasant, and the forecast is predicting temperatures close to 70 F today. Daytime temperatures for the remainder of the week will be in the low to mid-60s and overnight temperatures into the 40s.
We have accumulated 191 GDD base 42 and 67.1 GDD base 50 so far for 2019. These accumulations are almost two weeks behind our 30-year averages, which are 356 GDD base 42 and 162.4 GDD base 50. The heat accumulation map on MSU’s Enviroweather is also reflecting our behind-average degree-day accumulation (Figure 1).
Conditions have also seemed to be on the wetter side, even if we have not received significant total rainfall amounts. The NWMHRC Enviroweather station recorded a wetting event on Monday, Wednesday, Thursday, and Friday last week, but only Thursday, May 9 recorded a noteworthy rainfall at 0.43”. More rain is predicted for Thursday, May 16. Enviroweather indicates the chance of rain for almost every day for the remainder of the week, but MSU meteorologist Jeff Andresen thinks the highest chances of rain are Thursday, May 16 and Sunday, May 19. Review Andresen’s weather report.
Tree fruits almost seem like they are suspended in time. We have documented little movement over the past week. We did see some movement with the warm conditions yesterday, and we suspect things are going to move quickly once we have more sustained warm weather. We are at tight cluster in apple, and we are starting to just see some white in sweet cherries. If we look at the weather forecast for the rest of the week, sweet cherries could be in full bloom by Monday, May 20. Full bloom in sweet cherry is 275 GDD base 42, and the current forecast predicts that we will be at 281 GDD base 42 on May 20. We also expect to see some bloom open in tart cherries, but the forecast predicts full bloom to be over Memorial Day weekend. However, forecasts can be a bit off, and bloom times will likely vary from this Monday’s forecast. The development will also move quickly as the trees have been in a holding pattern for the past two to three weeks.
We pruned sweet cherries yesterday at the Northwest Michigan Horticulture Research Center (NWMHRC). We were hoping the window of dry weather would minimize potential canker infections. We also took the opportunity to apply Promalin with the few days of warm weather. Ideally, we would like to see more than two days of warm weather to see activity of plant growth regulators, such as Promalin. However, we do have newly planted sweet cherries with very few branches and thought it was worth the time and effort to try Promalin.
If growers are interested in applying ReTain in sweet cherries, the recommendation is to apply that material early and of course, under warm conditions. We have data that show ReTain is excellent on the shy-bearing Regina. We also have data on increasing yields in Balaton with ReTain. Regina and other sweet cherries respond well to one pouch of ReTain applied at popcorn stage. Phil Schwallier’s data showed a 30% increase in set with one pouch. However, when a second pouch was applied at three days later at full bloom, we observed a 40 to 80% increase in set compared to the UTC. Our data shows that late applications (>30% bloom) will not result in increases in fruit set, so growers should err on the early side for ReTain applications.
Growers should be aware of bloom in sweet cherries—if the weather warms, bloom will come on quickly and the optimal time to apply ReTain may blow by before growers can spray ReTain. See Phil Schwallier’s article “Using ReTain to set fruit” for updated information on pears, apples and sweet cherries.
Late last week, the region experienced its second, long wetting period since the onset of apple scab season. The first significant scab infection event occurred the May 1-3 for any areas that were showing green tissue. The second scab event occurred from Wednesday evening May 8 and extended into May 10, resulting in a light to moderate infection across the region (Figure 2).
Although average temperatures were warmer during the second event compared with the first, temperatures were still cool. These infection periods are a reminder to growers that scab infections are possible in cold weather as this fungus can develop at temperatures as low as 32 degrees Fahrenheit as long as sufficient wetting has occurred.
There has been some new growth with the recent dry, sunny and warm weather. According to RIMpro, a larger proportion of scab fungal spores matured in this warmer weather, and RIMpro models are calling for a relatively larger spore discharge in the coming rain, particularly for the Benzonia and Williamsburg models where development is farther along (see links to models below). There is a chance for light rain predicted for Wednesday and Thursday this week followed by a few days of drier weather and then rain predicted again for Sunday and into early next week. Growers are covering for this upcoming wet weather. Fungicides should not be relied on for back-action purposes to minimize the chance for resistance development.
Tart cherry buds are at bud burst and sweet cherries are at early white bud. As mentioned previously, the region is behind in terms of degree days, but based on observations, trees seem to be developing faster than degree days are accumulating. At this current rate of development, we could see sweet cherry bloom late this week into early next week and bloom time pathogens could be a concern. At this time, the forecast is not calling for ideal American brown rot temperatures, but we could experience a long wetting period during bloom time that could be concerning for blossom blight. Additionally, new green leaves in cherries are susceptible to cherry leaf spot: sweet cherry is less susceptible to leaf spot compared with tart cherry.
Insect activity remains slow in our trap line. This week we found a few spotted tentiform leafminers. No black stem borers, American plum borers, nor oriental fruit moth were detected. We will deploy codling moth and San Jose scale traps this week. We will also put up San Jose scale pheromone dispensers (not yet commercially available) in sweet cherry at our cooperating field sites later this week.