MSU Extension Fruit & Nuts

Weather report

We have had a mixed bag of sunshine and rain over the past week. The most notable event was the rainfall we received on Saturday morning, July 24, as it was one of the biggest rain events we have seen in many years here at the Northwest Michigan Horticulture Research Center, if ever for some of us. The rain began early in the morning, and it rained extremely hard and caused tremendous amount of washout on dirt and farm roads as well as water covering main roads where creek and riverbanks overflowed. There was a flash flood warning in Leelanau County on Saturday morning.

The Northwest Michigan Horticulture Research Center Enviroweather station recorded 4.74 inches of rain on July 24. However, despite the intense rains at the station, other Enviroweather stations had varying amounts of rainfall; East Leland recorded 3.83 inches, Eastport 1.84 inches, Kewadin 1.59 inches, Northport 1.89 inches, Old Mission 3.96 inches and Williamsburg 3.44 inches. Surprisingly, the two most southerly Enviroweather stations only received 0.2 inches of rain at Benzonia and 0.11 inches of rain at Onekama.

We had trace amounts of rain again last evening and into this morning, July 26-27. However, the Petoskey Enviroweather station recorded 0.79 inches of rain. According to Jeff Andresen, more rain is coming on Thursday and perhaps some isolated rain showers on Saturday, July 31.

Daytime temperatures have been pleasant and have ranged from mid-70s into the low 80s degrees Fahrenheit. We had a warm day yesterday where the daytime high hit 85.3 F here at the station. Once the front moves through on Thursday, cooler conditions will prevail over the weekend. Next week, conditions should warm again. Andresen is predicting August to be warmer than normal. So far at the Northwest Michigan Horticulture Research Center, we have accumulated 2,204 growing degree days (GDD) base 42 and 1,420 GDD base 50.

Crop report

Tart cherry harvest is ongoing across northwest Michigan. Many growers to the more southerly areas are finished with harvest and many growers that had a short crop have also completed harvest for 2021. Harvest has been slowed because of labor shortages at the processor. Many processors are not running at full capacity (i.e., fewer lines, less hours), and as a result, growers have been placed on quotas to bring in their fruit. We have also identified issues with trucking this season. There has been a shortage of trucks/drivers to get fruit from the farms/receiving stations to the processing facility. Cherries have certainly sized with the rain, which was excellent as fruit was initially small because of the early droughty weather. Brown rot is evident in tart cherries, which is far less likely than this disease showing up in sweet cherries.

Apples are sizing beautifully. The Galas at the Northwest Michigan Horticulture Research Center are at 55 millimeters and Honeycrisp are at 52 millimeters. Some apples are just starting to color. Apple harvest is predicted to be earlier than last season. Comments from the Ridge area is that McIntosh may be up to 10 days earlier than last season.

Pest and disease report

Diseases

Despite all the rain we received at the Northwest Michigan Horticulture Research Center on July 24 (4.74 inches), this event only resulted in a low infection period for cherry leaf spot. We only had six hours of wetness, so that amount of rain in that short amount of time is notable. As many growers are finishing up harvest, we recommend trees keep their leaves into September to minimize chance of damage if we have a hard winter.

Chlorothalonil or captan are good choices for a post-harvest application. If growers have been clean of leaf spot until now, these orchards may not need a post-harvest application. We rated our leaf spot efficacy trials, and even the UTC is surprisingly clean this season compared to past seasons, particularly those with wet springs that really jump start leaf spot infection early. This dry spring minimized early infections, and the weather reduced the amount of leaf spot we are seeing in orchards later into the season.

We have observed higher than normal levels of powdery mildew in both apple and cherry. This disease is favored by hot and dry conditions, both of which we have had this season. This disease often takes a back seat to leaf spot and apple scab, but this year, this disease is much more of a concern. There is a lot of mildew on new shoot growth in tart cherries, and as a result, powdery mildew makes the leaves brittle and are easily removed during the shaking process. Unfortunately, there is little growers can do to eradicate this disease once we can see mycelial growth on the leaves.

Insects

We have taken down our cherry trap line at the Northwest Michigan Horticulture Research Center.

We did not catch any oriental fruit moth again this week in apple. Codling moth numbers were an average of about two moths per trap, which is similar to what we have been seeing at the station for the whole season. We have also heard from many area consultants and growers that there are few codling moth in traps this season. In many cases, growers have not hit the start of the second generation based on first generation biofix.

However, San Jose scale male catch increased considerably this week. We caught no males last week, and our traps counts jumped to an average of 76 males per trap this week. Growers should be proactive in apple as this is the stage of this insect that can injure fruit. Again, we detected no apple maggot in our traps this week. We did detect an average of 4.3 obliquebanded leafroller moths in our trap at the station. We also found zero black stem borers in our traps.

Spotted wing Drosophila (SWD) numbers are finally increasing in the region. We have caught more flies in SWD traps at grower farms, and our trap numbers have also increased in traps at the station. Most of the trap numbers in our unsprayed tart cherry block at the station have been ones, twos and occasionally other single digit numbers in our five traps. On July 21, we caught a total of four flies in our five traps. Those numbers increased the following day, and our trap counts rose to 13 flies in those five traps. On July 23, we saw another slight increase in fly count, and we captured 19 flies.

In addition to trap count increase, we could visibly see flies in the unsprayed block, and we even observed females laying eggs into fruit. This observation is not unusual, but it is unusual to see this level of activity after so many days of rain, high humidity, and warm conditions for such a long stretch of time. Additionally, the SWD risk model has been predicting we have been at high risk at the research station since June 28, almost a month previous.

The number of SWD larvae in fruit has also risen. As mentioned in previous reports, we have sampled fruit for larvae daily from June 21 – July 23. We collect five samples of 30 fruit around each trap tree for five trap trees for a total of daily 750 fruits. We detected our first larvae on July 6 when we detected two in our sample. All fruit samples prior to July 6 contained zero larvae (from June 21 through July 5). We found no larvae the following day on July 7, then we detected 12 larvae on July 8. From July 9-20, we detected larvae in daily samples, but all samples were single digit larvae. However, on July 21, the larval number jumped up to 22. Then on July 22, the number of larvae in our 750-fruit sample increased to 69. On July 23, we detected 70 larvae in our sample.

This increase in the number of larvae infesting tart cherry fruit is notable as it went from single digits to 22 to about 70 in two days. Also notable is that our trap counts increased as the number of larvae increased: zeros to single digits from June 21 until 13 flies on July 22 and 19 flies on July 23. The fly counts increased as larvae increased, but the traps did not appear to provide early detection type information. By the time trap counts reached into double digits, we already had found 69 larvae infesting fruit.

SWD continues to be a mystery, and 2021 highlights that it is a difficult pest to comprehend and ultimately to manage. We will delve into data further during the fall and winter months to better understand this insect’s population growth and what appears to be a delay in building their populations and their infestation potential in 2021. Our daily sampling, datalogger data that records daily weather conditions, and fly and larvae detections will provide more insight on this pest as well as how we can modify the current risk model to guide grower decision-making.