MSU Extension Fruit & Nuts

Weather report

The weather has been very summer-like over this past week. Conditions were warm and dry. We even had some optimal windows for making spray applications as winds were down, and growers were able to get good coverage on the crops. Daytime temperatures were mostly in the low 80s degrees Fahrenheit with some days topping out in the 70s. Nighttime temperatures started out cool at the beginning of last week but warmed into the 60s through the latter part of the week and into the weekend.

Thus far, we have accumulated 998 growing degree days (GDD) base 42 and 586 GDD base 50. We are catching up to our 25-plus-year old averages, which are 1118.6 GDD base 42 and 637.8 GDD base 50.

The week was predominately dry until later in the weekend. We had minimal rainfall on Saturday, June 20, and the Michigan State University Enviroweather station at the Northwest Michigan Horticulture Research Center recorded only 0.12 inches of rain. Sunday was dry, and the rain moved back in on Monday where the weather station recorded 0.36 inches of rain. The rain is also ongoing today, June 23. This wetting event was a nice constant rain that was much needed as soil conditions were drying out.

The weather forecast is for continued showers this morning and cooling off and becoming windier this afternoon. MSU agricultural meteorologist Jeff Andresen is also predicting that these windy conditions will continue over the next week, which will make spraying difficult. Weather will cool over Wednesday and Thursday, and rain moves back into the region on Friday and Saturday, June 26 and 27. However, rains may be lighter and more variable in the north compared to the southern regions of the state. The longer term forecast for the first week of July will move into a period of warmer and wetter than normal conditions. Review Andresen’s report below.

Crop report

Fruit continues to size. Cherries, both sweet and tart, only grew 1 millimeter over the past week. However, this rain will likely help increase size. The tart cherry crop is thinning out a bit in some blocks, but some growers still think they have a good crop. The Balaton crop is reported to be excellent across the region. Sweet cherries are also looking thinner than they had before heading into last weekend.

Apples made a big jump in size over the past week, and both Honeycrisp and Gala are approaching the “hail Mary” phase of thinning where the fruit are too big to have much effectiveness with thinner applications. Honeycrisp fruitlets at the station increased from 10 millimeters in size to 20 millimeters, and Gala increased from 11 millimeters to 19 millimeters over the past week.

According to the precision orchard management measurements we have conducted, we are close to the number of fruitlets desired for the season (Figures 1 and 2). We applied a pint of Sevin and 8 ounces of NAA on Friday, June 19, as our only thinner applications for 2020. We were concerned as we saw intensive fruitlet drop earlier in the season. However, by using this model, we observed we were not at the optimal fruit load on Thursday, June 18, and as a result thinned the following day on Friday with what looks to be good results (Figures 1 and 2).

Pest report

It has been roughly eight weeks since biofix (April 26-27, depending on location), and according to the RIMpro models, the last ascospores of primary apple scab season were released during yesterday’s rain. The rain came during the day, which is usually related to good spore release. Some growers, particularly those in cooler locations or areas with less rainfall, are playing it safe and planning to add a fungicide in the tank to be covered for the rain coming this weekend, as it is predicted to be a longer stretch of weather with a good amount of precipitation forecasted. If scab got a foothold during primary, fungicide programs will need to continue beyond primary scab to prevent fruit infections.

• Benzonia
• East Leland
• Williamsburg Tower

We have observed and received few reports of fire blight in apple blocks so far this season but continue to monitor for symptoms as there are a few strikes in commercial blocks. Little fire blight at this time is welcomed after the high EIPs during bloom and the earlier windy weather that could have resulted in a trauma event. Recent rainy weather did not bring high winds, which was good news for avoiding trauma blight. Trees have not reached terminal bud set at this time and hence, there is still a risk of trauma blight if we receive severe weather in the future. Fortunately, the near-term forecast continues to look unthreatening.

Nectria has been reported and this fungal pathogen produces symptoms such as flagging and browning of leaves that resembles fire blight. Branches with nectria are often associated with a stem leftover from the previous season. Additionally, nectria symptoms usually begin from the base of the branch and brown toward the terminal end. This is in contract to trauma blight infected shoots that flag at the terminal end and die from the tip to the base.

Few new cherry leaf spot lesions are showing up in managed blocks, which reflects effective programs and good windows for staying covered between rains. However, there are some active lesions and conidia present on older leaves. Rain over the weekend, June 20, triggered cherry leaf spot infections across the northwest and while some areas dried up, parts of northern Leelanau county received spotty light rain that kept the disease clock running on Enviroweather.

Rain on Monday, June 22, started the clock again for much of the region, and high infections are currently ongoing across the region. Symptoms of infection from these events will show up at the end of June into early July, if infection occurred. Growers will need to recover after this wet period to be protected for rains coming this weekend. The current forecast is calling for a good chance for extended wetting that could bring almost three-quarters of an inch of rain from Friday afternoon into a good portion of Saturday. If this forecast for warm and wet weather holds true, this event will be another heavy leaf spot infection.

Codling moth catches jumped significantly this week with traps at the station catching 20 to 40 moths; these numbers are much higher than what we usually catch at the station. Egg hatch began late last week and growers with moderate to high pressure started taking action against the hatching larvae. Since biofix (June 1), we have accumulated about 330 GDD base 50. The delayed timing (i.e., about 350 GDD base 50 post-biofix) is quickly approaching on Wednesday or Thursday this week for treatment in low pressure blocks or where an ovicidal material was applied at 100 GDD base 50 post biofix.

Aphids were new on the scene this week. We found winged and immature aphids on terminals of young trees. The immature aphids are very small and difficult to see at this time.

We have continued to observe mites on both apple and cherry at the station. We found two spotted spider mites on cherries previously, and we are now seeing these mites on apples as well. European red mites have been found on both apples and sweet cherries. Mites pierce and suck contents out of plant cells which results in leaf stippling and bronzing. When mite populations are high, feeding can cause leaves to abscise prematurely. Warm and dry weather like the conditions we have experienced for much of the season so far is especially favorable for mite development.

Obliquebanded leafroller are in the pupal stage and adults are starting to emerge in parts of the region. We have not caught moths at the Northwest Michigan Horticulture Research Center at this time, but moths have been captured in the Yuba area, which is slightly ahead of the station in terms of degree days.

The first flight of male San Jose scale is ending. Crawlers are not yet active. In conjunction with visual observations, we are using the Washington State University San Jose scale degree day model to estimate crawler activity. Based on this model and our biofix (June 1), the beginning of crawler activity could start this week. However, our observations suggest that it can be difficult to observe the early stages of crawler emergence, particularly in blocks with low pressure. Different chemistries used against this pest work in different ways. For example, some insecticides such as those in the pyrethroid class must come into contact with the crawlers to be effective. Hence, these materials typically target timings when a good number of crawlers are active. Other materials such as insect growth regulators can be used at earlier timings when crawler emergence is just beginning. As the crawlers are exposed to growth regulators, their normal developmental process is interrupted.

Rose chafers are still active, feeding and mating. The beetles skeletonize leaves but they will also feed on developing fruitlets. The beetles can quickly re-infest blocks that have been treated previously so continue to monitor for the beetles, particularly in new or young blocks and orchards adjacent to hay fields. The beetles live for about three weeks, and it has been roughly a week and a half since we first observed them feeding in the region’s orchards. Thankfully, there is only one generation of rose chafers per season.

Lesser peach tree borer trap numbers remained consistent with last week and we are at the degree day timing for peak activity of the flight of these moths. We are also approaching the degree day timing for the first greater peach tree borer moths to begin flying, but we have not detected these moths in traps at the Northwest Michigan Horticulture Research Center. American plum borers were not detected in our traps this week. The traditional borer trunk spray material, chlorpyriphos, can cause phytotoxicity on sweet cherry foliage.

Adult black stem borer beetle activity has slowed substantially over the last two weeks, and trap counts are down considerable. However, growers have been reported collapsing trees, and when they cut open the trunks, galleries of eggs and larvae are present. The eggs and larvae are currently protected from spray materials inside of trees in galleries created by their mother. The best way to know when the beetles emerge is to monitor for them visually and by using traps. Once adult beetles emerge, target adults with a pyrethroid and follow up again one week later.

Spotted wing drosophila (SWD) numbers took a jump last week in the northwest region as well as across the state. More than half of the traps in the region and the state have detected SWD, and based on previous work with SWD, this pest is likely active in all fruit crops across the state. Therefore, we are no longer including tables for risk when flies are not present as it is too risky to assume that no flies are present when such a high percentage of traps across the state are detecting SWD.

Currently, all the northwest region is at “low risk” according to the beta SWD model (Table 1). Warmer parts of our region are approaching the 954 degree day indicator that moves us from low to medium risk of fruit infestation in tart cherries. For this model, full bloom is the biofix, and we begin accumulating degree days base 39.2 after biofix to indicate the risk for SWD infestation. The risk for infestation will continue to heighten as fruit ripen and GDD reach and exceed 954 GDD base 39.2 F post full bloom.

Please note that because different sweet cherry varieties ripen at different timings, the indicators for sweet cherry may not be applicable to all varieties. Use good judgement and previous experience to estimate when to begin SWD programs in sweet cherry blocks.