MSU Extension Fruit & Nuts

Weather update 

This week is exceptionally hot. With highs in the 90s and evening lows in the 60s, keep in mind that working in this weather can be dangerous. This Michigan State University Extension article on understanding and preventing farm employee heat-related illness details what can be done this week. Also, keep the Michigan State University (MSU) Enviroweather potential evapotranspiration model in mind this week when making irrigation decisions. 

For a detailed look at the upcoming weather forecast, MSU agricultural meteorologist Jeff Andresen has weekly 15 - minute talks posted every Tuesday. 

Crop update 

Sweet cherries are being harvested in the area this week. Given the hot temperatures, we are anticipating a tighter harvest window than normal.  

For apples, consider implementing sun scald protection. Sunburn necrosis can occur when daily temperatures are in the 90s, like this week. This article on apple sunburns from Washington State University details how this damage can occur and how to prevent it. Protective measures include protective netting, overhead water applications and sprays like kaolin clays, calcium carbonates or lipophilic waxy matrices.  

We typically report GDD accumulation starting at March 1, but this is not a typical year. We reached the required chilling hours on Jan. 15 to move into ecodormancy and start accumulating GDD. This means the heat we received in February mattered for this year’s crop. As such, I will be reporting GDD from Jan. 1 rather than March 1 this season.     

Growing degree days are quickly moving in line with third cover timings. Keep this in mind for insect controls this week.  

Pest and disease update 

Apple scab 

We are moving towards the end of the primary apple scab season, but it isn’t quite over. Last night, there was a wetting event from 1-2 a.m. This event did not have a risk for infection (11%), but it was associated with one ascospore. 

I caught two ascospores per rod following the rain from 5 p.m. June 8 to 9 a.m. June 9. This was a moderate risk event with 184% progress towards infection.  

I will continue to scout for spores until we get two consistent zero spore collections. With the scattered showers in the area this past week, please note that I am basing my spore collections off of the Sparta Enviroweather station.  

Please remain covered with fungicides until primary scab season is concluded and no secondary lesions can be detected with careful scouting. I have seen scab lesions in multiple blocks throughout the region, despite consistent spray programs.  

No matter how few spores we catch, they all matter for controlling secondary scab. While one ascospore can create one lesion, each lesion can produce up to 100,000 conidia! Spraying now can prevent many sprays later in the season.  

Fire blight 

We are past the window of primary concern for shoot blight timing now. Most shoots are hardening off.  

If you have any fire blight infections, please reach out to me at brow1111@msu.edu. The Sundin lab is looking to sample different bacterial isolates throughout the state this year.  

Powdery mildew 

Primary powdery mildew lesions from overwintering inoculum have ceased sporulation. However, secondary powdery mildew is continuing to spread. I am seeing powdery mildew take over whole shoots. Keeping control over powdery mildew now will prevent it from becoming a wider spread issue later in the season. This hot, dry weather is excellent for powdery mildew. 

Fruit rot 

Start incorporating control measures for bitter rot and black rot now. Mummies and fallen fruit contain inoculum for this season. They are beginning to sporulate and infect at this point in the season. Frog eye leaf spot from black rot is now visible across different blocks in the area.  

Sooty blotch and fly speck 

While these two fungal diseases appear later in the growing season, their control window starts now. Summer diseases need springtime control.

Cherry leaf spot 

Cherry leaf spot is much more widespread than typical years, even in orchards with consistent programs. We are now seeing some early defoliation associated with cherry leaf spot, which can both harm this year’s crop and the overall tree health. Keep in mind that cherry leaf spot lesions look like small, reddish-purple dots (2–3 millimeters across). A key symptom is white, fluffy fungal growth on the underside of the lesions. These lesions can overtake leaves and lead to defoliation. We are especially seeing these symptoms in leaves that also have bacterial canker symptoms. While this disease is primarily thought of in tart cherry production, it can certainly impact sweet cherries too. 

Bacterial canker of stone fruits 

Bacterial canker of cherry (caused by Pseudomonas sp.) and bacterial canker of peach and plum (caused by Xanthomonas sp.) is popping up across the state this season.  

Both of these diseases are associated with “shot hole” in the leaves, brown dry lesions on the fruit and cankers in the trees. This can also be associated with gummosis production, which is a general stress response by the trees.  

If you are seeing infections now, consider summer pruning to remove developing cankers, copper sprays at leaf drop, and dormant copper next spring.  

Cracking from rain or wounding in sweet cherries leads to higher likelihood of American brown rot infection. As fruit gets closer to harvest, susceptibility to brown rot increases. Growers should consider if brown rot management is needed. Using Indar may not be effective, even at the highest labeled rate of 12 ounces per acre. Previous research and resistance screening of American brown rot isolates in west central and northwest regions found that most screened fungal isolates had functional resistance to Indar. Find more information about this study. Effective materials for managing American brown rot include Merivon, Miravis and Cevya. Full coverage of all rows will be important to manage this disease. 

Insects 

Codling moth first generation flight is coming to an end this week, according to the Rimpro model. Egg hatch will also cease this week and the associated L1 larvae are present in the area. 

Obliquebanded leafroller adults are active this week. My date of sustained catch is June 10. As such, egg hatch is expected to begin June 21 with peak egg hatch June 22.  

No green fruitworms were caught in my traps this week.  

The second generation of oriental fruit moth is emerging this week with peak adult emergence from June 17 to 20. Peak egglaying is expected this week on June 23.  

The second generation of spotted tentiform leafminer is high this week.  

Potato leafhoppers are currently in the area in high quantities. Keep an eye out for them in sticky traps and on the undersides of leaves. Look out for V-shaped, burnt leaf margins,or cupping of young terminal leaves.  

Green apple aphids are present in some blocks this week.  

Woolly apple aphid colonies are still active and growing in orchards. If you used a spirotetramat treatment during the petal fall, first cover timing window, be sure to wait 10–14 days after application to make sure the product has taken effect before planning another spray.  

San Jose scale crawlers are active in south Michigan this week. I expect crawlers are active in the area, but I have been catching parasitoids only. Keep in mind that controlling this round of San Jose scale crawlers will be easier than controlling future generations this summer.  

European red mites are active in the area with visible bronzing in some blocks. If you have mite damage, be sure to stay on top of irrigation to limit stress in impacted trees.  

The first spotted wing drosophila are in the area this week. As cherries move towards ripening, ensure they are covered for spotted wing drosophila.  

For more information about regional reports, please visit the Michigan State University Extension website.