MSU Extension Fruit & Nuts

Weather update and IPM impacts

This past week has been exceptionally hot and wet with 2.46 inches of rain. This upcoming week is expected to cool off, but the rainy conditions will continue.

This weather has some integrated pest management (IPM) impacts to note. The high volume of rain has degraded spray residues, and the consistent rain and high winds will limit spray windows this week. However, Wednesday and Thursday (June 26-27) are predicted to be good spraying conditions with both clear skies and low winds.

The warm and wet conditions are ideal for disease growth. This will be especially important for fungal issues like secondary scab, fruit rots and summer diseases in apples. Additionally, between the hail and high wind conditions, this presents a potential for trauma blight, which will be discussed later in this article.

In cherries, this weather can exacerbate bacterial canker, cherry leaf spot and American Brown Rot. The volume of rain can also contribute to cracking in cherries as sweet cherry harvest continues this week.

On the insect front, these rain events are likely to cause an increase in Japanese beetle and rose chafer populations in the next week.

Crop update

Sweet cherry harvest is wrapping up in the area this week. Early peach varieties will be ready for harvest as early as next week.

Apples have moved into the cell expansion phase of growth. Please keep in mind that the recent rains and the associated potential runoff will likely have leached fertilizer out of the soils, especially mobile nutrients like nitrogen.

Growing degree day accumulation is now moving between third cover and fourth cover timing.

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.     

Disease update

Apple scab

We are now at the end of primary apple scab season. This was a tough season to control with consistently windy conditions and unreliably predicted rain events. We are seeing small amounts of secondary scab throughout the region now. Please be sure to thoroughly scout prior to stopping scab controls.

Below is the full report of my primary apple scab trapping for this season. If you are seeing secondary scab, please consider the predicted lesion dates compared to scouting reports to identify any holes in this spring’s spray program.

* = Light infection risk, ** = moderate infection risk, *** = heavy infection risk, no asterisk (*) = no infection risk.

Fire blight

The wind and potential hail from the storm today, June 25, can cause damage to the trees that will allow bacteria to enter and infect. This is called trauma blight. Prompt sprays after the storm will limit the development of disease. Consider streptomycin sprays. Agri-Mycin has a preharvest interval of 50 days, which should not interfere with apple harvest. The heat this week may result in russeting or phytotoxicity if copper sprays are used.

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

The rain this week has reduced powdery mildew’s ability to spread in the air. However, once the rain clears, we will once again have consistent severe risk of powdery mildew development. At this point, the overwintering primary powdery mildew has ceased. However, this is a cyclic disease and if it has established a foothold it will continue to spread all summer long. While some powdery mildew won’t kill trees, it can rapidly get out of control and reduce photosynthesis throughout the canopy.

Fruit rot

Start incorporating control measures for bitter rot and black rot now. Mummies and fallen fruit contain inoculum for this season. They are sporulating and infecting at this point in the season. Frogeye leaf spot from black rot is now visible across different blocks in the area. The rain and heat this week will be very beneficial to the spread of these diseases.

Sooty blotch and fly speck

While these two fungal diseases appear later in the growing season, their control window is 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 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.

American brown rot

Cracking from rain or wounding in sweet cherries leads to higher likelihood of American brown rot infection. As fruit get closer to harvest, susceptibility to brown rot increases. 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

Insect update

Spotted wing drosophila has high populations this year. The warm and wet conditions this spring following a mild winter are prime for spotted wing drosophila development. Additionally, the high wind conditions and rainfall have presented challenges for getting spotted wing drosophila treatments on this season. Please refer to “The 2024 outlook for spotted wing drosophila in Michigan cherry orchards” for more information on spotted wing drosophila and their control.

Please note that this weather is conducive with increased populations of Japanese beetles and rose chafers. We anticipate a spike in their populations in the next week.

Codling moth first emergence of second generation of adults is expected June 28. The larvae for this generation will finish hatching this week. Consider this article on codling moth management.

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 on June 23. The populations are maintaining this week with the first emergence of second generation adults outside of the seven-day forecast range.

No green fruitworms were caught in my traps this week.

The second generation of oriental fruit moth is emerging with peak adult emergence from June 17–20. Peak egglaying was on June 22.

The second generation of spotted tentiform leafminers 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.

San Jose scale crawlers are active in south Michigan this week. I expect crawlers are active in the area, but I have been catching only parasitoids. 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.

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