MSU Extension Fruit & Nuts

Weather update and integrated pest management impacts 

The Sparta Enviroweather station received 1.81 inches of rain last week and another 0.75 inches of rain is anticipated for this upcoming week. We are expecting another warm week with highs ranging from 75 to 85 degrees Fahrenheit. Luckily, no rain is currently predicted for July 4. 

These rainy conditions are ideal for fungal growth. Please ensure you have covers on for apple diseases including bitter rot, black rot, sooty blotch and fly speck and secondary apple scab. Stone fruit fungal diseases such as cherry leaf spot and American brown rot will also be encouraged by this weather.  

Consider the rainfastness of your products and the recent rain events when timing sprays. 

Crop update 

Sweet cherry harvest is wrapping up in the area this week.  

Apples have moved into the cell expansion phase of growth. 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 accumulations is now moving into 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 

While primary scab season is over, keep an eye out for secondary scab. 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.  

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. Some sites have had strong powdery mildew pressure this season.  

Fruit rots 

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. Frog eye 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. The mycelium associated with this disease is currently not visible, but it is growing. 

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 gets 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 disease. We are seeing this disease on both tart and sweet cherries this season.  

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.  

Japanese beetle and rose chafer leaf damage is appearing in orchards this week. As they emerge, keep an eye out for leaf skeletonization.  

Codling moth first emergence of second generation of adults was on June 29. The larvae for this generation are now hatched. 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 on June 21 and peak on June 24. The populations are high again 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 to 20. Peak egg laying was June 22. The first emergence of generation three adults is expected outside of the seven-day forecast. Shoot tip damage associated with oriental fruit moth is visible throughout orchards now.  

The second generation of spotted tentiform leafminers is high again 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. Terminal leaf cupping with leaf hoppers present underneath the leaves are common this week.  

Woolly apple aphid colonies are still active and growing in orchards, but populations have dipped with the recent heavy rains.  

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