Tree Fruit Disease Update: Preventing Late Season Disease Issues

Some issues that should be on the grower radar during this time: pre- and postharvest fruit rots and Marssonina blotch. Here are some nuggets of wisdom to deal with these potential headaches:

Preventing pre- and postharvest apple fruit rots

Small shoulder cracks are being observed on Honeycrisp picked for the maturity ratings. Such wounds can be an easy opening for fungal spores to cause fruit rots. In addition, we are all aware of how susceptible Honeycrisp is to bitter rot; these shoulder cracks make the fruit more vulnerable. Since bitter rot is the primary concern, our research to date has shown conventional fungicides vary in efficacy, even within a fungicide mode of action group.

Fruit with shoulder cracking can be vulnerable to fruit rots after harvest. Photo: C. Walsh, University of Maryland

Our data this summer support Merivon (FRAC group 7 + 11; 0-day PHI) to be the most effective late-season control. Merivon also affords rot protection during storage. The predominant fungal species causing bitter rot in our region is not susceptible to trifloxystrobin, which is found in Luna Sensation and Flint Extra; however, the species is susceptible to pyraclostrobin, which is found in Merivon.

General management techniques

Keep the following in mind to reduce postharvest fruit rots

• Bruised or wounded fruit are susceptible to postharvest fruit rots, such as blue mold and gray mold. While harvesting, handle fruit carefully when picking and transferring fruit from bag to bin to avoid bruising or wounding.
• The more mature a fruit, the more susceptible it is to storage diseases. Harvest fruit at proper maturity.
• Inoculum sources for rot pathogens causing disease in storage (if already not hitching a ride on the fruit) come from plant and soil debris. Use clean bins and minimize the amount of soil and plant debris brought in on bins.
• Warm temperatures encourage pathogens to grow. Keep fruit cool after harvest, i.e., keep bins in the shade.
• If delivering to a packinghouse, minimize the time between harvest and delivery of fruit.

Managing Marssonina Blotch

Marssonina blotch, caused by Marssonina caronaria, is found on leaves and fruit of apple. Last year, Marssonina blotch caught growers off guard late in the season when trees were observed to be losing leaves during September. In 2019, several orchards have been already observing Marssonina blotch on apple trees as early as July. This is a troubling disease since it can lead to premature defoliation, which is a tree stressor. Depending on how temperamental Mother Nature will be over coming weeks, growers will need to be mindful of keeping trees protected, especially during frequent heavy rainfall events. This is an easy disease to manage with conventional fungicides.

Leaf spots first appear on the upper surface of mature leaves mid to late summer. They are 5-10 mm in diameter, grayish, brown, and tinged purple at the periphery. Small black fungal fruiting bodies are often visible on the surface. When lesions are numerous, they coalesce, the surrounding tissue turns chlorotic, and defoliation results. Defoliation begins about two weeks after the symptoms appear. This fungus can also infect fruit; however, we have yet to observe it in the field. Considering many orchards in the region experienced a wet spring and frequent rain events during the summer, the disease pressure is high this year.

Marssonina infection can stress apple trees causing premature defoliation (left). Disease pressure is high during the 2019 season for Marssonina blotch on apple trees (right). Photos: K. Peter, Penn State

Most cultivars are susceptible to this disease. Consequently, be mindful of your fungicide applications through harvest. Once the disease becomes established, it is very difficult to get under control, and trees will defoliate prematurely quickly. To date, we have observed, Rome, Honeycrisp, Gala, and apple scab resistant varieties to be very susceptible if fungicides applications have not been maintained. As a result, reapplication of fungicides after major rain events will be critical.

If you already actively control for apple scab, you will also control Marssonina blotch year to year since M. caronaria also overwinters in fallen leaves. Disease control is managed through orchard sanitation, pruning, and fungicides. Removal of overwintered leaves on the ground may reduce the inoculum level. Conventional fungicides commonly used for early and summer apple diseases easily keep this disease in check. Anecdotally, sulfur has provided limited control and may only suppress the disease. Consequently, organic growers may struggle the most with Marssonina blotch. Sanitation and dormant copper sprays will be important in these orchards.