Apple Disease - Marssonina Blotch

Marssonina blotch (also known as apple blotch) is controlled easily with fungicides; however, excessive rainfall may cause lapses in control. Marssonina blotch is found on the leaves and fruit of the apple. The disease has also been noted on pear trees. The disease is widely distributed and has been reported in North America, Europe, and Asia. It was found in Pennsylvania in 2017. The disease can cause severe defoliation, which can start as early as August. 

Marssonina Blotch Symptoms on Apple Trees

Leaf spots first appear on the upper surface of mature leaves around the middle of June. They are 5 to 10 millimeters in diameter, grayish-brown to black, and tinged purple at the periphery. Small, black acervuli are often visible on the surface. When lesions are numerous, they coalesce, the surrounding tissue turns chlorotic, and defoliation results. Severe defoliation may start in early summer. Cultivars do not differ significantly in susceptibility to the disease. Fruit infection is uncommon and occurs only in trees with numerous leaf infections. Clear, brown spots appear on the surface of the fruit; they become oval, depressed, and dark brown with age; and are almost black at harvest. Fruit infections can be latent and appear during storage. The surface of the fruit is somewhat indented, and small, black acervuli are visible in the lesions.

Diplocarpon coronariae Disease Cycle

Primary infections are initiated by ascospores produced on overwintered leaves. Mature ascospores begin releasing around late bloom through petal fall, and will discharge for about three to four weeks. This is the primary infection period. Rain is required for spore release. Infection of leaves by conidia takes place most frequently at 68 to 77°F, with a minimum of four hours of leaf wetness. Primary symptoms can often appear in mid- to late-June, usually on mature leaves. Defoliation can begin as early as two weeks after the symptoms appear. If the disease becomes established during the primary infection period and is not effectively controlled, conidia will continue to spread throughout the trees throughout the season (secondary infection).

Marssonina Blotch Management

Disease control is managed through orchard sanitation, pruning, and fungicide applications. Removing overwintered leaves from the ground may reduce the inoculum level. Using urea in the fall to help leaf decomposition is encouraged. Chemical control should begin in earnest around late bloom/early petal fall and last until early to mid-June. Prior to rain events during this time, use FRAC Groups 3 and/or 7 fungicides, tank-mixing with a rainfast mancozeb or captan. These are the most effective fungicides. Do not use FRAC Group 11 fungicides during the early season, as this mode of action is ineffective for Marssonina blotch control. Save FRAC Group 11 fungicides for the cover sprays to control fruit rots. For organic producers, micronized sulfur (10-12 pounds per acre) is the most effective organic fungicide to date for suppressing the disease. Research is ongoing to find additional organic products for Marssonina blotch control.