During this year’s harvest season, I’ve received several phone calls and e-mails about trees showing premature defoliation and foliar disease symptoms. Upon further discussion with growers and scouts, the symptoms are consistent with an emerging disease concern in apples, apple blotch disease, formerly referred to as Marssonina leaf blotch. Apple blotch disease is caused by a fungal pathogen (Diplocarpon coronariae), which can infect leaves, fruits, and twigs and cause significant defoliation. The disease most commonly appears in orchards that have received low or no fungicide applications. One of the first reports of apple blotch disease was in Wisconsin in 1903, so the disease is not new to the state, but it has not typically been considered a significant disease of apple.

Why are we seeing apple blotch disease show up later in the season?

I suspect that fungicide applications ceased as harvest operations ramped up. This likely resulted in an unprotected period where the fungus could infect leaves (and possibly fruits) and cause rapid defoliation. Further complicating this scenario were the late season rain events that many saw in August and September. Any residual fungicides would have been washed off by the rain and the lack of additional fungicide sprays during harvest may have made conditions ideal for the pathogen to spread and infect.

Symptoms

Most symptoms of this disease will present on foliage. Foliar symptoms begin on mature leaves as circular black or brown spots (Figure 1), the fungus forms spores inside these leaf lesions. As these lesions expand leaves quickly turn yellow to brown (Figure 2) and defoliation occurs (Figure 3). Defoliation can reduce tree vigor, fruit quality, and yield. Foliar symptoms following infection seem to occur more readily on mature leaves, but young leaves can also be symptomatic. Disease symptoms on apple fruit are not common and likely only appear when disease pressure is very high; fruit infections show up as small, black circular lesions on the fruit (Figure 4).

Disease cycle and favorable environmental conditions

The fungus survives and overwinters in fallen leaves, and in the spring spores of the fungus release during rain events or in high humidity. Optimal temperatures for the fungus range from 68 to 77°F. Fungal spores land on leaves and lesions may take over 1 week to develop; mature leaves tend to develop symptoms first. Leaves eventually turn yellow, and defoliation occurs about 2 weeks after infection. Secondary infections can occur when spores are produced on newly formed lesions. Symptoms of the disease can present early in the season (June and July), but defoliation later in the summer and into fall, is also common when conditions are warm and wet. The duration of spore dispersal is not well understood and likely depends on climatic factors and may vary by region. Recent studies in Pennsylvania apple orchards suggest that spore dispersal was high in July and August. Wisconsin’s climate is different than Pennsylvania’s so keep in mind that peak spore dispersal events may vary in the upper Midwest.

Management considerations for apple blotch disease

• The pathogen can be managed with the conventional fungicides that are typically applied for early and late-season diseases. Fungicides can wash off during rain events, therefore fungicide reapplications may be necessary to prevent apple blotch disease. Consider the extension of fungicide programs if there are conducive conditions (rain and warm weather) later in the season.
• Fungicides in FRAC groups 3, 7, and 11 provide good control of the disease. Additionally, captan provides very good control of apple blotch disease.
• Sanitation – if you currently use flail mowing or urea to enhance leaf litter breakdown on the orchard floor for apple scab control, this practice will also be beneficial for apple blotch disease management as this pathogen also overwinters in leaves.
• Neighboring Malus species could be additional sources of inoculum, especially those receiving low or no fungicide inputs.
• Empire, Rome, and Honeycrisp varieties appear to be more susceptibility to apple blotch disease. Some scab-resistant varieties are also susceptible to apple blotch disease.
• Fungicides for apple scab control often work for the apple blotch fungus, but if you do not make apple scab fungicide sprays because you use scab-resistant cultivars there could be a gap in your fungicide spray program.
• Since the interval and duration of spore dispersal is not entirely understood, fungicide recommendations are still being refined. The pathogen can produce both primary and secondary spores suggesting that control may be needed for both types of inoculum if primary infections are not controlled early in the season.

*Mention of a fungicide product is not an endorsement.

References:

• Penn State Extension Publications
• Khodadadi, F., Martin, P.L. et al. 2022. Characterizations of an Emerging Disease: Apple Blotch Caused by Diplocarpon coronariae (syn. Marssonina coronaria) in the Mid-Atlantic United States. Plant Disease 106:1803-1817.

This article was posted in Apples, Disease and tagged Apple Blotch Disease, Apples, disease, Leslie Holland.