As harvest operations ramp up, fungicide applications often drop off, leaving orchards unprotected. Late‑season rains wash off residual fungicide, providing ideal conditions for apple blotch to take hold, resulting in rapid foliar infection and defoliation.

Symptoms

• Lesions: Circular black or brown spots on mature leaves, progressing to yellowing and leaf drop.
• Fruit: Only under very high disease pressure do small black circular spots appear.

Disease Cycle & Environment

• The pathogen overwinters in fallen leaves and releases spores in spring during rain and high humidity; optimal infection temperatures range between 68–77 °F.
• Infections may initiate in summer, and defoliation can follow ~2 weeks post-infection.
• Frequent spore dispersal opportunities may arise mid- to late‑season in warm, wet conditions.

Management Strategies

• Utilize fungicides in FRAC Groups 3, 7, or 9 during petal fall for early-season control.
  • Cevya (FRAC Group 3), is the first fungicide in the United States specifically labeled for controlling apple blotch disease.
  • FRAC Group 11 fungicides are less effective against apple blotch and should be saved for summer fruit rot management.
• Captan and fungicides in FRAC Groups 3, 7, and 9 provide very good control; consider extending spray programs later into the season when warm, wet weather persists.
• Organic fungicide options are limited with sulfur showing inconsistent results in field trials.
• Continue flail mowing or urea treatment of leaf litter to reduce overwintering inoculum.
• Improve canopy air flow through pruning and reduce moisture-holding conditions.
• Consider dolomitic lime application in fall (organic-compatible) to accelerate litter breakdown.
• 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.

Key Takeaways:

• Harvest timing + heavy rains = increased risk of apple blotch disease due to washed-off fungicides and spray gaps.
• Effective control involves both continued fungicide protection into harvest and robust sanitation/cultural practices.
• Captan and rotation with FRAC 3, 7, and 9 fungicides remain central tools.

Resources:

• https://ext.vt.edu/content/pubs_ext_vt_edu/en/SPES/spes-509/spes-509.html?utm_source=chatgpt.com
• https://extension.psu.edu/apple-disease-marssonina-blotch

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