I’ve seen several reports of blister spot in apple orchards this season, which aligns with the particularly wet conditions we’ve experienced. The excess moisture creates an environment conducive to the development and spread of the blister spot pathogen, making it more prevalent than in drier seasons. Blister spot may not always be top of mind for growers, as fire blight often takes precedence in bacterial disease management efforts. To address this, I am providing some information on blister spot in this article.

Symptoms

Blister spot symptoms appear as purplish-black lesions around the fruit lenticels, becoming most noticeable 2-3 months after petal fall. Initially, they manifest as small, green, water-soaked, raised blisters at the stomata on the fruit surface. As the lesions grow, they expand to about 4-5 mm in diameter and darken. Although these lesions are relatively superficial, only penetrating 1-2 mm into the fruit flesh, they can still be a concern for fresh market fruit, even though the fruit itself does not decay or rot from the lesions.

Pathogen and Disease Cycle

Blister spot is caused by the bacterium Pseudomonas syringae pv. papulans, which overwinters in apple buds, leaf scars, or infected fruit on the orchard floor. Infected buds may look healthy or display necrosis on leaf primordia (i.e., the tissues that form into the leaves). The bacterium can persist as an epiphyte (bacteria that live on plant surface without causing apparent harm), on apple leaves or fruit surfaces, as well as on weeds in the orchard. Wet weather conditions favor the disease, and it spreads through water. Traditionally, this disease has been thought to only be economically damaging on the variety ‘Mutsu’ (Crispin), however, other varieties are also susceptible to the disease.

Management

Since this is a bacterial disease, there are limited options for management, especially with the risk of antibiotic resistance. In the early 1980s, streptomycin was used in New York apple orchards to control blister spot until resistance led to control failures. Since many conventional orchards rely on streptomycin for fire blight management, alternative strategies may be necessary to address both diseases while reducing antibiotic resistance development risks. Applying streptomycin after bloom increases the chance of streptomycin resistance for the fire blight pathogen. Current recommendations for blister spot suggest applying bactericides 10-14 days after petal fall, with two additional applications at weekly intervals.

Looking ahead, with potential increases in blister spot reports during rainy seasons and growing concerns about fire blight in Wisconsin orchards, it’s crucial to explore effective management strategies for both diseases with a limited set of tools while minimizing antibiotic resistance risks. This may involve using alternative antibiotics to streptomycin during bloom for fire blight management in orchards also affected by blister spot. Early-season applications of fixed copper (around the green tip stage) have also been shown to reduce blister spot severity. Additionally, phosphite fungicides have been reported as effective in controlling blister spot.

If you suspect you may be dealing with Blister Spot, please submit a sample to the UW Plant Disease Diagnostic Clinic – https://pddc.wisc.edu/sample-collection-and-submission-2/

References

• Compendium of Apple and Pear Disease and Pests, 2^nd Edition. https://doi.org/10.1094/9780890544334

This article was posted in Apples, Disease and tagged Apples, Blister Spot, disease, Leslie Holland, Wisconsin Orchard Disease Update.