Blister spot is a bacterial disease of apple fruit caused by Pseudomonas syringae pv. papulans. This disease is of economic importance mainly on the cultivar ‘Mutsu’ (‘Crispin’) but can be seen on ‘Golden Delicious’ grown adjacent to ‘Mutsu’. Even though fruit grow to maturity and no detectable yield loss occurs, severe infection results in ugly fruit and greatly reduces fresh-market quality.
|Figure 1. Blister spot on ‘Mutsu’ fruit. Photo: T. van der Zwet, USDA.|
Blister spot lesions are first noticeable 2 to 3 months after petal fall as small, green, water-soaked, raised blisters that develop at fruit stomata (fig. 1). These spots result in purplish black lesions associated with fruit lenticels. As the fruit increase in size, the lesions expand to about 3/16 inch and become darkened. A mid-vein necrosis of ‘Mutsu’ apple leaves may occur prior to fruit lesion development (fig. 2).
The bacterium overwinters in a high percentage of apple buds, leaf scars, and diseased fruit on the orchard floor. Throughout the growing season, the bacterium can survive as an epiphyte on foliage and fruit in the orchard. Even though the highest populations of the pathogen have been found on ‘Mutsu’, the bacterium also has been detected on foliage and fruit of other apple cultivars. Young ‘Mutsu’ fruit show an increased susceptibility to infection for about 6 weeks, beginning about 2 weeks after petal fall.
|Figure 2. Blister spot infection on leaf mid-vein. Photo: T. van der Zwet, USDA.|
The disease is mainly a problem on the apple cultivar ‘Mutsu’. When ‘Mutsu’ is interplanted with other (normally resistant) apple cultivars (‘Red Delicious’, ‘Cortland’, and others), the pathogen may spread to these. Prior to the development of streptomycin-resistant strains of the pathogen, growers could control the disease with three well-timed antibiotic sprays, the first applied no later than 2 weeks after petal fall and the others applied weekly thereafter. Growers still use this strategy in orchards without resistant strains of the pathogen; however, resistant strains may develop after only a few years of antibiotic use. Once a strain develops resistance to the antibiotic, further use of antibiotic is ineffective.
Adapted by Alan R. Biggs, West Virginia University.
Original text prepared by T. van der Zwet, K. S. Yoder, and A. R. Biggs. The original version of this article appeared in The Mid-Atlantic Orchard Monitoring Guide (NRAES-75) and is reproduced with permission from the Natural Resource, Agriculture, and Engineering Service, Cooperative Extension, 152 Riley-Robb Hall, Ithaca, New York 14853-5701, U.S.A. (607) 255-7654.