I. Introduction: This bacterial
disease is of economic importance mainly on the cultivar Mutsu (Crispin) but can be seen
on Golden Delicious when 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.
II. Symptoms: Infections of blister spot are first
noticeable two to three months after petal fall as small, green, water-soaked, raised
blisters that develop at fruit stomata (photo 2-39). 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 (5 mm) and become darkened. A mid-vein necrosis of Mutsu apple leaves
has been observed prior to fruit lesion development (photo 2-40).
III. Disease Cycle: 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 has also been detected on foliage and fruit of other apple
cultivars. Young Mutsu fruit show an increased susceptibility to infection for about six
weeks, beginning about two weeks after petal fall.
IV. Monitoring: Earliest spots (photo 2-39) can be
detected near the calyx end of the fruit that face the sun and are on the periphery of the
tree, beginning about mid- to late June in southern fruit-growing areas and mid- to late
July in northern areas.
V. Management: The disease is mainly a problem on the apple cultivar Mutsu.
When Mutsu is interplanted with other (normally) resistant apple
cultivars (i.e. Red Delicious, Cortland, and others), the pathogen may
spread into these, also. Prior to the development of
streptomycin-resistant strains of the pathogen, the disease could be
controlled with three well-timed antibiotic sprays, the first applied
no later than 2 weeks after after petal fall, and the others applied
weekly thereafter. This strategy is still employed in orchards
without resistant strains; however, resistant strains may develop
after only a few years of antibiotic use. Once resistance to the
antibiotic develops, further use of antibiotic is ineffective.
Text prepared by T. van der Zwet, K. S.
Yoder, and A. R. Biggs
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