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Blister Spot on Mutsu (Crispin) Apples

Apple growers know the cultivar Mutsu (or Crispin) is highly susceptible to the disease Blister Spot. The disease is caused by a bacterium Pseudomonas syringae pv.papulans that is present in most orchards. Other cultivars like Golden Delicious, Red Delicious, Jonagold and Cortland are also susceptible to this disease, especially when grown near infected Mutsu trees.

The bacteria overwinter in buds, leaf scars and diseased fruit on the orchard floor from the previous season. Infected buds may appear healthy, but the bacteria multiply during the spring and are rain splashed throughout the orchard. Bacteria can survive and multiply on leaves, wood and weeds in the orchard without causing any disease symptoms. During late spring or early summer, a brief shower can splash bacteria onto the fruit to infect through the fruit pores or lenticels. Fruit are most susceptible from 2-6 weeks after petal fall. Symptoms may not appear until 2-3 months later, but at this time, bacteria can no longer infect the fruit. Initial infections appear as small water-soaked raised blisters associated with the lenticels. The first spots are often near the calyx of fruit from the sun-facing side of the tree. Lesions become purplish black, only expanding to 4-5 mm in diameter, and rarely penetrate the flesh (Figure 1). Lesions do not develop into fruit decay, but several (to 100+) blister spots can occur on a single fruit reducing the quality significantly.

Figure 1. Blister spot symptoms on a Mutsu (Crispin) apple.

Two products are registered to control Blister spot in Ontario, Aliette WDG and Copper 53W.
Aliette WDG is applied at petal fall, followed by 2 more applications at 7-day intervals. No more than 3 applications of Aliette are allowed per season. Research in New York and Ohio during the mid 1990's indicate that Aliette will suppress the severity of Blister spot infections under heavy disease pressure. Earlier applications may provide better control; however, the economics of 3 applications may not be justified if only suppression results. Early application at petal fall appears to be the most important.
Copper 53W should be applied not earlier than 10 days after petal fall to reduce injury. Hydrated lime should also be mixed with the copper as a safener. Do not mix Aliette with copper.

For more information consult OMAFRA Fruit Production Recommendations.

Learn more

• OMAFRA Fruit Production Recommendations, Publication 360

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