Blister Spot on Crispin (Mutsu) Apples

Most apple growers who grow the variety Crispin (or Mutsu) are concerned with the disease Blister Spot. The disease is caused by a bacterium Psuedomonas syringae pv. papulans and is probably present in most orchards. Research has shown that many cultivars including Golden Delicious, Red Delicious, Jonagold and Cortland are susceptible to this disease but Mutsu is highly susceptible and the variety impacted the most. The other susceptible cultivars usually become diseased in the rows planted beside a block of the highly susceptible Mutsu cultivar.

The bacteria overwinter in apple buds, leaf scars and diseased fruit from the previous season that are left on the orchard floor. Although infected apple buds may appear healthy, the bacteria multiply during the spring and are rain splashed to leaves and other plant surfaces throughout theorchard. The bacteria can survive and multiply on leaf surfaces including weeds in the orchard without causing any disease symptoms. During late spring or early summer, a brief shower is all that is required to distribute the bacteria onto the fruit where they infect through the pores or lenticels.

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

Young fruit are most susceptible beginning 2 weeks after petal fall lasting for about 6 weeks. Often symptoms do not appear until mid to late July or up to 2-3 months after petal fall at which time the bacteria can no longer infect the fruit. Initially the infection appears as small water soaked raised blisters associated with the lenticils on the fruit surface. The first spots are often detected near the calyx end of fruit that are produced towards the outside of the tree facing the sun. The lesion eventually becomes a purplish black lesion expanding no more than 4-5 mm in diameter on the fruit surface and rarely penetrates the flesh (Figure 1). The lesions do not develop into a fruit decay, however, several to over 100 blister spots can occur on a single fruit reducing the fresh market quality significantly.

There are two products registered for control of blister spot in Ontario, Aliette WDG and Copper 53W. Aliette WDG is applied at petal fall followed by two more applications at 7-day intervals. No more than three applications of Aliette are allowed per season. Results from a study conducted in New York and Ohio with Aliette during the mid 1990's indicate that this product will suppress the severity of Blister spot infections during years of heavy disease pressure but the economics of three applications for suppression may not be justified. It is possible that the timing of applications in the Ohio and New York studies was not early enough. Growers who choose to use Aliette for managing Blister spot in Mutsu apples are encouraged to apply the product at petal fall. Copper 53W is also registered for Blister spot control and should be applied not earlier than 10 days after petal fall to avoid causing injury. Hydrated lime should also be mixed with the copper as a safener. Do not mix Aliette with copper. Fore more information consult OMAFRA Fruit Production Recommendations, Publication 360.

Related Links

• OMAFRA Fruit Production Recommendations, Publication 360