Blister spot in Mutsu apples

It is that time of year that many apple growers begin to notice blister spot on Mutsu (or Crispin). The disease is caused by a bacterium Pseudomonas syringae pv. papulans and is probably present in most orchards. Research has shown that many cultivars including Golden Delicious, Red Delicious, Jonagold, Shizuka and Cortland are also susceptible to this disease but Mutsu is highly susceptible and the variety impacted the most. The other susceptible cultivars usually become infected in the rows planted beside a block of the highly susceptible Mutsu cultivar. However, this year there appears to be blister spot appearing in blocks of some susceptible varieties not planted near Mutsu blocks. The very wet weather conditions experienced this past June (2015) was ideal for the infection, spread and development of this disease in all susceptible varieties regardless of their proximity to the highly susceptible Mutsu block.

The bacteria overwinter in apple buds, leaf scars and diseased fruit that were left on the orchard floor from the previous season. Although infected apple buds may appear healthy the bacteria multiply during the spring and are rain splashed to leaves and other plant surfaces throughout the orchard. 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 fruit pores or lenticels. 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 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 (Figure 1). The lesions eventually become purplish black expanding no more than 4-5 mm in diameter on the fruit surface and rarely penetrates the flesh. Lesions do not develop into fruit decay; however, several to over 100 blister spots can occur on a single fruit reducing the fresh market quality significantly. In southwestern Ontario, a midvein necrosis of the leaves on tender shoots has been observed (Figure 2). Crusty brown lesions occur on the mid-vein of the lower leaf surface, causing affected leaves to become curled and puckered.

Figure 1. Blister spot often develop near the calyx end and sun facing surface of Mutsu (Crispin) apples.

Figure 1. Blister spot often develop near the calyx end and sun facing surface of Mutsu (Crispin) apples.

Figure 2. Crusty brown lesions occur on the mid-vein of the lower leaf surface and stem, causing the affected leaves to appear curled and puckered.

Figure 2. Crusty brown lesions occur on the mid-vein of the lower leaf surface and stem, causing the affected leaves to appear curled and puckered.

It is too late to control Blister spot once the spots appear. There are two products registered for control of Blister spot in Ontario, Aliette WDG and Copper 53W. Aliette WDG should have been applied at petal fall followed by 2 more applications at 7-day intervals. No more than 3 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. Copper 53W is also registered for Blister spot control and should have been applied 10 days after petal fall to avoid causing phyto-injury. Hydrated lime should always be mixed with the copper as a safener. Results from a trial conducted during the early 2000's indicated that applying Surround to suppress insect damage not only improved fruit finish and reduced heat stress but also appeared to suppress the severity of Blister spot on Mutsu fruit. For more information on the management of Blister spot consult OMAFRA Guide to Fruit Production, Publication 360.



For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca