Blister spot

Excerpt from Publication 310, Integrated Pest Management for Apples,
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Table of Contents

1. Introduction
2. Symptoms
3. Biology
4. Monitoring and management

Introduction

Blister spot, caused by the bacterium Psuedomonas syringae pv. papulans, is probably present in most apple orchards. Many cultivars including Golden Delicious, Red Delicious, Jonagold and Cortland are susceptible to this disease but Mutsu (Crispin) is very susceptible. Other susceptible cultivars usually become infected only when planted adjacent to an infected block of the very susceptible Mutsu cultivar. Infections occur especially if frequent rains occur during the mid to late July when the fruit is at a susceptible growth stage.

Symptoms

Initially symptoms appear as small water-soaked raised blisters associated with the lenticils (fruit pores) on the fruit surface. The first spots are often detected near the calyx end of fruit growing towards the outside of the tree facing the sun (Figure 4-162). Symptoms often do not appear until mid to late July after which time the bacteria can no longer infect the fruit. The lesion eventually appears as a brown blister with a purplish black margin expanding to no more than 4-5 mm in diameter on the fruit surface (Figure 4-163). Lesions rarely penetrate the flesh to more than 1-2 mm deep. The lesions do not develop into fruit decay, but several to more than 100 blister spots can occur on a single fruit reducing the fresh market quality significantly.

Figure 4-162. Blister spots are often initially detected near the calyx end of fruit growing on the outside of the tree canopy facing the sun

Figure 4-163. Blister spot lesions eventually become purplish black with a tan centre expanding no more than 4-5 mm in diameter on the fruit surface

In southwestern Ontario, a midvein necrosis of infected leaves on tender shoots has been observed (Figure 4-164). Crusty brown lesions occur on the midvein of the lower leaf surface, causing affected leaves to become curled and puckered (Figure 4-165).

Figure 4-164. Midvein necrosis of leaves on tender shoots infected with the blister spot bacteria

Figure 4-165. Crusty brown lesions on the midvein and petiole of the lower surface of blister spot infected leaf

The disease causes superficial blemishes to the fruit, making them unsuitable for fresh market. Occasionally, other fungi enter the blister spot lesion resulting in decayed areas on the fruit.

Biology

The bacteria overwinter in apple buds, leaf scars and diseased fruit left on the orchard floor from the previous season. Although infected apple buds appear healthy, the bacteria multiply during the spring and are rain splashed to leaves and other plant surfaces throughout the orchard. The bacteria survive and multiply on leaf surfaces including weeds in the orchard, without causing any disease symptoms. Warm, humid or wet conditions during the spring and early summer favour a build up of bacterial populations and subsequent infections of leaf veins and fruit. During late spring or early summer, a brief shower is all that is required to distribute the bacteria onto the developing fruit where they infect through the fruit pores or lenticels. Young Mutsu fruit are most susceptible beginning two weeks after petal fall lasting for about six weeks (late July). After this point the lenticels on fruit are no longer susceptible to infection by the bacterium.

Monitoring and management

When planting a block of Mutsu trees obtain disease-free nursery stock and avoid planting near older Mutsu blocks where blister spot is present. The cultivar Shizuka has similar appearance and quality of fruit as Mutsu but is much less susceptible to blister spot and is an alternative cultivar to plant in orchards with a history of blister spot. Avoid the use of overhead irrigation during the period of fruit susceptibility. A newly planted "clean" block of Mutsu trees often remains disease-free for a significant period of time. Eventually the blister spot bacteria spreads into the orchard, initially causing low levels of infections but if left unmanaged, 80% or more of the fruit can become infected. The level of damage may decline in some years due to weather conditions unfavourable for bacteria spread and disease development. Once in the orchard, blister spot cannot be eradicated, but proper management minimizes fruit damage each year. Recent research shows certain products with bactericidal properties are effective at reducing the level of infection. For more information on products registered for blister spot control, consult OMAFRA Publication 360, Fruit Production Recommendations.