Blister Spot

Table of Contents

1. Introduction
2. Description
3. Biology
4. Damage
5. Management

Introduction

Blister spot, Pseudomonas syringar pv. papulans, is a bacterial disease of apples, particularly the cultivar Mutsu (Crispin). Disease levels vary from orchard to orchard and year to year depending on the level of pathogen populations and local climatic conditions. Even though numerous cultivars are susceptible to blister spot when artificially inoculated, only a few cultivars have been observed to develop symptoms on the field. These include Golden Delicious, Red Delicious, Jonagold, Fuji, Cortland and Rome Beauty and only when located near Mutsu trees with high infection levels.

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Description

Disease symptoms are first observed as small lesions confined to the area around the lenticels on fruit in mid- to late July (Figure 1). The lesions continue to expand during the growing season. Near harvest they are 1-5mm in diameter, 1-2mm deep, and have blistered brown centres with a dark purple border (Figure 2). Some lesions are irregular in appearance, but most are round. They tend to be concentrated at the calyx end of the fruit or on the side exposed to sunlight. There may be a few to a hundred spots on a fruit. The lesions remain shallow and generally do not lead to fruit decay.

In southwestern Ontario, a mid vein necrosis of the leaves on tender shoots has been observed (Figure 3) particularly in nursery trees. Crusty brown lesions occur on the mid vein of the lower leaf surface, causing affected leaves to become curled and puckered (Figure 4). Elsewhere, blister spot has been found to cause spots on the leaves and cankers on the wood. However, these symptoms have not been observed in Ontario.

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Biology

Blister spot survives the winter on dormant buds of apple trees. Sometimes up to 60% of the buds in an orchard may harbour the pathogen. It is also known to overwinter in fruit lesions on dropped fruit on the orchard floor.

In the spring, when new foliage and blossoms first appear, the bacterium colonizes on leaves, flowers, and later fruitlets. Disease symptoms are often absent in spite of the presence of the bacterium. The bacteria multiply and spread by rain and insects to neighbouring foliage, trees and weeds. Other apple cultivars, as well as weeds such as leafy spurge, clover, dandelion, quackgrass and common mallow are known to support populations of blister spot bacteria on leaf surfaces.

Warm, humid or wet conditions during the spring and early summer favour a buildup of bacterial populations and subsequent infections of leaf veins and fruit.

Fruit infection occurs in the time period shortly after petal fall when the stomata on the fruit are susceptible to the bacterium. In Ontario this susceptible period occurs from two weeks after petal fall until late July. After this point, the bacterium can no longer penetrate lenticels, and infections are therefore less likely to occur.

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Damage

The disease causes superficial blemishes to the fruit, making them unsuitable for fresh market. Occasionally, Alternaria or other fungi may enter at blister spot infected lenticels resulting in decayed areas on the fruit.

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Management

Cultural practices will reduce the risk of blister spot. When planting a block of Mutsu trees, strive to obtain disease-free nursery stock and avoid planting near older Mutsu blocks where blister spot is present. Avoid the use of overhead irrigation during the period of fruit susceptibility.

A newly planted "clean" black of Mutsu trees will often remain disease-free for significant period of time. Blister spot usually arrives eventually, beginning with low levels of infections and increasing until 80% or more of the fruit show symptoms. The level of damage may decline in some years, due to weather conditions unfavourable for the disease. The development of blister spot cannot be eradicated, but can be managed to minimize fruit damage each year.

Recent research has shown that certain bactericides are effective at reducing the level of infection. Repeated sprays are needed, beginning shortly after petal fall.

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