Black Rot

Publication 310: Integrated Pest Management for Ontario Apple Orchards

Table of Contents

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

Introduction

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Description

Black rot may appear in apple orchards as several different forms. Leaf infections result in a disease called frog-eye leaf spot. The round lesions begin as small purple spots. As they enlarge they are characterized by light tan centres surrounded by a darker area and a purplish perimeter (Figure 1). Heavy infection causes leaves to eventually yellow and abscise. Sometimes this results in premature senescence of affected trees, predisposing them to winter injury.

Figure 1. Frog-eye leaf spot on leaves.

A second symptom of the formation of trunk or limb cankers. Affected areas are a reddish brown colour and slightly sunken (Figure 2). As the canker ages, the wood becomes shrunken and layers of bark peel back (Figure 3). The cankers can eventually cause whole limbs to die back.

A third symptom is fruit infections. After petal fall, small purple-black flecks appear on the fruit (Figure 4). These gradually enlarge into larger brown, dead but firm areas, often typified by the formation of concentric rings (Figure 5). Late in the growing season and in storage, black pycnidia (fruiting bodies) are often visible in the concentric circles (Figure 6).

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Biology

Black rot overwinters in cankers on twigs, branches, and trunks, and on mummified fruit (Figure 7). Cultivars characterized by mummified fruit, such as Cortland and Northern Spy, and where mummified fruit is present due to chemical thinning programs or fireblight, are particularly susceptible. Ascospore release begins early in the growing season, as buds begin to swell and open, and continues throughout the spring and summer. The heaviest ascospore release occurs for a four- to six- week period following petal fall.

The optimum temperature for leaf infections to occur is 26°C accompanied by 4.5 and 13 hours of leaf wetting, and fruit infections between 20°C and 24°C with nine hours of wetting. Risk of infections is low when temperatures are below 10°C (more than 24 hours of leaf wetness required), and none occur below 8°C. Leaf and fruit infections can occur simultaneously in the spring.

Figure 7. Fruit mummy - overwintering site for black rot spores.

Secondary infections, form conidia released from pycnidia, can occur throughout the summer. Whereas ascospores are mainly wind borne, conidia are most often spread by splashing droplets of rain. For this reason, summer infections are often characterized by lesions forming on growing fruitlets where near or in contact with mummified fruit.

Black rot is considered only moderately aggressive in regards to attacking healthy, unstressed trees. Cankers on twigs, branches and trunk most often occur when black rot spores contact wounds, such as from pruning cuts made after trees have broken dormancy, mechanical wounds (machinery), or physical wounds (hail). Severely cankered limbs can be weakened to the point that they may break under heavy fruit loads. Cankers eventually result in dieback of affected limbs. Young trees, affected at the trunk, can be killed while older trees similarly infected may continue to survive for a number of year.

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Management

Removal and destruction of affected wood is very important to limit the spread of black rot in the orchard. Fire blighted limbs, a target for colonization of black rot spores, should also be removed by dormant pruning.

Woodpiles of old apple wood must not be stacked in or near orchards since they can be a major source of disease inoculum. An attempt should be made to scout surrounding wood lots and identify hardwood trees infected with the disease. If possible these trees should be removed to decrease potential disease pressure. Make sure to get the neighbour's permission to but down trees if affected trees are on an adjacent property. Although labour intensive, an effort should be made to remove fruit mummies, especially on younger orchard blocks where future production potential is large.

Although EBDC fungicides (i.e., Dithane, Manzate and Polyram) are though to have weak activity on black rot, a full rate protectant program applied in the spring for apple scab has been generally effective in limiting early season black rot infections in Ontario apple orchards. Most commonly the disease has increased in incidence in orchards where early season rates of EBDC fungicides have been reduced, when used in combination with the sterol-inhibiting fungicides, that have no activity against black rot.

Therefore, in orchards where black rot is a problem, particularly where fruit and wood canker infections are present, the use of Nova should be avoided. Instead a full-rate protectant program of a captan formulation should be used for several years. Captan has good activity against black rot, but caution must be used to avoid using captan within two weeks of a Superior oil or Agri-mek plus oil application. A combination of Benlate and captan, applied for powdery mildew control after petal fall, will also control black rot infections.

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