Reports on Plant Diseases

Sooty blotch and flyspeck are two of the most common diseases of apple that often occur on fruit at the same time. Sooty blotch is a disease complex caused by the fungi Peltaster fructicola, Geastrumia polystigmatis, and Leptodontium elatius. Flyspeck is caused by the fungus Zygophiala jamaicensis. These diseases are wide spread in the Midwest. Since both diseases are controlled in the same way, they are usually considered together.

Because the fungi causing sooty blotch and flyspeck grow superficially on the surface of the fruit, losses are primarily through lowered fruit quality. Golden Delicious and Grimes Golden apples are quite susceptible to these diseases although all fruit varieties become infected. Infections are much more obvious on yellow-skinned apple cultivars.

The sooty blotch fungi infect the leaves, twigs, and fruit of about 25 different plants including ash, blackberry, bladdernut, citrus, crabapple, elm, hawthorn, maple, pear, persimmon, prickly-ash, raspberry, sassafras, sumac, sycamore, and willow (Table 1). The flyspeck fungus also infects a variety of plants including banana, blackberry, carnation, citrus, grape, Japanese persimmon, pear, plum, quince, raspberry, and sumac (Table 2). These diseases are very common during wet seasons.

Figure 1. Sooty Blotch on apple fruit.

Figure 2. Close-up of flyspeck on apple fruit.

Symptoms

SOOTY BLOTCH: Sooty blotch appears as sooty or cloudy blotches on the surface of the fruit. The blotches are olive green with an indefinite outline (Figure 1). The blotches are frequently a fourth of an inch in diameter or larger, and may coalesce to cover much of the fruit. The "smudge" appearance results from the presence of hundreds of minute, dark pycnidia that are interconnected by a mass of loose, interwoven dark hyphae. The sooty blotch fungus is generally restricted to the outer surface of the cuticle and can be removed by vigorous rubbing or bleaching. In rare cases, the hyphae penetrate between the epidermal cell walls and the cuticle.

FLYSPECK: Groups of a few to 50 or more slightly raised, black and shiny round dots that resemble fly excreta, appear on the apple fruit (Figure 2). The individual "fly specks" are more widely scattered and much larger than the pycnidia of the sooty blotch fungus. The flyspecks are sexual fruiting bodies (pseudothecia) of the fungus, and are interconnected by very fine hyphae. The blemishes can be removed by vigorous rubbing or bleaching.

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Disease Cycle

Both fungi overwinter on the twigs of many woody plants (Tables 1 & 2). The disease cycles, as well as the temperature and moisture conditions necessary for infection, are much the same for both diseases.

SOOTY BLOTCH: The pycnidia on host plants produce large numbers of spores (conidia) that ooze out of infections and collect in a gelatinous mass. The conidia are carried by air currents and windblown rain through orchards from late May or early June until autumn. The thick-walled, dark hyphae formed on apple and other twigs often break up into cell-like fragments. These fragments may be washed from twigs onto the developing fruit, where they initiate infections. After spread and enlargement, secondary infections occur on fruits. Current-season twig growth is also infected during the summer and early autumn.

The fungi grow in a wide range of temperatures, 64 to 80 F (18 to27 C). Humid weather is essential for infection and disease development. When May and June are cool and moist and are followed by a hot July and August, sooty blotch often does not appear on the fruit until close to harvest. Disease outbreaks are most severe when cool, rainy weather in the spring is coupled with late-summer rains and cool fall temperatures prior to harvest. Under ideal conditions, the incubation period from infection to the appearance of symptoms may be as short as 5 days. In the orchard, however, this period usually lasts 3 to 4 weeks on fruits that are 42 to 45 days old.

FLYSPECK: In late spring, this fungus produces both ascospores and conidia that are wind-borne into orchards from other plants. The incubation period in cool weather (65 F or 18 C) is about 15 days.