Sooty Blotch and Fly Speck

A. B. Groves.

Sooty blotch and fly speck are the common names for two diseases of the apple that can mar the appearance of the fruit.

Sooty blotch appears as rootlike spots or blotches. Fly speck makes dark spots that look somewhat like fly specks. Although the two diseases are not caused by the same fungus (sooty blotch is caused by Gloeodes pomigena and fly speck by Leptothyrium pomi), they are so commonly found in association and develop freely under the same favorable climatic conditions that they are usually described together.

Sooty blotch is also widely known by the name cloud and less commonly as sooty smudge, sooty spot, and inky spot. It appears as dark-brown to olive-green spots or smudges on the surface of mature pomaceous fruits.

Fly speck appears as well-defined, slightly raised, small, dark spots on the surface. The specks commonly occur in groups, seldom singly. The two diseases cause little or no actual injury to the fruit, but by disfiguring it they lower its market value. They are most troublesome in orchards on low land or in other places where air drainage is poor and the humidity tends to remain high. Both diseases are easily controlled, but the possibility of their occurrence cannot be ignored if blemish-free fruit is to be produced.

Sooty blotch occurs throughout the humid fruit-producing regions of North America, and has been reported from Great Britain and France. Specimens I received from Australia in 1932 appeared identical with types I then was studying in Virginia. The disease has been observed on wild crab apples in the mountains of West Virginia. Probably the diseases occur more widely than has been reported, but have received less attention than more serious disorders.

The sooty blotch spot on an apple consists of a thin fungal crust, or thallus. Profusely branched and connecting threads of rather thick-walled cells, which are brown to olive green, make up the thallus. It varies from a somewhat uniform thin mycelial mat with indefinite margins to a type with clearly defined margins and heavy aggregations of cells, known as plectenchymal bodies, scattered through it. The plectenchymal bodies may occasionally develop into pycnidia, although they usually fail to develop spores.

The fly speck thallus is dark brown to olive green and is sharply defined. The individual specks are much smaller than an entire sooty blotch spot, the fly speck thallus usually being about 250 to 300 microns in diameter. This is much larger than the individual plectenchymal bodies in a sooty blotch thallus, however. The individual fly speck spots show no connection one with another which is visible to the unaided eye. Microscopic examination reveals the individual specks of one group or colony to be connected by hyaline mycelial strands, however.

A careful examination of numerous sooty blotch specimens reveal differences in the gross morphological appearance of the spots. The variations may be grouped into a few characteristic types. After studying specimens from widely separated sections of the country, I classified them into four groups.

The most common is the ramose, or branched, type. Strong radial or fern-like growth is a common characteristic. Plectenchymal bodies vary from sparse to abundant and tend to be irregular in shape.

Spots of the rimate group, the second most abundant, have roughened cuticle in the infected area. The fungus penetrates the cuticle and grows most abundantly along the minute fissures produced. The heavy aggregations of fungal cells along the fissures are conspicuous enough to be readily detected through a hand lens.

The punctate, the third type, is typified by conspicuous darker dots or specks throughout the spot. The dots, plectenchymal bodies, although quite conspicuous, are smaller than those of fly speck, and the interconnecting mycelium is plainly visible.

The fourth and least conspicuous type is the fuliginous, or smoky smudge, type. The spots may be rather well defined or with indefinite margins, covering large areas of the fruit. Plectenchymal bodies are characteristically absent under normal conditions although they may form on fruits placed in a moist chamber. The fungal mat is thin and lacks conspicuous growth characters. The fuliginous types may be removed from the surface of the fruit with light scraping, but cannot be wiped off.

Sooty blotch and fly speck occur commonly and are of consequence only on the pomaceous fruits. The only extensive investigation of the natural host range of the sooty blotch fungus was made in 1930 and 1931 by R. C. Baines and M. W. Gardner, of Purdue University. They reproduced the disease on apple fruits through the use of pure cultures from 10 woody plant hosts: Hard, or sugar maple, Acer saccharum; the pawpaw, Asimina triloba; downy hawthorn, Crataegus mouis, leatherwood, Dirca palustris; white ash, Fraxinus americana, spicebush, Lindera benzoin; sycamore, Platanus occidentalis; slippery elm, Ulmusfulva; willow, Salix nigra; bristly greenbriar, Smilax hispida., bladdernut, Staphylea trifolia; and prickly-ash, Zanthoxylum americanum. The cultures were obtained from the fungus found on the young twigs.

Inoculum presumably comes from infected twigs. The greater abundance of spots on the upper side of the fruit and over areas close to the twigs indicates that the twigs are a source of inoculum, rather than fallen fruits from the previous season.

The sooty blotch fungus forms pycnidia abundantly. Conidia are produced less freely. I have found only a few conidia in examining many apparently mature pycnidia on the surface of apple fruits. A. S. Colby, of the University of Illinois, in 1919 reported the conidia to be almost hyaline, one-celled, of varying shape, and measuring 10-20 by 4-7 microns. He also found infection to be spread by single-celled mycelial fragments or chlamydospores.

Baines and Gardner reported spore production within the pycnidia on woody hosts other than apple. They found spore production to be abundant, the spores maturing and discharging in the late spring. The mature spores they found were bicellular.

C. F. Taylor and I. G. Bennett, of the University of West Virginia, made fruit-bagging experiments to determine the time of infection on unsprayed apple trees. They found relatively little fly speck before early July, although infection increased rapidly afterwards. All fruit that was unprotected before mid-July became heavily infected. Little infection occurred on fruits that were exposed only after mid-August perhaps, therefore, the normal infection period for fly speck lies between early June and mid-August.