Sooty blotch has been commonly characterized as a superficial disease: The fungus does not penetrate deeply into the host tissue. The superficial character of the disease cannot be correctly interpreted as being wholly outside the host cells, however.

Evidence of the cuticle-rupturing nature of the fungus is afforded through the manner in which the diseased spots lose moisture more rapidly than the surrounding tissue, and the spots become sunken when fruits are held for a few weeks after harvest. The entire fruit eventually shrivels when it is infected extensively.

Investigations I made in 1932 demonstrated the manner in which the sooty blotch fungus penetrates the cuticle of the host fruit and spreads through the epidermal cells immediately beneath the cuticle. Some types make only scattered penetrations of the cuticle; the main part of the mycelial mat is on the surface of the cuticle. The rimate types proliferated abundantly in the cuticular fissures created until the major part of the invading fungus was within or beneath the epidermal layer.

The sooty blotch fungus may be cultured and grows readily, if slowly, on various media. It is not very easy to isolate the fungus, though, because it is hard to free the thallus of contaminating organisms. Surface sterilization frequently destroys the fungus it is desired to isolate, particularly the fuliginous types. I have readily isolated the fungus by gently swabbing and wiping the diseased surface with sterile water and cotton-tipped swabs, cutting small pieces of diseased tissue out, and placing the tissue on the surface of a plain agar plate. The fungus soon produces a thin, pubescent layer of aerial mycelium. Bits of it are removed with very finely drawn glass needles and transferred to the surface of a plate of nutrient agar. The fungus produces a leathery and markedly heaped-up thallus in culture. The color varies from olive green to deep brown and gray. The submerged mycelium usually appears black. The colonies produce characteristic masses of spores, which at first are salmon pink but later become quite dark in mass. The spores bud freely, and it is difficult to find specimens of uniform size and form. Isolates vary in microscopic appearance, but there appears to be no correlation between these variations and thallus types encountered on the fruit.

SOOTY BLOTCH and fly speck never presented a difficult problem in the past. The fungicides commonly used for protection against more serious diseases were normally quite adequate against sooty blotch and fly speck. The diseases have tended to reappear, however, in well-cared-for orchards since the widespread shift from the inorganic insecticides and fungicides to the organic pesticides. The inorganic fungicides formerly used were principally copper salts and had a wide range of fungicidal effectiveness. The organic fungicides have a much more specific character.

Evidence and observation indicate that the reappearance of sooty blotch can be attributed partly to the lesser effectiveness of, or persistence of, the organic fungicides used on apples, and partly to a reduction in late-season spraying made possible because of the reduction in insect problems through the use of DDT and other organic insecticides. Growers have not tended to make summer applications of fungicides alone, spraying only where they considered it necessary also to combat insects.

The sooty blotch which has reappeared in well-sprayed orchards the past few seasons has been of the fuliginous type. Whether this indicates a greater resistance of this type to the ferbam and captan fungicides, or to an infection which occurs after the fungicide becomes ineffective and only the fuliginous type develops rapidly enough to become visible before harvest, has not been determined. Fly speck has also reappeared together with the fuliginous type of sooty blotch.

C. F. Taylor conducted extensive control trials on sooty blotch and fly speck. He tested many fungicides and made applications at intervals through the season. He found ferbam to be superior to bordeaux against fly speck but inferior against sooty blotch. Although ferbam applications in late May were the most effective, no application of ferbam was entirely satisfactory. Bordeaux mixture was most effective in mid-July. Sulfur was not a satisfactory fungicide in his tests. The use of sulfur is generally avoided in summer spraying, however, because of its tendency to cause sulfur sunscald when temperatures are high.

That fly speck usually appears earlier than sooty blotch suggests either an earlier time of infection or a more rapid rate of development following infection.

A. B. GROVES is a plant pathologist with the Virginia Agricultural Experiment Station. He has been stationed at the Winchester Fruit Research Laboratory since 1929. His field of specialization includes tree fruit diseases and the problems associated with their control.

Coryneum pustule on peach.