Scab or Black Spot on Peach

John C. Dunegan.

Peach scab is also called black spot or freckles; apt names because the black spots on a badly infected peach do make it look freckled.

Cladosporium carpophilum, the fungus that causes the disease, occurs throughout the world on peach twigs, leaves, and fruit. In the United States and probably elsewhere fruit trees grown in dry sections are rarely affected, but in more humid sections the fungus is so persistent that growers must undertake control measures every year to protect the fruit. The fungus occasionally attacks plums and cherries but is of little importance on them.

The disease appears on the fruit as small, greenish, circular spots, one-sixteenth to one-eighth inch across, which become visible about the time the fruit is half-grown. The spots usually are most numerous near the stem. At times they cause an excessive dropping of the fruit by killing the stem tissues. The spots slowly increase in size, turning olive green to black and velvety as the result of the mass of dark spores that form on the surface. If the spots are especially numerous, they may coalesce into a crustlike covering over most of the fruit, which interferes with the normal growth of the peach as it matures. Badly infected fruit may crack open to the pit and be destroyed by the brown rot fungus.

On twigs of the current season's growth, the fungus produces many small oval, brown lesions or cankers that may retard the growth of the twigs. Spores produced in the twig lesions the following spring start anew the cycle of fruit, leaf, and twig infections.

Toward the end of the growing season the scab fungus occasionally develops on the lower surface of the leaves as indefinite pale-green or brownish patches scattered irregularly over the leaf blade, midrib, and stem. The leaf infections are of little economic importance because the area involved is not extensive and only negligible defoliation occurs. The infections give the plant pathologist an easy way to get in pure culture an organism that ordinarily is hard to isolate free from contaminations.

The life cycle of the peach scab fungus is simple enough. It consists merely of a series of superficial spots alternating between the twigs (where the fungus overwinters) and the fruit, twigs, and leaves produced the following season. The process is an endless cycle; the fungus always is present on some part of the peach tree. Its very ubiquity would seem to make it difficult to control, and yet, in truth, it is one of the easiest. So readily can the fungus be controlled that the presence of scab spots on peach fruit is an indication of improper spraying procedures. This is one of the diseases where research has shown the way to uniformly satisfactory control year after year.

The secret of the control of the peach scab fungus lies in the proper timing of the spray application. When bordeaux mixture was the only spray material available, few growers made any effort to control the scab fungus compared to the injury that generally followed the use of this copper spray, the scab disease was the lesser of two evils. The demonstration in 1907 that a mixture of sulfur and lime could be used safely on peach trees made the control of the scab fungus practical. The demonstration in 1917 by G. W. Keitt that 40 to 60 days elapse from the time the spores first infect the peach until the Spots become visible furnished the final clue to the problem. Keitt showed that the fungicide must be applied within 3 to 4 weeks after the petals have dropped if the fruit is to remain scab free. Since the peaches are then very small and show no evidence of scab infections, it took some time to convince peach growers that the control application must be made early in the season. For years now a single application of sulfur as a spray or dust 3 to 4 weeks after the petals have dropped has been the standard procedure. Sulfur is also used in later sprays to protect the fruit from infections of brown rot. The sprays have little effect on scab infection on the fruit, except on late-maturing varieties, but they do prevent some twig lesions as a comparison of sprayed and unsprayed trees shows.

I have to admit that that control program merely prevents infection on the fruit. It does not eradicate the fungus the new shoots and branchesare infected by the end of the growing season and next year's fruit crop must likewise be protected by sprays. As the standard fungicides applied to peaches for scab and brown rot control do reduce the number of twig lesions, a grower could perhaps eliminate the lesions by additional sprays after the fruit is harvested. But that would not be economical, as it is much less expensive to apply one spray early each season to protect the fruit from scab infections than it would be to apply a series of postharvest sprays to prevent overwintering lesions from developing on the twigs.

THE PEACH SCAB FUNGUS is almost unique in the problem it presents: An ever-present fungus is so efficiently controlled by one application of a sulfur fungicide that it is uneconomical to make any attempts to eradicate the fungus.

JOHN C. DUNEGAN, principal pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, is in charge of investigations of the control of diseases of deciduous fruit trees. He has studied fruit-disease problems in the United States since 1921.