Spot Anthracnoses

by ANNA E. JENKINS

SPOT ANTHRACNOSES form an important frontier in present-day studies of plant diseases. A decade ago about 30 of the maladies had been recognized, a tenfold increase from 1925, when identification of citrus scab, now called "sour orange scab," drew attention to others of this class of diseases. Collectively they can be referred to as spot anthracnoses, although individually they are frequently called anthracnose, or scab, of the particular plant or plant group affected.

The continued investigation of the diseases has been of an exploratory nature, with dependence upon historical research. Many colleagues in various parts of the world have collaborated in the work that led to the present recognition of approximately 175 different spot anthracnoses. One of them, A. A. Bitancourt, director of research in plant biology, Biological Institute, Sao Paulo, Brazil, has discovered more spot anthracnoses and isolated in pure culture more of their pathogens than any other investigator. Fully 100 of these fungi have now been described.

But regardless of where one lives, one helps to pay the bills caused by losses from spot anthracnoses. Moreover, one may discover a new spot anthracnose lurking somewhere, perhaps not yet spread about. The range of plants affected extends from the ferns to the composites, usually considered the highest of the flowering plant families. Young tissues of leaf, stem, or fruit are attacked, and then only under damp conditions. Effects may be exhibited in various ways: Spotting of susceptible foliage or fruits, including failure of the fruit to develop, or its early fall (grape anthracnose, sour orange scab, lima bean scab, and so forth), defoliation to a greater or less degree (snowberry anthracnose, and pecan anthracnose in nurseries, for example).

Fortunately, lesions are usually small. Unfortunately, they may be exceedingly numerous. A safeguard for many hosts is their inate tendency to heal by formation of callus. This accounts for the somewhat wartlike leaf spots, limited more or less definitely to one side of the blade, as in citrus scab. Rose anthracnose furnishes a good example of a simpler type of leaf spot (necrotic) with complete killing of tissue. Stem cankers on some woody growth may be comparatively harmless and inconspicuous. Among small fruits, however, grape canes are particularly vulnerable to serious attack. Those of brambles (raspberry, blackberry) also suffer greatly. A capital advantage for the well-being of most plants is that the spot anthracnoses are notably restricted each to its own suscept (any plant or animal capable of being infected by another organism) or related suscept group—that is, the parasites are specific. For instance, the pathogen of avocado scab will not go to citrus, nor will that of sour orange scab attack avocado.

Research since 1925 has revealed that citrus trees suffer not from the attack of one scab alone, but from three; moreover, one of them affects sweet orange, formerly thought to be practically immune from scab. Among these kinds of citrus scab, sour orange scab, Australian citrus scab, and sweet orange fruit scab, only sour orange scab is known in the United States. Different strains within pathogens of these diseases have been proved by experiment with regard to culture characteristic and, in one instance, by R. E. D. Baker, of Trinidad, British West Indies, with regard to host selectivity.

Despite the ability of plants to ward off spot anthracnose attacks, we have only to read accounts of these diseases to realize that almost invariably they gain a place among the most important maladies of the crop concerned. For example, investigating sour orange scab in Florida, J. R. Winston of the Department reported in 1923 that citrus scab (sour orange scab) was "largely responsible. for the failure of the lemon industry in Florida, which prior to the introduction of citrus scab gave promise of becoming a profitable undertaking."

For nearly seven decades plant pathologists in this country have recounted more or less extreme losses from bramble anthracnose. The long-accepted basis for assuming that this disease is of European origin is erroneous, as C. L. Shear and I have recently shown. The earliest records of grape anthracnose, however, are clearly traceable to that continent and almost certainly to early Greece and Rome.

Spot anthracnoses are caused by fungi. These micro-organisms in the conidial stage are of the so-called form genus Sphaceloma A. In their ascus or perfect stage, where this is known, these pathogens belong to the genus Elsinoi of the Myriangiales.

In 1943 Dr. Bitancourt and I reported mango scab as a disease new to science. The known distribution of this disease is Florida, Cuba, Puerto Rico, and the Canal Zone. The 'pathogen we have now described as Elsinoj mangiferae. A similar, although apparently not identical, disease has been discovered on mango in Brazil ( the States of Amazonas and Sao Paulo). It appears that one hindrance to the earlier recognition of the scab was that it was not distinguished from the colletotrichum disease of mango. On the fruit, for example, the Elsinoi is superficial in its attack, producing small scabs or larger russeted areas. Instead of causing a more or less deep fruit decay, as is common, colletotrichum may produce superficial blemishes also in the form of russeting. Mango scab is said to be of minor importance in commercial groves in Florida, but is a serious disease in mango nurseries.

Stem and foliage scab of sweetpotato discovered in Formosa and the Amani Islands by Japanese plant pathologists has now been found in Guam and Brazil, as A. P. Viegas and I have reported.

Cinchona scab owes its discovery to United States Government botanists who were searching in Central and South America for cinchona, a strategic war material. It was particularly the curved distorted capsules that brought the disease to F. R. Fosberg's attention in Colombia in February 1943. On a subsequent exploratory excursion in November 1943 he was accompanied by W. H. Davis, plant pathologist. It was on the basis of Davis' specimens collected on that occasion that I made the original identification of what we now know as scab of cinchona. The published account describes in some detail the small raised, brown spots characterizing the disease, which are often produced in untold numbers on young leaves, stems, and capsules. In its present known distribution the disease affects Cinchona pubescens and C. officinalis in Colombia and C. delessertiana in Peru. A much wider distribution is certain to be demonstrated, particularly because of the geographical range of cinchona.

Under intensive culture in the Orient, scab, if present, should have been detected and reported long ago; possibly the pathogen (Elsinoe cinchonas) has not yet been carried there. The economic threat of such a disease-producing organism is difficult to evaluate; only the future will reveal when and where it can become a serious menace. Certain it is that its capabilities are apparent under natural conditions in the native home of the quinine plant.

Besides gray scab of willow (S. murrayae Jenkins and Grodsinsky), named in 1943, known to occur on five species of Salix and found in Rhode Island, Virginia, New York, California, and Washington, I shall name another spot anthracnose recorded in this country since 1942. This one may be called pome fruit spot anthracnose, or anthracnose of apple and pear. In June 1946 M. J. Forsell, L. W. Boyle, and I reported the discovery of this disease, caused by Elsinoe piri, in western Washington and Oregon in 1943-45. There were numerous findings of the disease on apple fruit and leaves and on pear leaves, and one on quince, that at Olga, San Juan County, Wash. In all cases only the conidial stage of the fungus (Sphaceloma pirinum) was present. The fact that this exotic malady has been present for a greater or less length of time in our Pacific Northwest without previous detection points to the significance of adequate surveys, together with accurate identification.

THE AUTHOR

Anna E. Jenkins is a mycologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. Her contributions deal particularly with new or little-known plant diseases and their pathogens. Among these are rose diseases of various etiology, blights of North American maples caused by Taphrina, and spot anthracnoses of various plants of economic importance. Dr. Jenkins holds three degrees from Cornell University.