Morphological Characters of Alternaria Mali Roberts1

MORPHOLOGICAL CHARACTERS OF ALTERNARIA MALI ROBERTS1 By JOHN 'W. ROBERTS Pathologist, Fruit-Disease Investigations, Bureau of Plant Industry, United States Department of Agriculture Some years ago Alternaría mali was described by the writer (16)2 and shown to be involved in a peculiar spotting of apple leaves. Alternaría malí does not originate these spots, but, gaining entrance through areas killed by Physalospora cydoníae (Sphaeropsis malorum), by chemicals such as sprays, or by other means, enlarges the dead spots, giving them a peculiar and characteristic appearance. A spot thus formed consists of the original dead area with semicircular or crescent-shaped enlargements about it (PI. i, B). One might conceive of such a spot as being formed by infections taking place indiscriminately within the tissues of the original dead area. The fungus growing at equal pace in all direc- tions from each of these infection centers would occupy circular areas, only part of which would project beyond the original spot. Where there is a single infection, in the center of a circular spot, the enlargement appears as a differently colored zone surrounding the original spot and concentric with it. By means of new infections the increase in size may be such that one spot may involve almost an entire leaf, appearing as a more or less, circular (original) spot surrounded by consecutive crescent-shaped enlargements. The striations or zones within the spots greatly resemble those produced by species of Alternaría on the leaves of other hosts. Crabill (7) does not agree that Alternaría malí is the cause of the. enlargement of spots initiated by Physalospora cydoníae. His inoculation experiments were apparently very few in number and his results indecisive. Of the writer's work Crabill states: "His photographs, however, show that the enlargements produced by artificial inoculation with A. mali are not at all typical. They are too uniformly spreading to resemble the clear-cut crescents of the typical frog-eye spot." Yet these photographs show a remarkable resemblance to figure 4 of his bulletin, labeled " Frog-eye leaf spots as they appear in midsummer/' As a matter of fact, both Crabill's photographs and those of the writer would have shown the crescents more clearly cut had the leaves been allowed to become older and dry before photographing. The writer's experiments involved more than 3,000 spots, and the results in all experiments involving Alternaría malí were quite decisive. The fungus was found constantly associated with characteristic spots on apple leaves from Virginia, Maryland, Tennessee, Arkansas, and Missouri. THE GENUS ALTERNARÍA The difficulties underlying attempts to identify species of Alternaria are appreciated by all those who have ever undertaken the task. Many 1 Received for publication February 20, 1924. 9 Reference is made by numbei (italic) to ' ' Literature cited, " p. 707-708. Journal of Agricultural Research, Vol. XXVII, No 9 Washington, D. C. Mar. 1, 1924 Key No. G-376 (699) yoo Journal of Agricultural Research vol. xxvii. No. 9 of the species are described so vaguely or so briefly that almost any mem- ber of the genus could be placed within them. Many investigators, finding an Altemaria on a new host, have made a new species of it and published a brief description wholly inadequate for purposes of positive identification. Elliott (9) has done much toward the establishment of the generic limitations of Altemaria and has sorted out some of the species into groups. He states: "No final disposition of the present specific names of Altemaria and Macrosporium can be made without a study of authentic specimens of each species. Most of the descriptions are far from being complete or definite enough to permit their being used for this purpose.'' Untü such work as Elliott suggests is done, the identification of species of Altemaria must in most cases be quite uncertain. Elliott's emended description of the genus is such that A. mali un- doubtedly belongs within it. His description is as follows: "Altemaria Nées. Conidiophores solitary or fasciculate, erect or subdecumbent, simple or branched, generally short, colored. Conidia muriform, often with few longitudinal septa, ovate, obclavate, or elongate, always with more or less definitely pointed apex, often long-beaked, colored, under favorable conditions forming chains. (Ex. A. tennis, type of the genus.)" The writer has studied Alternaria mali as found growing in dead spots on apple leaves and as cultivated on artificial media. In addition, Alternaria tennis, from Europe, and species isolated from the leaves of lilac, forsythia, and blackberry and from the fruits of apple, cranberry, and blueberry have been grown on artificial media and studied. All these forms, while resembling one another in many respects, revealed many points of difference. In some species the hyphae were almost colorless, forming a thin scarcely perceptible crust over the surface of a plate of corn-meal agar with an exceedingly scant production of conidia and no aerial hyphae. Others formed a dense, nearly black crust with a copious production of conidia and no aerial hyphae. Still others produced a black crust with abundant conidial production at its surface and an abundance of more or less flocculent aerial mycelium or a greenish- gray aerial mycelium so dense as to form a thick carpetlike growth over the surface of the culture medium. A few produced an abundance of dirty white aerial hyphae with a very scant growth in the medium or along its surface and a very scant production of conidia. In the species studied the conidia conformed to the generic description, that is, they were ovate, obclavate, or elongate. In most of these obclavate conidia predominated, but ovate conidia were found in cul- tures of all except one form which produced elongate forms exclusively. In the species producing practically no aerial hyphae, elongate conidia predominated. While the surfaces of the spores were usually smooth, in some forms verrucosity was quite common. An entire conidium or only a part of it often possessed a verrucose surface. If only a part of the conidial wall was verrucose, it was in a strip or band at right angles to the long axis of the conidium. Verrucosity or absence of verrucosity was not so much a difference between conidia as between chains of conidia, the conidia of individual chains usually being all verrucose or all smooth-walled. The presence or absence of verrucosity appears to be due to highly localized conditions of the immediate environment, if one may judge from selection experiments reported upon later in this paper. The conidia of all the species were produced in chains with the narrower end distal. De Bary (1), Jones (11), Elliott (9), and many others found Mar. i, 1924 Morphological Characters of Alternaria mali 701 this to be true of the species studied by them, and that the conidia of all species of Altemaria are so arranged is generally believed. However, in examining published illustrations of Altemaria, even of quite recent date, one finds drawings in which the broader end of the conidium occupies the distal position. It would be interesting to know whether or not such forms really exist. At the distal end of each conidium, except quite frequently the last one formed in the chain, there is a stalklike projection, variously known as the isthmus, beak, or appendage. It is not so deeply colored as the conidium and may even be hyaline or nearly so. It may be quite short and swollen or long and slender, nonseptate, uniseptate, or even multi- septate (fig. 1). It is formed by a growth or a budding of the conidium at a time when it is the youngest of the chain, that is, when it occupies the distal position in the chain. From this outgrowth the next spore in the chain is developed. In descriptions of species of Alternaria the term conidiophore is usually applied only to the outgrowth from the conidia-bearing hypha which supports the first formed conidium of the chain. In this sense it is considered as the conidiophore for the whole chain. The so-called isthmus or beak may also be considered as a conidiophore, not in relation to the conidium from which it is an outgrowth and to which it remains attached, but in relation to the distally situated conidium which grew out from it. This view is supported by the fact that it becomes separated from this conidium as most conidiophores become separated from the conidia which they bear. In one form studied the so-called isthmi or beaks tended finally to collapse and disin- tegrate, which is also a fairly common behavior of conidiophores. A con- sideration of isthmi as conidiophores also offers an explanation as to why the end spores of the chains often have no isthmi or have especially long ones. In the former case budding has not started; in the latter the long, septate hyphalike structures which may become new conidia and conidiophores have been formed. The same sort of budding, but at right angles, makes the branches .of the chain. The isthmi or conidiophores often do not have as thick walls as the conidia. The tip often appears to be swollen and turgid, a condition which facilitates the breaking up of the chain through lessening the line of contact between the isthmus and the immediately distal conidium. When, however, a breaking up of the chain does not occur at this stage, as often happens when the chain lies along the surface of the culture medium, it is finally brought about by the shrinkage and disintegration of the isthmi. In this way, also, they perform a separative function.

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