Preliminary Studies on Pleosphaerulina briosiana

Abstract

While studying parasitic fungi causing spots on alfalfa and related plants in the spring of I922, another very characteristic leaf spot was found, which, under some conditions of temperature and humidity, assumed enormous proportions. At first it was with difficulty that this spot was distinguished from those caused by Pseudopeziza med-icag-in-is and Pyrenopeziza med'icagcginis, which were in abundance at that time. However, it was soon seen that there were distinct differences which could be recognized with some study. The Pseudopeziza spot is finally irregular in outline, very dark, and always contains one or two raised apothecia. The Pyrenopeziza leaf spot is a less sharply defined yellow blotch with numerous pycnidia usually following the venation of the leaf. These spots are more fully described by Jones (5, 6). The new spots tend to be circular with a definite border, always much darker than the light-gray or light-brown interior, I to 2 mm. in diameter, and may be few and scattered or so many as practically to cover a leaflet. Under most conditions in the field no fruit bodies are found in them. When the perithecia have been found in the field, they were usually on leaves far advanced in infection and irregularly scattered in larger dead areas containing several of the typical spots. These spots are illustrated on Plate XXI, figures I and 2. Later, upon recognizing the cause of this leaf spot of alfalfa to be an ascomycete, producing large.muriform spores in perithecia, it was identified by the writer as Pleosphaerulina briosicana Poll. This determination was corroborated by F. R. Jones and J. M. Reade. In searching through the journals very little was found in regard to this fungus in America, and so this investigation was begun in the hope of throwing some light on its development and relationship to its host; particularly to find out just how and under what conditions infection takes place, the extent of its parasitism, and the manner in which this fungus spreads to new fields. This work has been done in the mycological laboratory of the University of Georgia under the direction of Dr. J. M. Reade, for whose helpful guidance I wish to express my gratitude.