STUDIES ON THE OVERWINTERING OF CERTAIN FUNGI PARASITIC AND SAPROPHYTIC ON FRUIT TREES

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Abstract
Evidence is submitted that some conidia of the black knot fungus, Dibotryon morbosum (Sch.) T. and S., are capable of overwintering in a viable condition on Prunus domestica. D. morbosum has also been isolated on numerous occasions during the winter from definite chlamydospores on buds and bark of P. domestica. Pure cultures of D. morbosum produced chlamydospores only when grown in close proximity to certain other organisms, or when certain chemicals were introduced into the Petri dishes. Chlamydospore formation was also induced on twigs of P. domestica when conidia of D. morbosum germinated in intimate association with a strain of bacteria originally isolated from the same host. Cladosporium carpophilum was isolated in several instances from chlamydospores on the bark of Prunus persica. This organism also produced chlamydospores both on culture media and on peach twigs. A histological study revealed the presence of two types of twig lesion. Early infections produced primary lesions during the summer, and secondary lesions during the following winter as a result of the migration of the mycelium beyond the barriers of periderm delimiting the primary lesions. Late infections, on the other hand, rarely induced periderm formation and did not produce secondary lesions during the winter. Taphrina deformans was isolated during the winter from the surface of buds on P. persica. Proof is submitted that spores of T. deformans can remain viable throughout the winter on twigs of the above-mentioned host. Other fungi, presumably saprophytes, representing the genera Coniothyrium, Cladosporium, Hormodendrum, Fumago and a fungus impossible to identify because it produced chlamydospores only, all developed from chlamydospores occurring on various hosts. The latter fungi all produced chlamydospores in culture. The evidence obtained indicates the probability that many more fungi over-winter as chlamydospores than have been reported to do so. Certain misapplications of the term chlamydospore in the various groups of fungi are dealt with. Owing to their unsatisfactory mycological status the importance of chlamydospores has hitherto been incompletely appreciated. The results of these investigations emphasize the importance of the application of a dormant spray on fruit trees.
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