Studies on Rhizoctonia Crocorum (Pers.) DC. and Helicobasidium Purpureum (Tul.) Pat.

Abstract

Fertile Helicobasidium purpureum has been found in very close association with root-rot characterised by infection cushions of Rhizoctonia Crocorum in distinct localities, on (a) red clover, (b) Mercurialis perennis, (c) Urtica dioica. In order to test the possible connection of the two fungi, they have been studied morphologically, culturally, and as to pathogenicity. Morphologically the hyphae of Helicobasidium are exactly similar to those of Rhizoctonia, having the same type of branching, septation, and absence of clamp connections. They also resemble one another in nuclear characters, both fungi having sometimes one but more often two nuclei per cell. Seven strains of Rhizoctonia Crocorum have been compared with numerous spore isolations of Helicobasidium as to cultural characters. Both fungi show considerable variation in colour and type of growth. Conidia belonging to the genus Tuberculina are frequently produced in cultures of Helicobasidium, and similar conidia have been found in strains of Rhizoctonia isolated from Urtica and from mangold. In cultures of Helicobasidium which originated from multiple spores there sometimes develop in subcultures strains which acquire the dark colour characteristic of the sterile Rhizoctonia strains, and which remain henceforth sterile. A similar variation has arisen in the Rhizoctonia isolated from Urtica, which at first produced only pale growth, with a tendency to form conidia. Successful inoculations, with the production of typical root-rot, have been obtained with four strains of Rhizoctonia, and with three of the dark-coloured strains derived from Helicobasidium spore cultures. The host plants used were various legumes and carrot. In every case precautions were taken to exclude the possibility of accidental infection, and adequate controls were used. The strains of Helicobasidium which have proved infective all originated from specimens associated with red clover. None of the strains from Urtica, nor the Rhizoctonia isolated from Urtica, has up to the present produced any infection on clovers or on carrot. Other strains of Rhizoctonia, however, have not given evidence of any specialisation in parasitism. The bearing of the observations made is discussed. While there have been inconsistencies in behaviour, it is possible that these are due to the fact that the organism is very variable, and that not all the strains are equally parasitic. It has also to be borne in mind that practically nothing is known as to the conditions for infection with Rhizoctonia Crocorum. The balance of the evidence is considered to favour the view that Helicobasidium purpureum (Tul.) Pat. is the perfect stage of Rhizoctonia Crocorum (Pers.) DC.