STUDIES ON THE PERENNIAL RUST PUCCINIA MINUSSENSIS

Abstract

The permanent infection of rust-free Lactuca pulchella plants by Puccinia minussensis may be initiated by inoculating the primary leaves of seedlings, or the rhizome buds of older plants, with either aeciospores or urediospores, or by packing teliospores into the leaf axils of mature plants and subsequently using the infected nodal portions as propagation cuttings. When a primary leaf of a seedling becomes infected, the binucleate mycelium progresses along the midrib and petiole to the stem and advances down it into the rhizomes but, before reaching the terminal buds of the rhizomes, its nuclear condition changes, as only uninucleate mycelium has been observed in the bud scales. Infected rhizome buds develop into young shoots that usually first produce pycnia, although, in cases of delayed mycelial development, telia appear on the more mature plant followed by pycnia on subsequent new axillary growth. The perennial mycelium seems to alternate from the binucleate to the uninucleate condition and vice versa, depending apparently on the maturity of the host tissue involved and the food supply available to the fungus.