Fungal growth, haustorial disorganization, and host necrosis in two cultivars of cowpea inoculated with an incompatible race of the cowpea rust fungus

Abstract

The development of the cowpea rust fungus was examined by light microscopy in two resistant cowpea cultivars. In cultivar TVu 4552, necrosis of the first invaded cell was usually rapid and correspondingly, only a few colonies grew intercellularly beyond the formation of an infection hypha. Electron microscopy suggested that secondary hyphae were initiated only at sites where the first-formed haustorium appeared relatively normal 24 h after inoculation, irrespective of the state of the host cell. Observations also suggested that host necrosis was not caused by the prior death of the haustorium. The few expanding colonies in TVu 4552 usually ceased their growth between 72 and 96 h after inoculation after a second wave of host necrosis. In cultivar Purple Hull Pinkeye, necrosis of the first invaded cell occurred more slowly and nearly all colonies produced secondary hyphae. Faster growing colonies often showed no necrosis until 72 h after inoculation, but at most infection sites, the average number of necrotic cells per μm of mycelium increased with time, although the average number of haustoria per μm of mycelium stayed constant. Fungal growth usually ceased at about 96 h after inoculation. The results show that in both cultivars there was a close correlation between the patterns of host necrosis and fungal growth, particularly at the early stage of fungal development. It is suggested that the direct or indirect effect of necrosis on haustorium development and function may be critical in determining its role in resistance to rust fungi.