Differences in symptom development in subterranean clover infected with Kabatiella caulivora Race 1 and Race 2 are related to host resistance

Abstract

Clover scorch (Kabatiella caulivora) is a severe fungal disease of Trifolium spp. contributing to the collapse of pasture swards across southern Australia during warm, humid spring weather. Host plant responses associated with resistance to the disease were determined in 2 cultivars of subterranean clover (T. subterraneum) separately inoculated with K. caulivora Race 1 or Race 2. Germination of conidia of both races reached a maximum 5 days post-inoculation on cv. Woogenellup (susceptible to both races) and 4 days post-inoculation on cv. Daliak (resistant to Race 1 but susceptible to Race 2). Germ tube growth of Race 1 was inhibited on cv. Daliak and the percentage of conidia penetrating leaf surfaces was lowest on this race–cultivar combination. Susceptibility was characterised by large petiole lesions, with invasive hyphae extending through the mesophyll tissue into the pith and then through the phloem tissue of vascular bundles, eventually causing the petioles to collapse. Resistance was characterised by small, black lesions with invasive hyphae extending no further than the fourth layer of mesophyll cells. A suberin-based material was observed beneath infected mesophyll cells in the incompatible interaction, beyond which no further growth of hyphae occurred. Race 2 caused a faster rate of host tissue necrosis than Race 1 and also the breakdown of starch grains in uninvaded petiole tissues. Starch grains in plants infected with Race 1 were evenly distributed in uninvaded tissue. Sporulation was rarely observed in the incompatible interaction but was common in compatible interactions within 15 days post-inoculation. These responses to K. caulivora can now be used as a breeding tool in evaluating and selecting improved resistance to clover scorch disease among breeding lines of subterranean clover.