Expression of resistance by tetraploid and diploid lucerne genotypes to Phytophthora medicaginis is not influenced by inoculum level or by temperature

Abstract

Phytophthora-resistant lucerne cultivars do not always perform well under conditions of high disease pressure in the field. To determine whether resistance expression remains stable under different infection intensities, tetraploid and diploid lucerne genotypes, genotypically defined for their reactions to Phytophthora medicaginis, were clonally propagated, and the influence of different reproducible inoculum levels (0·5 and 5·0 g dry weight mycelium/kg dry weight potting mix), the period of exposure to these levels (10–60 days), and temperature (16/22°C and 24/30°C) on disease expression was determined in controlled environments. Generally, expression of resistance by resistant genotypes, remained stable under these conditions. Biotic (e.g. Aphanomyces eutiches) or abiotic factors other than P. medicaginis may be responsible for the poorer than expected performance under fiield conditions in some instances, or the percentage of resistant plants in some cultivars currently classified as resistant is insufficient to provide buffering against productivity reductions under severe epidemics. Further research is needed to clarify the situation.