Estimation of inoculum potentials of Phytophthora megasperma f.sp. medicaginis in chickpea fields and the development of a glasshouse resistance assay

Abstract

A survey of chickpea (Cicer arietirium) fields in south-eastern Queensland revealed that Phytophthom megasperma f. sp. medicaginis (Pmm) was the major cause of root rot in 1988. Eleven sites were each sampled 3 times during the growing season and the levels of infective units of Pmm present in the soil samples collected from each site were determined using a modified baiting technique in combination with Most Probable Number (MPN) analysis. Inoculum potentials of field soils ranged from 0.01 to 10.95 infective units per g dry weight of soil. Pmm was usually not detected by baiting unless soil samples were collected from the vicinity of diseased plants, and inoculum potentials declined markedly in the absence of a growing chickpea crop. A glasshouse assay for resistance was developed using levels of inoculum that corresponded to the inoculum potentials encountered in naturally infested field soils. Infestation of potting mix at the rate of 13 oospores/g produced an MPN of 0.3 infective units/g. This screening technique differentiated between a partially resistant chickpea cultivar (CPI 56564) and a highly susceptible line (ICC 6334). Different levels of zoospores were also tested as the inoculum source, but variability in plant response was higher than that encountered with oospores.