Characterization of Phytophthora clandestina races on Trifolium subterraneum in Western Australia

Abstract

Phytophthora clandestina is a causal agent of root rot disease of subterranean clover in Western Australia (W.A). As a significant number of isolates of P. clandestina from W.A. could not previously be designated using existing differentials, a comprehensive set of subterranean clover (Trifolium subterraneum) cultivars was used as differentials to delineate a broader range of races of the pathogen. One hundred and one isolates of the pathogen collected from W.A. were screened on nine subterranean clover cultivars, of which seven were found to be useful as host differentials. A total of 10 races (in contrast to the five recognized previously) were defined and differentiated using octal nomenclature, presenting a clearer picture of the racial distribution of P. clandestina among W.A. isolates. Differences were found in the race populations between Australian states and are therefore important to the selection/breeding of cultivars for specific regions of Australia to counter the predominant race populations and for enforcing quarantine measures in relation to seed movements within and outside Australia. The octal nomenclature used provides a sound basis for follow-up studies and future race designations. Races 173 and 177 in this study were widely distributed and were the most common races in W.A., and together constitute 80% of the isolates characterized. While six of the seven host differentials were resistant to isolates belonging to race 001 and all were resistant to 000, it is of concern that only one differential was resistant to 157 and 173 and that none of the host differentials were resistant to 177. Our approach to P. clandestina race delineation is clearly conservative and is different from previous studies. The octal nomenclature we applied in this study is not only scientifically sound but also will facilitate rapid recognition and characterization of the races.

Severity of phytophthora root rot and pre‐emergence damping‐off in subterranean clover influenced by moisture, temperature, nutrition, soil type, cultivar and their interactions

“…A major driving factor in these interactions was the differences between the subterranean clover cultivars. Cultivar Seaton Park is the only cultivar of the four chosen for this study that has resistance to race 173 (You et al ., ,b). In contrast to cv.…”

“…This was expected as You et al . () demonstrated that cv. Meteora was moderately resistant to race 173 used in the current study.…”

“…The most important soilborne pathogens of subterranean clover include Phytophthora clandestina , various Pythium species such as Pythium irregulare , Aphanomyces trifolii and Rhizoctonia solani (Wong et al ., ; Barbetti et al ., , ; Ma et al ., ; Nichols et al ., ). Of these, P. clandestina is generally considered the most important (You et al ., ,b; Barbetti et al ., ), and it is also known to affect other forage legumes, such as Biserrula pelecinus , Hedysarum coronarium , Ornithopus compressus and O. sativus (Li et al ., ).…”

“…Subsequently, You et al . () characterized a total of 10 races of P. clandestina in Western Australia, of which races 173 and 177 were widely distributed and common in Western Australia, together constituting 80% of isolates characterized.…”

Urana subterranean clover (Trifolium subterraneum L. var. subterraneum)

“…Urana is susceptible to both Race 001 and Race 173 (formerly known as Race 0 and Race 1, respectively, and recently recoded by You et al 2005c), 2 of the most widespread races of root rot caused by Phytophthora (You et al 2005a). Urana is moderately susceptible to root rot caused by Pythium irregulare Buisman.…”