Onion stunting in South Australia associated with Rhizoctonia solani AG 8

Abstract

Patches of stunted onion plants are common within otherwise healthy crops in the “Mallee” region of South Australia. A number of fungi including species of Rhizoctonia, Fusarium, Pyrenochaeta and Pythium were isolated from the roots of stunted plants, but only R. solani anastomosis groups (AG) 2.1 and 8 consistently caused severe stunting when onion seedlings were grown in soil inoculated with these pathogens. High quantities of R. solani AG 8 DNA were associated with soil collected from stunted patches but the pathogen was generally low or absent in soil from healthy areas. DNA of AG 2.1 and 3 were common in onion field soil, but quantities were not correlated with stunted areas. AGs 2.2, 4 and 5 were rarely detected in onion field soil. Nematodes such as Pratylenchus, Paratylenchus, Paratrichodorus and Tylenchorhynchus were extracted from soil and/or onion roots but numbers were generally low and were similar in stunted and healthy areas. Overall these results suggest that onion stunting in South Australia is primarily associated with R. solani AG 8 although interactions with other fungi and/or nematodes may also be involved.