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.

Stunting of Onion in the Columbia Basin of Oregon and Washington Caused byRhizoctoniaspp.

“…At the lower temperature regime, R. solani AG 8 isolate Rh070943 collected from pea plants was markedly more virulent than the other two AG 8 isolates studied, causing severe reductions in plant weight at low inoculum densities. Some researchers also have observed considerable variability in disease response to isolates of this AG (22,23,52) while other researchers have not (18,19). The AG E isolate also significantly reduced onion emergence, leading to complete preemergence damping-off at high inoculum densities.…”

“…Stunting of onion in the Columbia Basin occurs when the crops are planted in combination with winter cereal cover crops (16), and a similar problem has been diagnosed recently in Australia (52). In the Columbia Basin onion production systems, the cover crops provide a wind barrier in the spring to protect emerging onion seedlings against wind-and sandblasting on the sandy soils typical of this region.…”

“…Previous studies have demonstrated that infections on onion can be initiated successfully by isolates of R. sokmi AGs 1,2-1, 3, 4, 5, and 8 (19,22,23,46,52,54); binucleate Rhizoctonia spp. AGs A, Ba, Bi, E, F, G, K, O, and R (8,18,19,22,23); as well as Waitea circinata var.…”

“…circinata, and binucleate Rhizoctonia AG E were found consistently to be pathogenic on onion seedlings, although virulence differed among isolates of a species and among species. R. solani AG 8 was also implicated as the main cause of onion stunting in Australia, where wheat and barley are used similarly to protect onion seedlings from wind-blown sand (52). R. solani AG 8 isolates caused severe stunting of onion at the low inoculum densities expected in agricultural soils (47) but did not reduce seedling emergence over a range of temperatures characteristic of the Columbia Basin between February and April, when spring-planted onion crops appear to be most susceptible to infection by Rhizoctonia spp.…”

Evaluation of Onion Genotypes for Resistance to Stunting Caused by Rhizoctonia solani AG 8

“…Anecdotal evidence from growers and crop consultants suggests that stunted patches can develop on any of the onion cultivars commonly grown in the Columbia Basin. The patches of stunted seedlings can range from <1 m to >25 m in diameter and, in severe circumstances, patches may comprise up to 15% of an onion field (du Toit, 2009;Wicks et al, 2011). Severely infected seedlings remain stunted relative to adjacent healthy plants throughout the season.…”