First report of Rhizoctonia solani anastomosis group AG-4 HG-I in the Lao PDR
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Rhizoctonia solani anastomosis group AG-4 HG-I is reported for the first time from the Lao PDR. It was isolated from gai lan (Brassica oleracea var. alboglabra) affected by collar rot, seedling death, root rot and stunting of older plants from the Paksong area of Champasak province. The anastomosis group was confirmed by sequencing and Koch’s postulates were fulfilled.
KeywordsChinese broccoli Chinese kale Kai lan Rhizoctonia collar rot Wirestem
Rhizoctonia solani is a species complex consisting of a number of anastomosis groups (AGs) that differ morphologically and phylogenetically with respect to host range and pathogenicity, susceptibility to different fungicides and geographic distribution (Gonzalez et al. 2001; González et al. 2006; Vilgalys and Cubeta 1994). They persist as hyphae in residue and/or sclerotia in soil and affect many crops, causing a wide range of diseases including foliar spots and blights, seedling death (damping-off), collar rot, root rot, crater rot, stalk and head rots of Brassica and other crops (Rimmer et al. 2007; Sneh et al. 1996). It is important to identify the AG causing a particular disease in order to determine the most effective integrated disease management (IDM) strategy and to contribute to the global understanding of the distribution and biology of R. solani.
The recovery of AG-4 HG-I from gai lan is consistent with the reports of the AG-4 group causing disease in B. oleracea and B. napus in North America (Keinath and Farnham 1997) and in cauliflower in Belgium (Pannecoucque et al. 2008). More recently, AG-4 HG-II was isolated from B. oleracea crops in the UK during an extensive survey (Budge et al. 2009). We have observed collar rot of seedlings and older plants and seedling death in several brassicas in Champasak province. Consequently, further studies are needed to determine the relative importance of AG-4 HG-I in brassica crops in this region.
We have also observed collar rot on other vegetable crops at the polyhouse farm where AG-4 HG-I was recovered, and at other farms in the province, indicating the possible presence of additional hosts for AG-4 HG-I and of other anastomosis groups of R. solani. The high incidence of Rhizoctonia collar rot and seedling death in gai lan is attributed to the current practice of raising seedlings in natural soil in ground beds in polyhouses where brassica crops, including gai lan, have been grown regularly for at least seven years. The production of seedlings in pathogen-free soil is being encouraged together with regular rotation to crops in other plant families. Besides infecting vegetables in the Brassicaceae, R. solani AG-4 is known to have a wide host range, infecting crops in the Chenopodiaceae, Fabaceae and Solanaceae (Ogoshi 1987) and Cucurbitaceae (Kuramae et al. 2003). Limited but potentially suitable rotation vegetable crops should be tested for susceptibility to AG-4 HG-I in the Alliaceae, Apiaceae and Asteraceae.
Financial support from The Crawford Fund of Australia is gratefully acknowledged. The authors also acknowledge the support provided by the Champasak Provincial Agriculture and Forestry Office. The first author is an Australian Volunteer for International Development, an Australian Government Program.
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