Title: Characterization of Rhizoctonia isolates associated with damping-off and crown rot of rooibos seedlings Authors
|Tewoldemedhim, Y -|
|Lamprecht, S -|
Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: November 21, 2010
Publication Date: January 19, 2011
Citation: Tewoldemedhim, Y.T., Lamprecht, S.C., Mazzola, M. 2011. Characterization of Rhizoctonia isolates associated with damping-off and crown rot of rooibos seedlings. Proceedings for the Southern African Society for Plant Pathology Annual Meeting. P. 51. Technical Abstract: Rhizoctonia species were reported to be an important component of the complex involved in damping-off of rooibos (Aspalathus linearis) seedlings and cause severe crown rot of seedlings in nurseries. However, no information is available on the anastomosis groups (AGs) of Rhizoctonia associated with damping-off and crown rot of rooibos seedlings. Therefore, the aim of this study was to characterize the AGs of Rhizoctonia associated with diseased rooibos seedlings in nurseries and to investigate a potential management strategy using soil amendment with compost. Isolation studies from 14 nurseries followed by ITS sequencing after PCR amplification and cloning of 69 isolates revealed that 65 of the isolates were multinucleate and four binucleate Rhizoctonia. The multinucleate isolates included Rhizoctonia solani AG 2-2 (72%), AG 4 HG-I (16 %), AG 11 (4%) and R. zeae (3%). Binucleate AGs included AG-B(o) (4%), and AG-K (1%). Rhizoctonia solani AG 2-2 was obtained from 11 nurseries, AG 4 HG-I from 3 while AG 11, R. zeae, AG-K and AG-B(o) obtained from one nursery each. This is the first report of these AGs on rooibos in South Africa. Pathogenicity and cross-pathogenicity studies on rooibos, lupin and oats showed that AG 2-2 and AG 4 HG-I were pathogenic on all three crops, while AG-11 was pathogenic on rooibos and lupin only. Rhizoctonia zeae and AG-K significantly reduced survival of rooibos only. This has important implications for crop rotation in nurseries since lupin and oats are used as rotation crops in rooibos nurseries and plantations. The AGs responded differently to soil amendment with compost. Survival of rooibos seedlings improved significantly when compost was applied to soil infested with AG 2-2 and AG 11, while survival was not affected by compost amendment in soils infested with AG 4 HG-I. It is therefore imperative to further investigate the use of compost amendments as part of a sustainable disease management strategy for the production of organic rooibos seedlings in nurseries.