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ARS Home » Pacific West Area » Wenatchee, Washington » Physiology and Pathology of Tree Fruits Research » Research » Publications at this Location » Publication #309179
Title: Rhizoctonia anastomosis groups associated with diseased rooibos seedlings and the potential of compost as soil amendment for disease suppression

Author
item TEWOLDEMEDHIN, Y - Agricultural Research Council Of South Africa
item LAMPRECHT, S - Agricultural Research Council Of South Africa
item Mazzola, Mark

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2015
Publication Date: 7/7/2015
Citation: Tewoldemedhin, Y.T., Lamprecht, S.C., Mazzola, M. 2015. Rhizoctonia anastomosis groups associated with diseased rooibos seedlings and the potential of compost as soil amendment for disease suppression. Plant Disease. 99:1020-1025.

Interpretive Summary: Rhizoctonia spp. are a biologically and genetically diverse group of fungi, many of which incite devastating diseases on a variety of economically important crops. However, there is little information concerning the specific identity of the population contributing to plant diseases in agriculturally important regions on the African continent. Rooibos, an economically important plant which is indigenous to South Africa, is grown for its health benefits and the demands for its herbal tea. Rhizoctonia cause significant disease problems in rooibos nurseries, but development of effective control measures requires information concerning the specific populations responsible for causing these diseases. A diverse collection of Rhizoctonia species were obtained from rooibos seedlings cultivated in soils obtained from multiple rooibos nursery soils in the Western Cape province of South Africa. Multiple species of Rhizoctonia were found to cause rooibos seedling death. In addition, several species from rooibos were shown to infect other crops commonly used in rooibos as a rotation or cover crop as a disease management protocol. Likewise, certain species recovered from these cover crops were shown to cause disease on rooibos. This information can be used to more effectively select cover crop species for use in rooibos nursery production systems that minimize the potential to build up inoculum of pathogenic Rhizoctonia spp. Different composts were evaluated for the ability to suppress disease of rooibos caused by Rhizoctonia spp. In general, disease suppression varied depending upon the compost utilized and also depended upon the species of Rhizoctonia introduced as the pathogen. As such, information from this study regarding the species of Rhizoctonia cause rooibos seedling death with be great importance for the effective and predictable use of compost as a disease control strategy in rooibos nursery systems.

Technical Abstract: Rhizoctonia spp. associated with rooibos in the Western Cape province of South Africa were recovered during the 2008 season by planting seedlings in rhizosphere soils collected from 14 rooibos nurseries. Seventy five Rhizoctonia isolates were obtained and 67 were multinucleate and 8 binucleate Rhizoctonia. The identity of these isolates to anastomosis group (AG) was determined through sequence analysis of the rDNA internal transcribed spacer region. The collection of multinucleate isolates included representatives of AG-2-2 (67%), AG-4 HG I (14 %), AG-11 (5) and R. zeae (3%). Binucleate AGs included AG-B(o) (4%) and AG-K (4%) and an unidentified binucleate Rhizoctonia (UNBR) (3%), with AG-2-2 being the most widely distributed (11 nurseries) Rhizoctonia. All AGs recovered from rooibos have been previously reported on crop plants in South Africa with the exception of R. zeae, however this is the first study to classify the Rhizoctonia AGs recovered from rooibos. In glasshouse bioassays, the most virulent Rhizoctonia AGs on rooibos and lupin were determined to be AG-2-2, AG-4 HGI and AG-11. Although plant damage was less than that observed for lupin and rooibos, oats were significantly affected by AG-2-2 and AG-4 HGI. Two composts sourced from independent suppliers were evaluated for disease suppression under glasshouse conditions. Compost amendment suppressed damping-off of rooibos incited by R. solani, however there existed both within and between species variation in level of disease control attained among the isolates tested.