Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/5/2006
Publication Date: 10/10/2006
Citation: Tewoldemedhin, Y.T., Lamprecht, S.C., Mcleod, A., Mazzola, M. 2006. Characterization of rhizoctonia spp. from cropping systems in the western cape province of south africa. Plant Disease. 90:1399-1406. 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 inmportant regions on the African continent. As crop rotation is a primary means of controlling Rhizoctonia incited diseases in these production systems, it is important to have information concerning these populations so that effective rotation sequences can be generated. The species and anastomosis groups of Rhizoctonia recovered from crops commonly used in rotations in the Western Cape province of South Africa, barley, canola, clover, lucerne, lupin, annual Medicago spp. (medic) and wheat were identified. The relative virulence of these different species to each of the rotation crops was determined. As these crops differed in susceptibility to the difference species of Rhizoctonia, suitable rotation sequences can now be devised for evaluation as a means to control diseases caused by these fungi.
Technical Abstract: Rhizoctonia spp. are important pathogens of a broad range of crop plants that are economically important to the farm economy of the Western Cape region of South Africa. However, there is little information concerning the identity and relative importance of these fungal pathogens. Isolates of Rhizoctonia spp. associated with barley, canola, clover, lucerne, lupin, annual Medicago spp. (medic) and wheat in the Western Cape were characterized by determining their anastomosis group (AG), in vitro optimum growth temperature, and pathogenicity towards emerging- and 14-day-old seedlings of all the aformentioned crops. The most abundant multinucleate AG was AG-4 HG-II (69%), followed by AG-2-1 (19%), AG-3 (8%), AG-2-2 (2%) and AG-11 (2%). The population of binucleate Rhizoctonia spp. was comprised of AG-K (53%), AG-A (10%), AG-I (5%) and unidentified AGs (32%). The optimal time for isolating Rhizoctonia spp. was at the flowering/seededpod stage (20-22 weeks after planting). Temperature studies showed that isolates belonging to AG-2-2, AG-4 HG-II and AG-K had signifcantly higher optimum growth temperatures than those from other AGs. In pathogenicity assays conducted on emerging- as well as 14-day-old seedlings, isolates of AG-2-2 and AG-4 HG-II were the most virulent on all crops. R. solani AG-2-1 was highly virulent on canola, moderately virulent on medic and lupin, weakly virulent on lucerne and barley and non-pathogenic on wheat. AG-11 isolates were moderately to weakly virulent on all crops, with the exception of barley and wheat. AG-3 was weakly virulent on canola, lupin and medic. AG-K was the only binucleate Rhizoctonia spp. capable of inciting disease in our assays. This is the first comprehensive study to elucidate the identity and potential importance of Rhizoctonia as yield limiting factor in crop production systems in the Western Cape province of South Africa.