Authors
Y. T.
Tewoldemedhin
,
University of Stellenbosch, Department of Plant Pathology, Matieland 7602, Stellenbosch, South Africa
;
S. C.
Lamprecht
,
University of Stellenbosch, Department of Plant Pathology and Plant Protection Research Institute (ARC), Private Bag X5017, Matieland 7599, Stellenbosch, South Africa
;
A.
McLeod
,
University of Stellenbosch, Department of Plant Pathology
; and
M.
Mazzola
,
United States Department of Agriculture-Agricultural Research Service, Tree Fruit Research Laboratory, Wenatchee, WA 98801
ABSTRACT
Isolates of Rhizoctonia spp. associated with barley, canola, clover, lucerne, lupin, annual Medicago spp. (medic), and wheat were recovered during the conduct of a 4-year (2000 to 2003) crop rotation trial in the Western Cape province of South Africa. These isolates were characterized by determining their anastomosis group (AG), in vitro optimum growth temperature, and pathogenicity toward emerging and 14-day-old seedlings of all the aforementioned crops. During the 4-year rotational trial, 428 Rhizoctonia isolates, in all, were obtained. 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. comprised AG-K (53%), AG-A (10%), AG-I (5%), and unidentified AGs (32%). The optimal time for isolating Rhizoctonia spp. was found to be at the flowering or seedpod stage (20 to 22 weeks after planting). Temperature studies showed that isolates belonging to AG-2-2, AG-4 HG-II, and AG-K had significantly 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. Rhizoctonia solani AG-2-1 was highly virulent on canola, moderately virulent on medic and lupin, weakly virulent on lucerne and barley, and nonpathogenic on wheat. AG-11 isolates were moderate 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 sp. capable of inciting disease in our assays. This is the first comprehensive study to elucidate the identity and potential importance of Rhizoctonia spp. as a yield limiting factor in crop production systems in the Western Cape province of South Africa.
