'n Studie van die patogene spesialisasie van Chalara elegans Nag Raj & Kendrick, die swartpeulvrotpatogeen van grondbone

Abstract

The taxonomy of the genus Chalara is concluded that the deposition of ( Corda) Rabenh. is discussed and it Thielaviopsis basicola (Berk. & Br .) Ferraris under the new name Chalara elegans by Nag Raj & Kendrick, is justified. In recent years c . elegans has become a serious problem in various ground= nut production areas in the Republic of South Africa. The cultural and pathogenic behaviour of c . elegans were determined in laboratory and green= house studies to identify different races of the pathogen. This was done to assess their importance in screening of breeding material for disease resistance. Thirteen isolates of c. elegans from different geographical areas in South Africa were obtained from infected groundnut, tobacco and citrus plants. The isolates could be differentiated according to in vitro growth characteristics. Groundnut isolates did not differ significantly on two different media, regarding temperature-, pH- and osmotic potential-effects, but differed significantly from the tobacco and citrus isolates. Only the citrus isolate was capable of growth at 30° C and one tobacco isolate grew optima 11 y at pH 6. A 11 other isolates (including the citrus isolate) grew optimally at 25°C and between pH 7 and 8. Soil was artificially infested with inoculum consisting of 150 propagules/g sand. The isolates were found to differ significantly in the in vivo studies. The groundnut isolates with similar pathogen i c fe a= tures differed significantly from the tobacco and citrus isolates which has distinct pathogenic features. Four different races of c. elegans were distinguished according to the pathogenic and cultural behaviour of the various isolates viz. race I : oli vaceous cultures, pathogenic to ground nut s, cotton and beans, represented by the groundnut isolates from Potchefstroom, i V Loskop, Delareyville, Vaalharts, Douglas and Fauresmith; race II: oliva= ceous cultures, pathogenic to cotton and beans, represented by the citrus isolate from Rustenburg; race III: gray cultures, intermediately pathoge= ni c to cotton, represented by the tobacco i so 1 ate from Brits and r ace IV: o 1 i vaceo us cu 1 tu res, nonpathogeni c t o grou nd nuts, cotton or beans, represented by the tobacco isolate from Potgietersrus .