Virulence of Potato Geographic Isolates of Rhizoctonia solani (AG-s)

The aim of this study was to test the virulence of seven geographic isolates of Rhizoctonia solani belong to AG-3 or AG-4, at the vicinity of Riyadh region (Al-karjand Dierab) and Hail, Saudi Arabia and from imported potato tuber from USA and Lebanon, on two potato cultivars (Spunta and Aphondiel) in a greenhouse at the College of Agriculture, King Saud University. Results indicated that all seven isolates of R. solani were pathogenic to the potato cultivars (Spunta and Aphondiel), but varied in the percentage of disease severity of black scurf, 80 days after inoculation. Isolates belong to AG-4 (isolate nos. 3 and 6) were highlyvirulent than those isolates belong to AG-3 (nos.I, 2, 4, 5, and 7) on potato tested cultivars and caused also a drastic reduction in yield, shoot and root dry weight and shoot length in comparison with those isolates belong to AG-3.