ETIOLOGY OF RHIZOCTONIA IN SHEATH BLIGHT OF MAIZE IN SICHUAN

Rhizoctonia isolates obtained from maize grown in commercial fields in 33 representative counties (or cities) in Sichuan province in China w ere characterized according to colony morphology, hyphal anastomosis a nd pathogenicity. Of 141 isolates, 116 were identified as R. solani, 2 3 as R. zeae and two as binucleate Rhizoctonia. The isolates of R. sol ani were assigned to four anastomosis groups (AG): AG-1-IA (101 isolat es, accounting for 71.6% of the total), AG-1-IB (2, 1.4%), AG-4 (9, 6. 4%) and AG-5 (4, 2.8%). The two isolates of binucleate Rhizoctonia bel onged to AG-K. On maize, isolates of AG-1-IA caused typical sheath bli ght symptoms. Lesions produced by isolates of AG-4, AG-5, AG-1-IB and AG-K were darker than those of AG-1-IA. Rhizoctonia zeae usually cause d discontinuous lesions with a dark brown margin and a brown centre on the leaf sheaths, as well as ear rot. Isolates of AG-1-IA were the mo st virulent to maize, with an average lesion length of approximately 1 5 cm. Isolates of R. zeae produced lesions approximately 12 cm long, w hile those of AG-4, AG-5, AG-1-IB and AG-K were progressively shorter. On potato dextrose agar (PDA; pH 6.4), the minimum temperature for my celial growth of R. zeae isolates was 14-18 degrees C, the maximum 38- 40 degrees C and optimum 30 degrees C. Isolates of R. zeae did not gro w on PDA (28 degrees C) at pH 2.0, the optimum for growth being pH 6.4 .