The interaction of binucleate Rhizoctonia (BNR) anastomosis group (AG)-A isolate W7, Rhizoctonia
solani AG-4 and cucumber seedlings were investigated to elucidate the mechanism of biocontrol of
Rhizoctonia solani by BNR. Hypocotyls of Cucumis sativus L. cv. Jibai were inoculated with a virulent
isolate of R. solani AG-4 isolate C4 and examined with light microscopy and scanning and transmission
electron microscopy. The hyphae of the virulent isolate colonized the outer surface of the hypocotyl and
penetrated the epidermal and cortical cells to the pith of hypocotyls. The hyphae of a non-pathogenic
species of BNR isolate W7 did not penetrate the cortical cells but instead colonized the outer epidermal
cells 12 h after inoculation. Accumulation of mucilage occurred on the surface after dense colonization
of the hypocotyls surface by the BNR. Observation of the hypocotyls surface pre-inoculated with BNR
and challenged inoculated with R. solani showed constricted and deformed hyphae of R. solani, which
were prevented to penetrate the epidermal cells. Pre-inoculating of BNR isolate caused profused
formation of the mucilage which lysed BNR hyphae and restricted the growth of R. solani. The mucilage
was identified as pectic substances. Analysis of pectin contents from dried hypocotyls tissues showed
highly methylated pectin with low uronic acid contents in the seedlings inoculated with R. solani and
unprotected by BNR while BNR- treated seedlings showed less methylated pectin and high amount of
uronic acid. Furthermore, the BNR-inoculated seedlings showed significant (P=0.05) increase in
calcium, indicating high amount of wall-bound cations in cell walls. Pectic substances accumulation
and increased calcium in the cucumber tissues greatly contributed in the protection of cucumber
seedlings against Rhizoctonia damping off by a non-pathogenic species of Rhizoctonia.