VARIATION IN SENSITIVITY TO FUNGICIDES AMONG ISOLATES OF Rhizoctonia spp. INVOLVED IN DAMPING- ," OFF OF COTTON SEEDLINGS

Five isolates of multinucleate Rhizoctonia so/ani and one isolate of binucleate
Rhizoctonia sp. were evaluated in vitro for sensitivity to certain fungicides i.e., Rizolex
T, Monceren T, Maxim, Premis, Beret MLX, and Tachigaren. All the R. so/ani isolates
were tolerant to Tachigaren, while the binucleate isolate of Rhizoctonia sp. was
sensitive; In the contrary, all the isolates were highly sensitive to Rizolex T. The
efficiencies of the same funqicides-in reducing damping-off of cotton cultivar Giza 89
were evaluated under greenhouse conditions in soil infested with each individual
isolate at two inoculum densities. Analysis of variance showed highly significant
. effects of both fungicides and isolate, while the fungicide x isolate interaction was a
nonsignificant source of variation. in percentage of seedling mortality, regardless of
the inoculum density. Due to the nonsignificant interaction between fungicide and
isolate, a least significant difference was used to compare between the general
means of fungicides. These comparisons showed that Rizolex T and Beret MLX were
the most effective fungicides in controlling damping-off at the lower inoculum density,
while Rizolex T and Premis were the most effective ones atthe higher inoculum
density. Tachigaren was effective in controlling the disease only at the higher
inoculum density; however, it was the least efficient fungicide. There was no
correlation between the in vitro efficiencies of the fungicides and their in vivo
efficiencies when fungicidal efficiency was evaluated based on the effects of each
fungicide on individual isolates. On the other hand, a highly significant correlation
between in vitro and in vivo efficiencies of the fungicides was observed only when
fungicidal efficiency was expressed as the mean efficiency of each fungicide over all
the tested isolates. Regression analysis showed that in vitro mean of fungicidal
efficiency accounted for 88 and 86% of the explained (model) variation in mean of
fungicidal efficiency under greenhouse conditions when the inoculum densities were
0.1 g and O.Sg/Kg soil, respectively,