Abstract: The gray mold disease on fruits,vegetables and flowers caused by Botrytis cinereais rather common and difficult to control.Oxalic acid(OA)is considered a critical factor in the infection through lowering pH in the infected area.Trichodermaspp.is a valuable bio-control agentin preventing gray rot on the plants.The fungusis known to be capable of eliminating OA.However,whether or not OA elimination is the reason that renders Trichodermaspp.its ability to ward off the gray rot remains unknown.To better understand the mechanism,this study firstly determined the effect brought about by T.harzianum LTR-2on the ambient pH when the plant was under an OA stress.Then,the antagonism effect of LTR-2against B.cinerea on pH and OA elimination was examined.Furthermore,the correlation between the inhibition rate and the ambient pH was statistically analyzed.The results showed that LTR-2could raise the pH when the OA concentration was less than 50mmol·L-1 in the surrounding area.Either in a PDA co-culture or plant tissue,LTR-2exhibited the ability to reduce acidification caused by the presence of B.cinerea.Statistical analysis showed a linear correlation between the acid-reduction rate of LTR-2and the pH on the medium or the ambient area on the plant.In a test,LTR-2validated its ability toalleviate OA on the cucumber petals infected by B.cinerea.The results obtained from current experimentation seemed to imply that elimination of OA on the plant could be one of the key factors in the control mechanismof gray mold disease involving LTR-2and B.cinerea.