【Objective】 The objectives of this study were to compare influence of allelochemicals from root exudates of watermelon and rice plants on the growth of FON and to clarify the mechanism of disease suppressiveness in the intercropping system. 【Method】 Root exudates of watermelon (REW) and rice (RER) plants were collected by nutrient solution culture. The effects of REW and RER on FON were determined by culture medium method. The phenolic acids of REW and RER were identified by HPLC. Exogenous phenolic acids were also used to investigate the effect of some phenolic acids, which existed in REW and RER, on FON. 【Result】Compared with the control, addition of 1.0 mL and 5.0 mL of REW increased the numbers of germinating spores by 46.9% and 59.2%, respectively, while the spore reproduction was enhanced by 10.8% to 84.6% when REW was added in the range from 0.1 to 2.0 mL. In contrast, addition of 1.0 mL and 5.0 mL of RER decreased spore germination by 14.3% and 6.1%, respectively, while addition of REW from 0.1 mL to 2.0 mL decreased spore reproduction by 4.6% to 37.5%. Salicylic acid, p-hydroxybenzoic acid and phthalic acid were identified in both RER and REW by HPLC analysis, but ferulic and p-coumaric acid acid were detected exclusively in REW and RER, respectively. Moreover, the content of p-coumaric acid was the highest among phenolic acids in RER, accounting for 37.9% of the total amount of phenolic acids, which was 1.4-fold as high as that in REW. According to the effects of exogenous phenolic acids on spore germination, spore reproduction and mycelial growth of FON, p-coumaric acid had the most significant inhibitory effects, followed by salicylic acid. In detail, exogenous p-coumaric reduced spore germination by 9.1% to 70.5%, spore reproduction by 24.1% to 100.0% and mycelial growth by 2.5% to 47.5%, as compared with the controls. In contrast, ferulic acid, p-hydroxybenzoic acid and phthalic acid increased spore germination and spore reproduction to some extent. Among of them, ferulic acid had the obvious promoting effects and could enhance spore germination by 28.6% to 114.3% and spore reproduction by 17.7% to 54.8% when its concentration ranging from 40 to 160 mg•L-1. 【Concluion】 It was concluded that the root exudates from rice plants were fungistatic while those from watermelon were pathogen-promotive. This finding provided a scientific basis for clarifying the mechanism of disease suppressiveness of watermelon in the watermelon/aerobic rice intercropping system.
