Abstract:Grey mold (Botrytis cinerea Pers.) is a pathogenic fungus of tomato botrytis, which occurs seriously in continuous cropping soil and causes heavily economic losses in tomato production. Farmers have to grow tomato annually in the same soil since lack of soil resources in China. Chemicals such as carbendazim, thiophanate-methyl and benlate have been used to control tomato botrytis for decades and a large amount of these chemicals have thus remained in both tomato fruits and soils, which is harmful to both human being and the environment. On the other hand, the pathogenic Botrytis cinerea has also developed resistance to these chemicals. As a result, an increasing attention has been paid to develop safe, effective, and stable bio-agents to control tomato botrytis in recent years. Studies have reported that Pythium oligandrum could inhibit or kill plant pathogenic fungi, including Botrytis cinerea, Phytophthora nicotiana and Pythium aphanidermatum. The oospores of P. oligandrum have been hence used to control black shank disease of tobacco, grey mold of tomato, and root rot of cucumber. However, the bio-control effect of the oospore agent is unstable and unsatisfactory because their germination and hyphal growth are significantly influenced by temperature, humidity, sun light, soil types, rainfall, and agricultural measurements. To address the importance of metabolites produced by P. oligandrum in bio-controlling of plant fungal diseases, in both greenhouse and field experiments we tested the hypothesis that P. oligandrum fermentation broth could influence the growth and botrytis control of tomato. For the greenhouse pot experiment, four treatments had been examined: plants inoculated or not inoculated with pathogen plus culture solution or plus fermentation broth. Meanwhile, tomato seedlings were respectively sprayed with water (control), fermentation broth or culture solution, before the occurrence of botrytis in the field. In the greenhouse pot experiment, the fermentation broth promoted the growth of tomato seedlings and total plant biomass production by 9.5% and root biomass by 15.4% compared to the control (pathogen inoculation plus culture solution). Chlorophyll concentrations in the leaves, root activities, and nutrient absorption (nitrogen, phosphorus, and potassium) in the field seedlings were also increased by spraying the fermentation broth. Meanwhile, the disease incidence was reduced by 57.2% and disease index by 60.3% with the fermentation broth supply under pathogen inoculation, leading to 60.3% of the relative control efficacy. Further results showed that the fermentation broth could protect the cell membrane of leaves against the pathogen damage, since less malondialdehyde, one of the oxidative products of cell lipids, was detected. In addition, the fermentation broth stimulated enzyme activities related to plant disease resistance such as superoxide dismutase, polyphenol oxidase, and phenylalanine ammonialyase in the leaves of tomato seedlings. The relative control efficacy of the fermentation broth reached 71.2% in the filed tomato seedlings. In summary, the fermentation broth of P. oligandrum have multiple functions, including the induction of the resistance to grey mold, the decrease in botrytis occurrence, and the promotion of plant growth, which are different from other bio-control agents that can control diseases only. Our research showed the potential of the metabolites produced by P. oligandrum as an effective, stable and safe bio-control agent. Further investigations are needed to clarify the effectiveness of metabolites from P. oligandrum in disease control.