Studies On Biological Characteristics And Antagonists Of Stem-end Rot In Citrus

Postharvest disease may cause great losses, which was controlled mainly by synthetic fungicides traditionally. With public's growing concern of the human health conditions and attention to the environmental pollution, pesticide residues, harmful to health and increased systemic resistance associated with pesticides usage in orchards, there is a great need for alternative approaches. Biological control with antagonists, has been proved to be one of safe, toxicless biological agent as an alternative to the pestcides.Two pathogen strains Diplodia natalensis Evans and Phomopsis cytosporella Penz. et Sacc. which caused stem-end rot of Citrus were obtained from surface of the natural rotted Citrus fruits. And the biological characters of them were studied. Two antagonist strains Pg2-5 and H-20 showed well inhibitory efficacy against postharvest stem-end rot of Citrus. The main results were as below:1. D. natalensis and P. cytosporella were separated from surface of the natural rotted Citrus fruits. The results of biological characteristics showed that, there was significant difference in colony form, growth rate of mycelia and yield of conidia in different culture media, among which potato sucrose agar played the best role in D. natalensis. Yield of conidia of D. natalensis can reach 9.08×10⁷ cells·mL^-1, while that of P. cytosporella was 3.26×10⁸ cells·mL^-1 in the Czapack and clover medium. Suitable temperature for the growth of mycelia was 25℃～30℃, optimum temperature was 30℃, and optimum temperature for producing of conidia was 28℃. Lighting did not effect the growth of the pathogen and yield of conidia evidently. The lethal temperature of conidia of D. natalensis and P. cytosporella were about 75℃and 70℃maintaining 10 min separately, and the inhibition effect of 25% imidazole was the best among the tested pestcides.2. Isolates which could control Diplodia and Phomopsis stem-end rot were screened separately. 9 strains exhibited inhibitory activity to D. natalensis Evans and 7 strains exhibited inhibitory activitory to P. cytosporella by primary and further screening in vitro. It was found that isolate Pg2-5 was most effective against D. natalensis and H-20 was the best one against P. cytosporella by in vivo challenge inoculation. The disease incidence rates were 20.00% and 30.00% separately, which were 56.67%, 50% lower than the control correspondingly.3. Inhibition effects of antagonists with different concentrations were tested. The results showed that: the biocontrol activity was positively correlated to the concentration of antagonistic yeast, negatively correlated to the time of incubation at an extent. When concentration of pathogen was 10⁷ cells·mL^-1 and antagonist concentration was 10⁸ cells·mL^-1, the disease incidence rate of Pg2-5 against Diplodia stem-end rot was 20.00%, and that of H-20 was 30.00%, while the control of that of two diseases were both 76.67%. When concentration of pathogen was 10⁷ cells·mL^-1 and antagonist concentrations were 10⁷ cells·mL^-1 and 10⁶ cells·mL^-1 separately, though the disease incidence rates went climb, they were significantly lower than the control.4.Antagonistic isolates were identified. Isolates Pg2-5 and H-20 were identified by morphological and physiological testing. Pg2-5 belonged to Candida, Candida ernobii and H-20 was Pichia Pichia anomala.