Rice Blast Disease Control Mechanism And Fungicide Resistance Of Tricyclazole

Tncyclazole, a non-fungicidal chemical, which has no direct inhibitory effects on growth or germination at disease control concentrations but can strongly inhibit melanization both in mycelia and appressoria of fungi, is applied in agricultural practices only for rice blast disease control. Some researches report that trcicyclazole is an anti-penetrant chemical, which prevents penetration into host epidermal by infection peg from appressoria in Magnaporthe grisea through blocking melanin biosynthesis in appressoria. This mean tricyclazole is a stringent protectant that can only be used before the infection initiation. But, rice blast disease is a typical multi-cycle disease that occurs all the rice growth season and a wonderful efficacy can be achieved by 2-3 treatments per season. Meanwhile, this disease control mechanism also cannot explain why tricyclaozle could not control disease caused by Colletotrichum lagenarium, for which melanin is also an indispensable pathogenicity factor just as in M. grisea. Moreover, polyketides pathway in fungi is the origin of many metabolites, toxins, and fats for example, other than melanin. And thiazole structure that shows special anti-fungi activity is contained in tricyclazole in contrast to the other melanin biosynthesis inhibitors (MBIs) such as pyroquilon. All these suggest that tricyclazole might have some else action modes besides anti-penetration originated from devoid of enough mechanic force in infection peg because melanization is blocked and glycerol cannot accumulate in appressoria.Anti-fungi activities of tricyclazole were assessed first. Results showed that tricyclazole could strongly inhibit pigmentations including melanization in many plant pathogenic fungi, Gibberella zeae, and Fusarium oxysporum f.sp. vansinfectum for example . Meanwhile, tricyclazole had no direct inhibitory effect on growth, spore germination and appressoria formation and direct inhibitory effect on melanin biosynthesis in C. lagenarium was observed. But there was significant difference in sensitivity of melanin biosynthesis to tricyclazole between isolates. The minimum inhibition concentration (MIC) of hyphal melanization(MIC-H) by tricyclazole in isolate NJ-1 was≤ 1 μ g/mL, but isolates NJ-0 with a MIC-H 50μ g/mL. Tricyclazole also had inhibitoryeffect on sporulation and penetrating into cellophane. A satisfied control efficacy could be obtained when cucumber plants were treated with tricyclazole at a concentration much higher than ten folds of MIC-H. These suggested that inhibiting melanization was the major action mode for tricyclazole and the low sensitivity of melanin biosynthesis in C lagenarium to inhibitor tricyclazole was one of the major reasons that lead to the unsatisfied anthracnose disease control.Effect of tricyclazole on secondary infection by rice blast fungus and mechanism of this inhibitory effect were studied by imitating the orbicular process of secondary infection in laboratory. Results showed that the curative efficiency of tricyclazole against rice blast was feeble. The efficiency of 10μg/mL tricyclazole spraying treatment 48 hours after inoculation was only 28.58%. But the secondary infection to nearby untreated healthy plants caused by disease plants treated with 10μg/mL tricyclazole as above was 63.28% less serious than that caused by the untreated disease plants. Further, inhibiting sporulation. hindering liberation and reducing pathogenicity of spores from the treated disease plants were the prominent mechanisms of this effect. This indicated that suppress of secondary infection by tricyclazole could partially counteract the devoid of curative effect.Involvement of active oxygen species especially of superoxide anion radical (O₂^-) in disease control by tricyclazole was studied. Results showed that tricyclazole could significantly induce O₂^- production in rice during rice-blast pathogen interaction but not for cucumber- anthracnose pathogen. O₂^- scavengers, mannitol and ascorbic acid, could directly inhibit the in vitro growth rate of both pathogens and...