Mechanism of Inhibitory Effect of Tricyclazole on Secondary Infection by Spores of <i>Pyricularia oryzae</i>

Abstract

Several fungicides used for the control of rice blast disease have been revealed to inhibit melanin biosynthesis in Pyricularia oryzae at the concentration not affecting vegetative growth of the f ungus. 1, 2) The fungicides including tricyclazole [5-methyl-1, 2, 4-triazolo(3, 4-b) benzothiazole] inhibit appressorial penetration probably due to the lack of melanization in appressoria.3) The other modes of action of tricyclazole in rice blast control were reported as inhibition of secondary infection by spores resulting from suppression of spore formation4) and the decrease of spore pathogenicity4) and from the production of antifungal substance in rice plants. " In this paper we studied the effect of tricyclazole on the appressorial penetration of spores of P. oyyzae, formed de novo in the presence of the chemical.Pyricularia oyyzae Cav., P-2 successively cultured on a V-8 juice agar medium" and on rice plants was used. Rice plants (Oryza sativa cv. , Asahi #4) were grown in pots (9 cm in diameter) in a greenhouse at 25+3C under natural day light. To obtain spores formed on the rice leaves treated with tricyclazole, four week-old healthy plants were put in a moist chamber at 27C together with infected plants as an inoculum for 4 days and then treated with the chemical solution (100 ml per pot) using spray gun; spores formed during incubation for inoculation were washed away during the treatment. The plants treated with the chemical solution were dried and placed in a moist chamber at 30C overnight for formation of spores. Spores formed on the leaves treated with tricyclazole and those formed on the V-8 juice agar medium containing the chemical were collected with a brush and washed three times with deionized sterile water containing 200 , age ml streptomycin (DWS) by centrifugation. The spores were incubated on cellulose membrane and penetration from appressoria was observed as described. 3) The spores formed on the V-8 juice agar medium containing tricyclazole at concentrations higher than 0. 5 ug/ml produced hyaline appressoria on the cellulose membrane resembling those produced from spores incubated in the presence of tricyclazole. The hyaline appressoria did not develop infection hyphae from appressoria and failed to penetrate cellulose membrane (Table 1).Spores formed on the leaves sprayed with 100 ug/ml tricyclazole produced hyaline appressoria with no ability to penetrate cellulose membrane ( Table 2). The frequency of appressorial penetration increased with decreasing tricyclazole concentrations used for the leaf treatment.The results presented in this report indicate that tricyclazole moved systemically into spores formed de novo on the rice leaves treated with tricyclazole and on the V-8 juice agar medium containing the chemical and that tricyclazole concentration in the spores should be high enough to inhibit melanin biosynthesis in appressoria produced from the spores and to inhibit appressorial penetration.This mode

Impact of tricyclazole and azoxystrobin on growth, sporulation and secondary infection of the rice blast fungus, Magnaporthe oryzae

“…Okuno et al observed that reduction in secondary infection after tricyclazole treatment in field conditions may be partially due to reduced formation of conidia on mycelia exposed to tricyclazole. They hypothesised that the effect of tricyclazole on secondary infection was caused by its deposition on Magnaporthe conidia, and at the onset of infection it inhibits appressorium melanisation.…”

“…It was developed more than 35 years ago and has been shown to specifically inhibit the two reduction steps in the DHN‐melanin biosynthesis pathway . The direct effect of tricyclazole on the capability of germinating conidia to penetrate host epidermis is well documented,, , whereas reduced secondary infection observed in the field was inferred to be due to reduced production of conidia or to lower virulence of conidia produced on tricyclazole‐treated lesions , , . Strobilurins, a class of mitochondrial respiration inhibitors, belong to the group of quinone outside inhibitors (QoIs) which inhibit mitochondrial respiration by binding at the Qo site of cytochrome b .…”

Inhibitory Action of Biofungicide Physcion on Initial and Secondary Infection of Magnaporthe oryzae

“…However, physcion did not affect the process at the low concentrations of 1.25-20 mg/l. This response of M. oryzae to physcion was different from its responses to tricyclazole and azoxystrobin, which seriously inhibited conidiation of the fungus (Okuno et al 1983;Kunova et al 2013). This character of physcion might be an obstacle for its effective control on rice blast because it is a polycyclic disease in fields.…”

Effect of Pyroquilon, an Inhibitor of Melanin Synthesis, on Sporulation and Secondary Infection of Magnaporthe grisea

“…Moreover, our study demonstrated that spores which had formed on the pyroquilon-containing meditmi did not show a normal infection behavior and that their infection behavior was impaired even in the absence of pyroquilon. An inhibitory effect on secondary infection of M. grisea has been reported for another inhibitor of melanin synthesis, tricyclazole (Okuno et al, 1983). However, as shown in Table 2, the inhibitory effect on penetration observed in exp.…”