Bioactivity And Molecular Toxicology Of Azoxystrobin Against Colletotrichum Capsici

Azoxystrobin, developed by Syngenta(Zeneca) company, was the first product of thestrobilurin fungicides. It combines high level and broad-spectrum of fungicidal activitywith excellent safety in the environment. Strobilurin fungicides inhibit mitochondrialrespiration in fungi at a specific site on complexⅢand break energy synthesization offungi. Strobilurins were called QoIs as they combined at Qo site of cyt b. This kind offungicides was widely used because of their novel action mechanism and non-cross-resistance with any other presented fungicides. However, the frequency of spontaneousresistance mutation was high according to several research reports. It's easy to formresistant-population under high fungicidal selection pressure.Capsicum anthracnose (Colletotrichum capsici and C. gloeosporioides) is one of theserious diseases on China capsicum production. Aimed to control the popularity ofcapsicum anthracnose, fungicidal workers are searching for new fungicides. So, it isessential to embark on bioactivity and toxicology at molecular biochemical level ofazoxystrobin to Colletotrichum capsici.Azoxystrobin worked alone did not apply good inhibition on mycelial growth and sporegermination. Sensitivity of Colletotrichum capsici to azoxystrobin could be reduced by theinduction of alternative respiration. In the absence of a host, alternative respiration provideda＞95-fold rescue potential from azoxystrobin inhibition during the germination of conidiaderived from five wild-type isolates of Colletotrichum capsici. And for mycelial growth,the rescue potential was 20-35 folds. SHAM had synergistic effect on the bioactivity ofazoxystrobin. But, azoxystrobin could strongly inhibit conidia production and delay thepigment formation.Methods of inoculation including spraying, smearing and piercing the sensitive seeds andfruits of capsicum with conidia suspension were compared. As a result, piercing the ripenfruits was the best way, so this method was used to test the protective and curative activityof azoxystrobin. Pronounced effect of alternative respiration during conidial germination invitro was not apparent when capsicums were treated with azoxystrobin prior to inoculation with conidia suspension. Azoxystrobin possesses excellent protective, curative, eradicativeand antisporulant activity. When the capsicums were inoculated 1 day after a 10μg/mLazoxystrobin treatment, the control efficacy was high to 95.5%; while the capsicums wereinoculated 1 day before a 10μg/mL azoxystrobin treatment, the control efficacy was only85%. Germination vitality and pathogenicity of conidia derived from the lesions underazoxystrobin treatment decreased, but the sensitivity to azoxystrobin changed little.Both C. gloeosporioides and C. capsici cause capsicum anthracnose. 78 single-sporedisolates causing capsicum anthracnose collected from five different places of HainanProvince were identified into Colletotrichum gloeosporioides and C. capsica based on themorphological characteristics. Of these isolates, 94.9% were C.gloeosporioides and 5.1%were C.capsici. Sensitivity of these isolates to azoxystrobin was determined with sporegermination test on water agar (WA) in vitro. Results showed no resistance isolates wereobserved after using azoxystrobin for several times, though the sensitivity of the isolateschanged a little, still not prominent. The sensitivity of the offspring of the most sensitiveand most tolerant isolates differentiated, respectively, but there were exactly inheritedsensitivity differences between the most sensitive and most tolerant isolates.Oxygen consumption test of mycelia of ColIetotrichum capsici showed that azoxystrobinstrongly inhibited mycelial respiration after 1h of the treatment at 10μg/mL. As treatmenttime went on, the respiration inhibition on the mycelia decreased. Results showed that thesensitivity of mycelium respiration to azoxystrobin decrease was not caused by induction ofalternative respiration or degrade of the fungicide in medium. Transcription levels of cyt bgene in mycelia of Colletotrichum capsici what was treated with azoxystrobin at 10μg/mLfor 0, 1, 12, 48 hrs respectively was consistent with the effect of azoxystrobin on mycelialrespiration. When the mycelia was treated with azoxystrobin for 1h, the transcription levelsof cyt b gene decreased, and 12 hrs later, the transcription strengthened, after the myceliawas treated for 48 hrs, the contents of mRNA of cyt b gene was even more than the control.This result showed there might be some relationship between the responses on mycelialrespiration and transcription levels of cyt b gene. Test of transcription levels of alternativeoxydase(AOX) gene of Colletotrichum capsici mycelia what was treated with azoxystrobinfor 0, 1, 12, 48 hrs respectively showed that after 12 hrs treatment, the transcription levelsof AOX gene decreased prominently. But 48 hrs later, the transcription levels of AOX generecovered, with almost no differences between the treated one and the control. Azoxystrobin inhibited mycelial growth in Botrytis cinerea, Rhizoctonia solani andMagnaporthe grisea respectively; it also inhibited spore germination, conidia production inBotrytis cinerea, Magnaporthe grisea and sclerotia forming in Rhizoctonia solani.Moreover, it lagged pigment biosynthesis in Magnaporthe grisea somehow. SHAMenhanced inhibition by azoxystrobin. Oxygen consuming test of the mycelia showed theconsistence in responses between Botrytis cinerea, Magnaporthe grisea and Colletotrichumcapsici when treated with azoxystrobin and SHAM, while in Rhizoctonia solani,azoxystrobin expressed inhibition all along..