| Colletotrichum leaf disease,mainly caused by Colletotrichzun gloeosporioides(C.gloeosporioides),is a serious disease of rubber tree in most rubber-growing countries.C.gloeosporioides has a variety of lifestyle patterns,including necrotrophic,hemibiotrophic,latent or quiescent and endophytic patterns.Therefore,biological control and cultivation management cannot effectively alleviate the occurrence of Colletotrichum leaf disease(CLD),and the control of CLD of rubber trees currently still depends on few fungicides such as zineb,carbendazim and prochloraz.To delay high risks of selecting for resistant fungal pathogens,it is important to test the baseline sensitivity and efficacy of the fungicides from different classes agasint C.gloeosporioides isolates for use on rubber tree to manage CLD.Penthiopyrad is a new succinate dehydrogenase inhibitor(SDHI)that recently has been registered for use as a fungicide in many countries.The current reported studies are still lacking in the systematic bioassay of penthiopyrad against C.gloeosporioides.Thus,the aims of the present research were to screen out the new fungicide for tropical crop diseases and solve resistant problem.The definite mode of action of SDHIs is helpful to show the effects of SDHI fungicides better.The results are as follows:91 isloates were separated from the immature leaves of rubber trees with typical symptoms of CLD from fields in Hainan,Yunnan,and Guangdong provinces of China by the tissue separation method.These isolates exhibited stable growth and normal morphology and were used to establish the group of C.gloeosporioides.The EC50 values of penthiopyrad against 91 C.gloeosporioides isolates ranged from 0.243 to 3.377 mg/L,with a mean EC50 value of 1.685 mg/L.The frequency distribution of the EC50 values for penthiopyrad was described by a unimodal curve.The baseline sensitivity curves were approximately normally distributed.No mutants resistant to penthiopyrad in the tested strains were found.C.siarnense isolate CS23 and C.fructicola isolate CF26 were used to determine inhibitory effects of penthiopyrad against C.gloeosporioides.Penthiopyrad greatly decreased the mycelial weight,showing that penthiopyrad significantly inhibited the vegetative growth of C.gloeosporioides.Penthiopyrad greatly decreased the mycelial weight of the isolates CS23 and CF26 grown by 38.56%and 65.19%.The distribution of septum of mycelia was not affected by penthiopyrad,but the mycelia were severely distorted,and the number of top branches increased with penthiopyrad treatment.As a SDHI fungicide,penthiopyrad inhibited the activities of SDH and energy synthesis of pathogens.As expected,penthiopyrad inhibited the SDH activities,causing a decrease in the ATP content of C.gloeosporioides.Penthiopyrad also exhibit strong activity against the mycelial growth of pathogens by inhibiting SDH activity,resulting in damaging the cell membrane.Penthiopyrad at 100 mg/L hardly inhibited conidium germination of C.gloeosporioides in vitro.Penthiopyrad positively and stablely inhibited conidia germination,tube growth and appressorium formation of C.gloeosporioides in vivo.Only 26.90%and 20.19%conidia formed appressorium at 12 h inoculation on penthiopyrad treated leaves with 5 and 10 mg/L penthiopyrad.Penthiopyrad also increased the Infection time of C.gloeosporioides on rubber tree leaves.The protective and curative activity of penthiopyrad against CLD with mycelium and spore infection of the isolate CS23 was determined on the light green leaves of rubber trees.The protective and curative effects increased with increasing concentrations of penthiopyrad.The protective and curative activity of 100 mg/L penthiopyrad against CLD with mycelium infection reached 98.53%and 66.79%,respectively.At the time of spore infection,the disease index of CLD was reduced to 3.70%with 100 mg/L penthiopyrad,indicating that penthiopyrad showed the good protective and curative effects.At the same concentration,the protective effect of penthiopyrad was higher than the curative effect. |
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