Effects Of Chemical Inducers On Resistance Induction Of Cucumber Against Diseases

The reduced systemic resistance of benzo(1,2,3)thiadiazole-7-carbothioic acid S-methylester (BTH), phosphate (K2HPO4), isonicotinic acid(INA) and Ferrous sulfate(FeSO4) were screened and bioassayed to Cladosporium cucumerinum, Pseudoperonospora cubensis,　Sphaerotheca fuliginea and Colletotrichum lagenarium in cucumber. The inhibition activities in vitro of these inducers, the histopathology of the host plant and the ultrastructural changes were investigated in order to reveal their resistance mechanism. The results showed that BTH and K2HPO4 expressed good effects of induced resistance and could be used as the inducers against the fungal diseases in cucumber. BTH could induce the cucumber to render resistance against Cladosporium cucumerinum, Pseudoperonospora cubensis,　Sphaerotheca fuliginea and Colletotrichum lagenarium at various concentrations, among which the induced resistance in cucumber treated with 0.3~0.7mmol/L BTH had the obvious effects for these diseases. After treatment with BTH in cucumber, the areas and numbers of lesion reduced, the disease severity decreased. The colony thickness of the powdery mildew reduced, and infection type was lower than control. The cucumber expressed apparent induced resistance effect when the leaves were inoculated at the first day after treatment with BTH, and the effect even better when inoculation at the third day. The duration of the induction could remain at least 9 days. K2HPO4 could also induce the resistance in cucumber at various concentrations. The disease severity decreased after treatment with K2HPO4 Comparatively; the effects sprayed with K2HPO4 were not as good as those with BTH. As for the different diseases, the best induced concentration of K2HPO4 varied, and the expression of induced resistance was later than BTH. The third day expressed better effect usually after induced and duration of the induction could remain about 5 days. At the same time, the result also showed that BTH and K2HPO4 had induction of systemic resistance in cucumber against downy mildew, powdery mildew and anthracnose. The colony diameters in Cladosporium cucumerinum and Colletotrichum lagenarium on the culture medium with BTH and K2HPO4 and their influence on the conidium germination of the two fungi were measured. The results demonstrated that BTH had no direct inhibition to the spore germination, mycelial growth and colony form at all the concentrations. K2HPO4 showed no inhibition to the mycelial growth, but had some degree of inhibition to the spore germination, and the greater the density, the stronger the inhibition. Histopathology of BTH and K2HPO4 on the development of Colletotrichum lagenarium was examined by light microscopy. The result showed that BTH had no direct inhibition to the spore germination. However, K2HPO4 had some degree of inhibition, and some of appressoria became malformed. The form of infection vesicle could be restricted obviously after treatment with BTH, whose effect was better than that of K2HPO4. The growth of infection hyphae was slowly and the number was reduced in treated leaves. Especially in BTH treated tissues, only a few of hyphae could be observed. Moreover, they concentrated in epidermal cells. The changes of the pathogen and the host during the expression of induced resistance by BTH and K2HPO4 in cucumber were observed through electron microscopy. The results indicated that hyphae treated with inducers were inhibited, and necrotized. The main expression in cell included the vacuolation of hyphal cells, organelle disintegration, and cellular tissue collapse. The reaction of the host cells to the pathogen invasion included the cell wall thickening and staining more densely, some papillary material deposit under cell wall, some host cytoplasm were disorganized and chloroplast vacuolated. Eventually, the host cells were disintegrated and necrotized.