Evaluation of Effect of Different Fungicides on Fungus Podosphaera leucotricha, the Causal Agent of Apple Powdery Mildew Disease

Introduction: Apple (Malus domestica, Borkh) is considered one of the most common popular and favorite deciduous fruit trees cultivated in Iran. Various harmful factors affect the performance of this fruit. Apple powdery mildew disease is one of the most important apple diseases that has a worldwide distribution and causes disease and is caused by the fungus Podosphaera leucotricha Ell. Et Ev. This fungus is an obligate parasite and it can attack to leaves, flowers, fruits and twigs. In the beginning of spring the disease appeared on leaves which are the most susceptible organs. The disease appears on the upperside of infected leaf as powdery white lesions and eventually the infected part of leaf turn brown and infections on the underside of infected leaf result in chlorotic patches. Infected leaves become crinkle, curl and drop prematurely. Although blossom and fruit infections are less common, they are important because infected fruits are small and stunted if they do not drop. P. leucotricha survives the winter as mycelium in vegetative tissues or in infected flower buds. The primary infection starts when infected buds break dormancy and fungus resumes growth and colonizes developing shoots. Spores growing on infected shoots spread nearby and initiate secondary infections. Also it causes early loss of leaves and stop the growth of diseased branches and as a result, the loss of yield. In heavily infected trees, rust can be seen on fruit surface. Powdery mildew infection usually occurs at relative humidity above 70%, and on days when humidity is low, the infection usually occurs at night or in the early morning hours when the humidity is high. Although the use of effective fungicides can control the disease well but appearance of resistant strains of pathogens to reduce fungicide efficiency in controlling disease and producers' access to effective fungicides from various chemical groups, while helping gardeners to reduce the economic damage caused by the disease, reduces the possibility of pathogen resistance to fungicides. The aim of this study was to determine the efficacy of Boscalid + Pyraclovastrobin (Bellis ® WG38%) (Manufactured by BASF Co.) (With doses of 0.4, 0.7 and 1 ml L-1) compared with Tri-floxystrobin + Fluopyram (Luna Sensation ®) (with a dose of 0.2 ml L-1), Tri-floxystrobin (Flint ® WG50%) (0.2 ml L-1) and Tri-floxystrobin + Teboconazol (Nativo ® WG50%) (With a dose of 0.2 ml L-1) to control apple powdery mildew disease. Materials and Methods: For the experiment, the apple orchard of Golden Smooth cultivar with a history of powdery mildew in Ardabil province was selected. The experiment was carried out in a randomized complete block design (RCBD) with 8 treatments and 4 replications. Control treatments were without any spraying and with water spraying. Treatments were applied at three stages (full green bud stage and followed up at pink flowers stage and 10 days after the 2nd spraying). Ten days after the first symptoms of the disease were observed on the control treatments, samples were taken from the sheets and the percentage of the disease incidence and disease severity percentage were calculated. After calculating the incidence and disease severity of apple powdery mildew for each plot, the corresponding values in SAS statistical software were analyzed and the means of both traits were compared by Duncan's multiple range test at one percent probability level. Results and Discussion: The results of analysis of variance of the data obtained from the evaluation of the leaves of the treated trees showed that the effect of treatments on reducing the percentage of disease severity and disease incidence is statistically significant. The results showed that Bellis ® fungicide with a concentration of 0.1 and 0.7 ml L-1, Luna Sensation ® 0.2 ml L-1, Nativo ® 0.2 ml L-1 and Flint ® with a concentration of 0.2 ml L-1 had a high efficiency of controlling apple powdery mildew disease. The efficacy of new Bellis ® fungicide with concentrations of 1 and 0.7 ml L-1 was 76 and 60 percent, respectively. According to the obtained results, Bliss fungicide with a dose of 0.4 per thousand had poor efficacy in controlling the severity and occurrence of the disease and was not statistically significantly different from the control treatments. The results also showed that there was no significant difference between the control treatments (treatment with water spraying and treatment without spraying). Conclusion: Because both Bellis ® concentrations are effective in controlling the disease, therefore in order to protect the health of the fungicide users, the consumer of sprayed products and the environment as well as reduction in costs, the preferred dose is 0.7 ml L-1.