Spore Production Status Of Apple Marssonina Leaf Blotch And Chemical Control Of Marssonina Coronaria In Shaanxi Province After Overwintering

Apple Marssonina leaf blotch is one of the diseases that causes early defoliation of apples.When severe,it can cause 87.7%of leaves to fall off（Lee et al.2018）,affecting fruit yield,quality,and flower bud differentiation in the coming year.In recent years,apple Marssonina leaf blotch has gradually become a major disease in the main apple production areas of China,occurring to varying degrees in various regions of Shaanxi,and the degree of harm is increasing year by year.At present,there are still many unclear areas in our research on apple Marssonina leaf blotch.The overwintering site and overwintering morphology of the brown spot pathogen,as well as the spore production and formation conditions of conidia and ascospore discs after overwintering,need to be explored and studied.Therefore,this study first studies and explores the overwintering survival dynamics and field spore production dynamics of the brown spot pathogen,and clarifies its overwintering patterns.Furthermore,research will be conducted on the screening of efficient,low toxicity,and safe chemical control agents to systematically improve the effectiveness of apple Marssonina leaf blotch control.The main research results of this paper are as follows:（1）Clarified the overwintering morphology of Marssonina coronaria in Shaanxi region.The study found that before overwintering in late autumn,a large number of microspores and a small number of conidia were formed on the diseased leaves of apple Marssonina leaf blotch,and no ascospore was found.A large number of component spores and ascospore can be observed on the visible diseased leaves in the early spring of next year,indicating that the brown spot pathogen in Shaanxi overwinters in the form of mycelium clusters,and may form a large number of conidial and ascospore disks during the overwintering process.（2）Clarified the soil depth at which pathogenic fungi can overwinter,as well as the quantitative relationship between the production of conidia and ascospores in different depths of soil.The spore production rate of diseased leaves conidia among different treatments is:0cm above ground>1 cm below ground>3 cm below ground>5 cm below ground.It indicates that the diseased leaves on the surface of the orchard are the most susceptible to spore production and are the main primary source of infection of apple Marssonina leaf blotch in the following spring;The detection rate of sporangium of diseased leaves among different treatments was:5 cm underground>3 cm underground>1 cm underground>0 cm above ground.（3）Clarified the time required for the germination of conidia produced by overwintering diseased leaves,and the effect of soil depth on the germination rate of conidia.The soil depth has a significant impact on spore germination.The relationship between its germination rate is:0 cm above ground≈5 cm below ground>3 cm below ground>1 cm below ground.（4）By conducting indoor toxicity tests on different types of pesticides,the inhibitory effects of several pesticides on the growth of brown spot pathogen were determined.The results showed that pyraclostrobin had good inhibitory effects on the germination of conidia and mycelial growth of Marssonina coronaria,and had both protective and therapeutic effects.Its EC₅₀ values were 1.796μg/m L and 0.466μg/m L,respectively.The inhibitory effects of difenoconazole,tebuconazole,and picoxystrobin on mycelial growth are better than those on spore germination,indicating that they have good therapeutic effects.Among them,difenoconazole has the best inhibitory effect on mycelial growth,with an EC₅₀ value of 0.016μg/m L;Metiram has a significant inhibitory effect on the germination of conidia,but a poor inhibitory effect on mycelial growth,with a good protective effect,with EC₅₀ of 1.349μg/m L and 23.438μg/m L,respectively.（5）Protective agents with excellent control effects were screened through field experiments.Among the six protective preparations,2.5 times the amount of bordeaux mixture and 2 times the amount of bordeaux mixture had the best control effect,with control effects reaching 94.247%and 93.489%,and there was no significant difference between the two.The80%mancozeb with 800 times the liquid has a better prevention effect;The overall prevention effect of metiram and nano-copper hydroxide is relatively low;The control effect that can be killed is the worst,only 37.230%.（6）Based on the results of field control and indoor toxicity testing,it is recommended to use protective fungicides such as 2 times the amount of Bordeaux solution and 800 times the amount of Metiram during the spore infection period and early stages of the disease for control.During the peak period of the disease,internal absorption fungicides such as 42.4%pyraclostrobin-fluconazole suspension 2000-fold solution,20%fluconazole amide-Benzimidazole suspension 4000-fold solution,and 25%propiconazole 2000-fold solution are selected for prevention and treatment.But it is necessary to avoid the use of single fungicides for a long time and try to rotate different fungicides for prevention and control.