Etiology Of Black Spot Disease On Bagged Apple Fruit

Black spot disease happened only on bagged apples. It has caused seriously economic losses every year in most of apple growing regions in China. Although the spot may happened on different part of apple fruit surface and the symptom is various in color or size, it is generally called black spot disease by farmers. At present, the study of the diversity of the pathogens is incomplicated and the division of the symptoms is confused. It is necessary to study the relationship between pathogens and the disease, which can provide reference for future study.From 2014 to 2016, 515 diseased apple samples were collected from Shaanxi province, and 618 isolates were isolated from diseased tissues. The results were as follows:We divided the symptom of this disease into four different types based on the symptom on Fuji apple fruit which is the most susceptible cultivar. TypeⅠ: black spot on fruit calyx. TypeⅡ: brown spot on fruit surface. TypeⅢ: ashen spot on fruit surface. TypeⅣ: black spot on fruit surface. The rates of the type were 8.7%,64.6%,18.8% and 7.9% respectively.Through the pathogenicity test we found that the pathogen is diversity, inclouding eight genera: Trichothecium, Alternaria, Acremonium, Sarocladium, Phoma, Fusarium, Epicoccum, Penicillium. Among these, Trichothecium has the highest isolation rate(40.2%), followed by Alternaria and Acremonium, reaching 27.5% and 21.2% respectively.Based on phylogenetic analysis and morphological features, we identified there is one species in Trichothecium: T. roseum; one species in Aternaria: Al. alternata sensu lato; two species in Acemonium: Ac. sclerotigenum and Ac. mali(new species); three species in Sarocladium: S. terricola(new record and new pathogen), S. liquanensis and S. mali(new species).The primary pathogens are different in different symptom types. 81.1% of T. roseum was isolated from TypeⅠ; In Type Ⅱ, T. roseum has the highest isolation rate, reaching 42.6%, followed by Al. alternata sensu lato(29.8%) and Ac. sclerotigenum(17.7%); In Type Ⅲ, Ac. sclerotigenum has the highest isolation rate(49.37%) and species in Sarocladium also incloued in this type; In Type Ⅳ, Al. alternate sensu lato, Al. liquanensis is the primary pathogen and the isolation rate reached 79.4%.Spore suspension of five pathogens(T. roseum, Al. alternata sensu lato, Ac. sclerotigenum,S. liquanensis and S. terricola) were used to inoculate apple fruit in different growth stage. When inoculated on immature fruits(in May and June), little black spots(1 mm) were formed. When inoculated at fruit enlargement period(in August and September), brown spots showed on the surface of the fruits and the diameter was 3-5 mm. Only the three major pathogens(T. roseum,Al. alternata sensu lato,Ac. sclerotigenum) can cause disease on unwounded mature fruits when inoculated in the laboratory. And when the fruits were wounded all pathogens can cause disease even though the symptoms were different for different pathogens.Ac. sclerotigenum was used to do conidial germination and inoculation experiments at different temperature and relative humidity. The results showed that there were 92% spores germinated at free water, but the germination rate can reach 30% only if the relative humidity above 97%. It means that the sports can germinat in relative high humidity, and free water is not necessary. As for the inoculation rate, it rose as the raise of relative humidity, and it can reach a high rate only if the relative humidity above 94%. Spores can germinate well from 20°C to 30°C. When the temperature was above 35°C, the germination rate fell significantly. It means that high temperature is not good for spores to germinate. As for the inoculation rate, it can reach more than 80% in the temperature of 10°C- 35°C.