Effect of environmental factors and apple varieties on the pathogenic activity of virulent and attenuated Alternaria mali strains

【Objective】The apple early leaf blight disease is one of the major diseases caused by the pathogen of Alternaria mali, which has widely distributed in the main apple- producing regions worldwide and posed a serious threat to both the yield and quality of apple. This disease is caused by A. mali strains with varying degrees of virulence. Virulent A. mali strain leads to spot leaf blight disease, which affects leaves and younger shoots, influencing the growth of flower buds and fruit. Attenuated A. mali strain causes the target spot disease, which mainly affects the leaves and results in large lesions on the fruit. Environmental factors, especially temperature and humidity, play the significant role in impacting the disease occurrence. The aims for the present study were to clarify the effects of different environmental factors (temperature, relative humidity and light conditions) and apple cultivars on the latent periodand pathogenicity of both virulent and attenuated A. mali strains.【Methods】In this study, the spray inoculation method was used to evaluate the effects of different environmental factors and apple varieties on the pathogenic activity of virulent and attenuated strains of A. mali. Healthy apple branches with consistent growth and diameter were collected and placed into sterile flasks containing sterile water, with five branches per flask. The virulent and attenuated strains of A. mali were inoculated by spraying spore suspensions (1×105 conidia ·mL- 1 ) onto the front and back of leaves growing on the branches, with sterile water as the control. In the temperature experiment, the treatment and control groups were cultured with a relative humidity of 90% and at temperatures of 15 ℃, 20 ℃, 25 ℃, 30 ℃ and 35 ℃ (16 h light/8 h dark), with three repetitions for each group. In the humidity experiment, the inoculated branches were cultured at 25 ℃ under 16 h light/8 h dark conditions, and the relative humidity was adjusted by modifying the concentration of H2SO4 to 60%, 70%, 80%, 90% and 100%. In the light condition experiment, the inoculated branches were placed at 25 ℃ and 90% humidity and exposed to various light treatments: continuous light, continuous darkness, 12 h light/12 h dark cycles, 3 h UV irradiation and 21 h light. In the variety experiment, branches of Starkrimson, Fuji and Golden Delicious were placed in sterile water, and the A. mali spore suspension was sprayed onto the leaves, with three repetitions for each variety, and the branches were cultured at 25 ℃ with 90% humidity under 16 h light/8 h dark conditions. After inoculation, the incubation period was observed every 6 hours and the disease index were recorded after 7 days.【Results】Different temperatures, relative humidity and light conditions all exerted varying degrees of influence on the pathogenic activity of virulent and attenuated strains of A. mali, with the virulent strain exhibiting a significantly shorter incubation period and higher disease severity than the attenuated strain averagely. Under different temperature conditions, at 30 ℃, both the virulent and attenuated strains of A. mali had the shortest incubation periods, being 54 h and 72 h, respectively. Within the temperature range from 20 ℃ to 30 ℃, disease severity was more pronounced after inoculation, with the virulent strain showing the highest pathogenic activity at 25 ℃, where the disease index reached 23.06 and the attenuated strain showing the highest activity at 30 ℃, with a disease index of 17.53. However, at 15 ℃, the incubation period was the longest for both strains, exceeding 100 h and the disease severity on the leaves was the slightest, with disease index of 5.86 and 8.42, respectively. Under different humidity conditions, with 60% relative humidity, the incubation period for both strains was the longest, 120 h, while at 100% relative humidity, the incubation period was the shortest, only 72 h. Additionally, with 60% relative humidity, the disease index for both strains were the lowest, being 10.88 and 9.42, respectively, while with 90% relative humidity, the disease index were the highest, being 19.01 and 12.50. Under different light conditions, both strains had the shortest incubation periods under alternating light/dark cycles, UV irradiation and continuous light. The virulent strain had a 60 h incubation period under these conditions, while the attenuated strain had a 72 h incubation period. However, under alternating light/ dark cycles, the virulent strain had the highest disease index of 22.59, while under UV irradiation and continuous light conditions, the attenuated strain had the highest disease index of 21.24. Moreover, significant differences were observed in the pathogenic activity of the virulent and attenuated strains among different apple varieties. The pathogenicity of both strains on Fuji was generally lower than that on Starkrimson and Golden Delicious, with disease index of 14.13 and 8.30, respectively. Compared to the virulent and attenuated strain pathogenicity on Starkrimson, their pathogenicity on Golden Delicious was stronger, with the highest disease index being 16.82 and 22.09, respectively. The disease resistance evaluations showed that Fuji exhibited moderate resistance to the attenuated strain of A. mali and resistance to the virulent strain, while Starkrimson and Golden Delicious both displayed resistance to both strains.【Conclusion】Different temperatures, relative humidity, light conditions and apple cultivars had varying degrees of influence on the incubation period and pathogenicity of virulent and attenuated strains of A. mali. It was found that when the temperature was between 25-30 ℃, relative humidity was 90%-100% and the light conditions included alternation of light and darkness, UV irradiation and continuous light, the incubation period of A. mali virulent and attenuated strains on Starkrimson was shorter, and pathogenicity was stronger. Additionally, Fuji showed better disease resistance to these strains. These findings can provide a theoretical basis for the scientific and rational control of apple early leaf blight disease.