Abstract:【Objective】The apple early leaf blight disease is one of the major diseases that caused by the pathogrn of Alternaria mali, which has widely distributed in the main apple-producing regions worldwide and poseed a serious threat to both the yield and quality of apple. This disease is caused by A. mali strains with varying virulence. Virulent A. mali strain lead 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 affecting the leaves and resulting in large lesions on the fruit. Environmental factors, especially temperature and humidity, play the significant role in impactinh 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 period and pathogenicity of both virulent and attenuated A. mali strains.【Methods】This study used the spray inoculation method 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) onto the front and back of the branches' leaves, with sterile water as the control. In the temperature experiment, the treatment and control groups were cultured at a relative humidity of 90% 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 ℃ with 16 h light/8 h dark conditions, and 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 ℃, 90% humidity and 16 h light/8 h dark conditions. After inoculation, the incubation period was observed every 6 hours and disease indices were recorded after 7 days.【Results】Different temperatures, relative humidity and light conditions all have 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. Under different temperature conditions, at 30 ℃, both the virulent and attenuated strains of A. mali had the shortest incubation periods, 54 h and 72 h, respectively. Within the temperature range of 20 ℃ to 30 ℃, disease severity was more pronounced after inoculation, with the virulent strain showing the strongest pathogenic activity at 25 ℃, where the disease index reached 23.06 and the attenuated strain showing the strongest 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 lightest, with disease indices of 5.86 and 8.42, respectively. Under different humidity conditions, at 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, 72 h. Additionally, under 60% relative humidity, the disease indices for both strains were the lowest, 10.88 and 9.42, respectively, while at 90% relative humidity, the disease indices were the highest, 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, 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 on different apple varieties. The pathogenicity of both strains on Fuji was generally lower than on Starkrimson and Golden Delicious, with disease indices of 14.13 and 8.30, respectively. Compared to the virulent and attenuated strains' pathogenicity on Starkrimson, their pathogenicity on Golden Delicious was stronger, with the highest disease indices 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 have 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 is between 25-30 ℃, relative humidity is 90-100% and the light conditions include full light, 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 provide a theoretical basis for the scientific and rational control of apple early leaf blight disease.
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