Effects of Environmental Factors on Spore Germination of Valsa mali

Apple Valsa canker, caused by Valsa mali, is a serious disease affecting apple trees. Saprophytic colonization is a crucial stage in the infection process, with spore germination being a prerequisite for this phase. The effects of nutrients, temperature, moisture, pH, and light on the germination of both conidia and ascospores of V. mali were investigated under controlled environmental conditions. The results showed that the germination of V. mali spores requires supplementary nutrients, and compound nutrients are more conducive to spore germination. In distilled water, neither conidia nor ascospores germinated, resulting in a germination rate of 0%. In a sugar solution, the maximum germination rate remained below 75%. However, germination rates reached 100% in media supplemented with bark extract, fruit extract, or potato dextrose culture medium. The temperature range for conidial germination was 3.1 to 34.8°C, with an optimum of 25.6°C. Conidial germination required a duration of wetness or relative humidity exceeding 97% to be sustained for at least 7.6 h. The suitable pH range for conidial germination was 4 to 8. In contrast, ascospore germination occurred over a broader temperature range of 0.6 to 41.7°C, with an optimum of 25.6°C. Ascospore germination required a relative humidity above 90% for at least 6.8 h and was supported by a wider pH range of 4 to 10. Light significantly inhibited conidial germination but had no observable effect on ascospore germination. These results can be used to construct a prediction model that estimates the amount and timing of V. mali colonization on branch surfaces based on environmental and nutritional conditions. This prediction model can then inform the timing and methods for implementing apple Valsa canker control programs more effectively.