Grape downy mildew is heavily influenced by climatic conditions. The disease thrives in cool and humid environments. For oocysts, the minimum germination temperature is 13°C, with an optimal range of 25°C, and a maximum of 33°C. Sufficient moisture is essential, particularly after spring rains. Sporangia germinate best between 10–15°C, with a minimum of 5°C and a maximum of 27°C, and they also require water droplets for activation. Mycelial growth occurs most effectively between 7–8°C and 29°C. Spore formation requires high humidity (95–100%) and temperatures ranging from 13 to 28°C. Under dry and high-temperature conditions, sporangia can only survive for 14–16 days. The infection process is most active between 12–13°C and 29°C, with an ideal range of 18–24°C. Young leaves need at least 70–80% humidity to become infected, while older leaves require 80–100% humidity. In general, grape downy mildew tends to be more severe in areas with less wind, frequent fog, heavy dew, and higher rainfall during spring and autumn.
Orchard conditions also play a significant role. If the orchard is consistently cold and damp, with dense foliage and low trellis structures, poor air circulation and limited light penetration can lead to increased disease severity. This environment creates the perfect setting for the pathogen to spread and thrive.
The susceptibility of grape varieties varies significantly. American grape cultivars are generally more resistant to downy mildew, while European types are more prone to infection. These differences in resistance can be attributed to various factors, including genetic makeup, leaf structure, and physiological characteristics. Additionally, the mineral composition of the plant plays a crucial role. Grape plants that have higher levels of calcium tend to be more resistant to disease. The balance between calcium and potassium in the cell sap is particularly important: when the ratio of calcium to potassium is greater than 1, the plant is more resistant; if it is less than 1, the plant becomes more vulnerable to infection. Understanding these factors helps growers make informed decisions about variety selection and cultural practices to manage the disease effectively.
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