Objective  To study the germination and infecting dynamics of sporangia of Peronophythora litchi under different temperature(θ) and wetness duration(t), construct mathematical models based on θ and t, and provide references for precision prediction, prevention and control of litchi downy blight.

Method  Under controlled temperature and wetness, the effects of temperature (15 to 30 ℃) and wetness duration (3 to 24 h) on sporangia germination of P. litchii and its infection on litchi were studied. Using the modified Weibull model, the sporangia germination model and disease severity model with good accuracies were fitted in SAS software. The corresponding contour risk prediction maps were drawn.

Result  Sporangia germination and disease severity were significantly affected by temperature, wetness duration and their interactions. The sporangia germination rate and disease severity increased along with the prolongation of wetness duration. At the temperature of 25 ℃, the sporangia germination rate reached the highest for all wetness duration treatments. In the same wetness duration, the disease severity of litchi fruit was the highest at 25 ℃. When the temperature was 15−30 ℃ and the wetness duration was 2−3 h, the disease severity of litchi leaf was above 0.2. The sporangia infected fruits seriously and quickly, and the disease severity of fruit was above 0.6 under the temperature of 22−30 ℃ and the wetness duration of 3−24 h. The disease severity models of leaf and fruit due to infection byP. litchii were f(t, θ)=1−exp−(0.194 3×t)²/cosh(θ−27.769 6)×0.927 7/2 andf(t, θ)=1−exp−(0.469 3×t)²/cosh(θ−24.556)×0.170 9/2, respectively.

Conclusion  The occurrence of litchi downy blight depends on temperature and wetness duration. Our models can be used to guide the time and frequency of fungicide application for disease prevention and control.