An infection model of apple white rot based on conidial germination and appressorium formation of Botryosphaeria dothidea

Regression models for determining infection periods of apple white rot were developed based on conidial germination and appressorium formation of Botryosphaeria dothidea. A total of 120 apple fruits were inoculated with the fungal conidial suspension and subjected to 6 temperatures and 10 wetness periods. Conidia germinated and produced appressoria, exhibiting swollen tips of germ tubes on the fruit surface. Conidial germination (G) increased with temperature (T) and wetness period (W), and was described as G = -89.273 + 7.649T + 7.056W - 0.109T² - 0.085W² - 0.066TW (R² = 0.75). Less than 2 hr of wetness period were enough for conidia to germinate at 25 to 30°C. Effects of temperature and wetness period on appressorium formation (A) could be explained as A = -1.540 - 2.375W + 0.045W² + 0.213TW (R² = 0.17). The relationship between conidial germination and appressorium formation (AJ was described as Ag = 0.381 - 0.227G + 0.005G² (R² = 0.67), suggesting that conidial germination may have to reach approximately 43.7% to initiate appressorium formation. Using the regression equation for conidial germination and the criterion of 43.7% conidial germination, an infection model was developed to determine infection periods based on temperature and wetness period. The infection model with the criterion of 43.7% conidial germination was apparently more conservative than the appressorium formation model in determining possibility of apple infection. The infection model seemed sensitive to variable weather conditions, suggesting possible use of the model for timing fungicide sprays to control white rot of apples in practice.