Effect of Environmental Conditions on Germination and Survival of Teliosporesand Basidiospores of the Pear Rust Fungus (Gymnosporangium asiaticum)

Pear rust, caused by Gymnosporangium asiaticum, is an important disease of pear (Pyrus spp.) in China in areas where cypress (Juniperus spp. and Sabina spp.), the alternate host, is present. Controlled environment experiments were conducted to study the effects of temperature, relative humidity (RH) and duration of free water on germination of teliospores and basidiospores of G. asiaticum. Teliospores, sampled from Juniperus chinensis, germinated at temperatures ranging from 5 to 28 , with an optimum between 16 and 20 . For teliospores to germinate, telial horns required free water: soaking in water for 30 sec triggered teliospore germination. After initial wetting, RH had little effect on germination of teliospores except at extreme temperatures (5 and 30 ) where germination needed near-saturation moisture. The minimum time for telial horns to produce basidiospores was about 3 h at optimum temperatures. Teliospores in all telial horns germinated at 12--4 , but at 8 and 28 only those from a third of telial horns germinated. Basidiospores germinated at 5--0 with the optimum around 15 ; they needed free water or saturated moisture to germinate. Germination in free water was about eight times greater than at 100% RH. Logistic models described well the germination dynamics of basidiospores. Basidiospore survival declined exponentially with storage time, and linearly with increasing temperature and decreasing RH. Basidiospores survived for at least six days in dry conditions with RH as low as 45%.