Effect of temperature and inoculum density on disease intensity of phytophthora parsiana

The effects of inoculum density and temperature on the disease intensity of Phytophthoraparsiana on almond seedlings were investigated. Almond seeds (Rabie and Kaghazi cultivars) were placed in moist vermiculite at 4°C for 45 days. Germinated seeds were sown in a soil: sand mixture (2:1 v/v) and grown in greenhouse (18°C-25°C). One-month-old seedlings were transferred from the greenhouse to the growth chambers set at 15, 18, 20, 25,30 and 32°C. The seedlings were subsequently inoculated either with mycelium of P. parsiana grown for 4-6 weeks on vermiculite amended with hemp seed extract or with zoospore (103, 104, 105 and 106 ml-1) by root dip method. The effect of temperature, inoculum density and their interaction on seedling mortality was measured. The results indicated that all three factors had significant effects on seedling mortality. While the highest disease incidence (100% mortality in almond seedlings) occurred at 30°C and 32°C, no mortality was observed at 15°C and18°C. Increasing temperature from 20°C to 30°C and inoculum rate from 103 to 106 zoospores ml-1 increased disease incidence significantly. Higher temperatures and inoculum densities also caused significant increases in the colonization level of the crown, main and lateral roots as well as reductions in the fresh and dry root weights of the seedlings.