Sugar beet leaves covered by sexual (cleistothecia) and asexual forms (mycelia and conidia) of Erysiphe betae were gathered at harvest time from a field next to Ghazvin Sugar Factory and maintained under natural outdoor conditions. In order to determine the function of cleistothecia and also conidia in the survival of E.betae some experiments were performed. The results indicated that ascospores were unable to be released in petri dishes but their release under natural conditions occurred after 4 months. Under In vitro conditions ascospores did not germinate but on the leaves germination was somehow and rarely possible, however these ascospores were degraded after 7 days and didn’t exhibit any pathogenicity. Conidia could cause pathogenicity within 3 but not after 4 months. The period after inoculation until appearence of disease symptoms was lenghthened with aging of conidia. The results for conidial germination showed that fresh conidia had 80 percent germination on the lamme but it decreased sharply after 2 weeks and reached 0 percent after 4 weeks. Although germination of fresh conidia on leaves was not more than 46 percent but they germinated well within 2 and 4 weeks.The results for the experiment to observe the first appearance of the disease in the field suggested that the first conidia were trapped by spore-trap in early June and the first symptoms appeared in 20 days. The conclusive results showed that ascospores had no function in the survival of the fungus. Although conidia maintained their pathogenicity for after 3 months but due to the 8- month interval between sowing time and appearance of disease symptoms in temperate zones(Karaj and Ghazvin) it doesn’t seem that these conidia have a role in transferring disease from one year to another.
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