Study on the overwintering of cleistothecia and conidia of Erysiphe betae causal agent of sugar beet powdery mildew in Iran.

Sugar beet leaves covered by sexual (cleistothecia) and asexual forms (mycelia and conidia) of Erysiphe betae were gathered at harvest time and maintained under natural outdoor conditions. In order to determine the function of cleistothecia and also conidia in the overwintering of E. betae some experiments were performed. The results showed 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 rarely possible, however these ascospores were degraded after 7 days and didn't have pathogenicity. Conidia could induce pathogenicity after 3 but not 4 months. The period after inoculation until appearance of disease symptoms increased with aging of conidia. The results for conidial germination showed that fresh conidia had 80 percent germination on glass slides but it decreased sharply after 2 weeks and reached to 0 percent after 4 weeks. Although their germination on the leaves was not more than 46 percent of fresh conidia but they had good germination after 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 20 days later. The conclusive results showed that ascospores had no function in the survival of the fungus and air-borne conidia are the main source of primary infections.