It is a worldwide-distributed obligate parasite. Pathogen’s sporangia germinate only on wet leaf surface, enters the leaf via stomata, and establishes a particular form that enables the pathogen to feed and transfer signals from host cells. Its sexual reproduction spores are Oospores, while the asexual reproduction spores are called Zoospores. Downy mildew is a severe fungal-like disease, especially in areas with frequent rainfalls and high humidity. Pathogen’s spores are dispersed by wind (conidia). Symptoms of downy mildew may be confused with powdery mildew spots, in early stages. Downy mildew symptoms on cucurbits differs from downy mildew of other vegetables. An outbreak of the disease is expected when leaves maintained wet for at least 5 to 6 hours and when enough inoculum is available for the initial infection. In warmer environments, the parasite survives at volunteer and/or cultivated plants in the field, while in colder areas it survives in plants of greenhouse or conidia are coming with the air from warmer (south) climates. It causes devastating crop losses of cucumbers in greenhouses. Despite the fact that downy mildew on cucumbers is an important pathogen of cucurbits in Europe, America had controlled the disease successfully until 2004 with resistant varieties. Now, the resistant varieties are no longer effective, so the problem reappeared. Moreover, fungicide- resistant populations have been found. The most important way to identify the pathogen is by observing its sporangia (signs) on the leaf’s underside during wet conditions.
Leaves: It creates angular(typical symptom on cucumber, squash, pumpkin leaf) or round light green at first and then chlorotic spots/ lesions enclosed in leaf veins(interveinal chlorosis) that eventually become necrotic. Sometimes, symptoms look like virus (mosaic) or powdery mildew infection. Spots differ in their morphology and size, between Cucurbitaceae hosts. A white cottony fungal sporulation that turns black or deep purple(depends on sporangia age), appears on the underside of the leaf (warm/humid night) (under every spot). Before the appearance of the growth, the pathogen develops water-soaked lesions underneath them. Extensive symptoms can lead to leaf necrosis, curling and defoliation. (“Wildfire” symptom- leaves appeared as if they had been burned)
Hosts: cucumber, pumpkin, zucchini, melon, watermelon
Palti J, Cohen Y. Publication of the Agricultural Research Organization No. 196-E, 1980 series. Vol. 8, Phytoparasitica. 1980.
“Vegetable diseases”- Panagopoulos G.C