Symptoms of co-infection of Iris yellow spot virus (IYSV) and downy mildew on onion scapes (flower stalks).

Symptoms of Iris yellow spot virus (IYSV) and downy mildew were observed in June 2008 on hybrid seed onion scapes. IYSV has become a production constraint to both onion bulb and seed crops in the Treasure Valley (southwest Idaho and eastern Oregon). First described from this region in the 1990s, serious sporadic epidemics have been reported in recent years. Characteristic symptoms of IYSV on scapes include straw-colored, dry, necrotic, spindle- or diamond-shaped lesions. Some lesions have distinct green islands surrounded by necrotic borders. Downy mildew (Peronospora destructor) is characterized by pale-green, yellowish to brownish areas of irregular size and shape (oval to cylindrical) on infected seed stalks. These symptoms could be confused with those caused by thrips damage. The fuzzy, velvety growth of the causal fungus appears under conditions of high humidity, leading to alternating yellow and green regions of tissue. As the lesions expand and coalesce causing extensive drying of the tissue, secondary saprophytic organisms such as Stemphylium, Cladosporium, and Alternaria spp. rapidly colonize the scapes (causing purple to dark spots) masking downy mildew. Co-infection by these two obligate pathogens, followed by saprophytes, can often lead to girdling and toppling of scapes.