296 M. BABADOOST
contaminated water (e.g., pound with run-off water from infested
fields), and fungicide application at weekly intervals beginning with
the first sign of the disease. Well-drained fields should be selected and
soil moisture should be kept as low as possible. Soil movement between
fields should be avoided. Infected plants should be removed from the
field and destroyed.
B. Downy Mildew
Downy mildew, caused byPseudoperonospora cubensis,isoneofthe
most important diseases of cucurbits (Cohen 1981; Waterhouse and
Brothers 1981; Thomas 1996; Colucci and Holmes 2010). The disease
occurs in temperate and tropical areas with high leaf wetness periods.
Without effective control measures, downy mildew causes considerable
losses in cucurbit crops. In 2004, the Eastern United States experienced
a major downy mildew epidemic on cucumber that resulted in an eco-
nomic loss of $16 million (Colucci and Holmes 2010). The disease is a
major concern for cucurbit growers throughout the eastern half of the
United States.
Downy mildew was first reported by Berkeley and Curtis in 1868
in Cuba (Colucci and Holmes 2010). The pathogen was namedPer-
onospora cubensis. In 1902, Berlese subdivided the genusPlasmopara
Schroeter and placedPeronospora cubensis Berk. and Curt. in the
subgenusPeronoplasmopara. In 1903, Rostovtsev in Russia proposed
the new genusPseudoperonosporaand since thenPseudoperonospora
cubensishas been the accepted name of the cucurbit downy pathogen.
- Symptoms.Downy mildew affects cucurbits plants of all ages
(Cohen 1981; Sherf and MacNab 1986; Thomas 1996; Colucci and
Holmes 2010). Symptoms are almost exclusively confined to the leaves,
although there are reports of sporulation of the pathogen on melon fruit
and floral parts. The appearance of symptoms varies considerably on
different cucurbit crops (Palti 1974; Thomas 1996).
Symptoms of downy mildew appear as small, slightly chlorotic to
bright yellow areas on the upper leaf surface; the color is less evident
on the corresponding lower leaf surface (Colucci and Holmes 2010).
Lesions appear first on the older leaves and appear progressively on
the younger, more distal leaves as these leaves expand. Symptoms on
cucumber and squash are angular lesions and are bound by the leaf
veins. On watermelon and cantaloupe, symptoms are irregular-shaped
lesions on the leaves and affected leaves turn brown rapidly. When con-
ditions favor sporulation of the pathogen, sporangia are produced on the
lower leaf surface giving the undersides of lesions a downy appearance,