- OOMYCETE DISEASES OF CUCURBITS 297
which may vary from colorless to deep purple, depending on the inten-
sity of sporulation. Severe infection results in leaves that are completely
dead and curled up. Death of the leaves exposes the fruit to sun scald,
which results in reductions in both quality and quantity of marketable
yield.
The key to identifying downy mildew is observing the signs (sporan-
giophores and sporangia) of the pathogen (Colucci and Holmes 2010).
Sporangiophores and sporangia are most noticeable during humid con-
ditions on the underside of the leaf. Color of sporangiophores and spo-
rangia ranges from colorless to gray-brown to deep purple. The color
depends on the density and age of the sporangia that darken with age.
Sporangiophores and sporangia can be observed using a 20×hand lens.
- Pathogen Biology. Pseudoperonospora cubensisis an obligate par-
asite (Palti 1975; Cohen 1981; Thomas 1996; Colucci and Holmes
2010). Dichotomously branched, aerial sporangiophores, 180–600×
5.4–7.2μm, are produced in small groups that emerge from stomata
of the infected host. A lemon-shaped sporangium, measuring 20–40×
14–25μm, is borne on each subacute tip of the sporangiophore. Each
sporangium has a papilla at their distal end. Sporangia appear smooth
under the compound microscope, but with the scanning electron micro-
scope the finely decorated surface of the sporangium is evident (Colucci
2008).
Sporangia produce zoospores, which are 10–13μm in diameter (Palti
1975; Thomas 1996). Moisture prompts the sporangia to release 5–15
zoospores. Zoospore encysts (produce a cell wall) and then form a
germ tube (50–95μm), which penetrates the stomatal opening of the
host plant tissue. Within the leaf, nonseptate, hyaline, intercellular
mycelium develops in the mesophyll and to a lesser extent in the pal-
isade tissues. Haustoria are formed within the host cells and allow for
the absorption of nutrients.
Pseudoperonospora cubensis rarely produces oospores; oospores
have only been reported from China, India, Israel, Italy, Japan, and Rus-
sia (Colucci and Holmes 2010; Cohen et al. 2011). Oospores are yellow
or hyaline, 22–42μm in diameter. Germination of oospores has not been
observed. - Host Range. Pseudoperonospora cubensisinfects only members of
the Cucurbitaceae family (Palti 1974; Palti and Cohen 1980; Thomas
1996; Colucci 2008; Colucci and Holmes 2010). Forty species in 20 gen-
era in the Cucurbitaceae family are known to be hosts ofP. cubensis.
Downy mildew occurs onCitrullus,Cucumis,andCucurbitaspecies
worldwide. Host specialization within members of Cucurbitaceae