126 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
Pseudoperonospora cubensis
The symptoms caused by P. cubensis are almost exclusively confined to the leaves,
although there are rare reports of sporulation on fruits and floral parts. The first evidence
of infection is small, slightly chlorotic to bright yellow areas on the upper leaf surface;
the colour is less vivid on the lower leaf surface. As lesions expand, they often coalesce,
resulting in necrosis of the infected leaves so that in a few days the entire leaf is dead.
This disease is commonly referred to as downy mildew (Agrios, 1997; Lebeda and
Wedrlechner, 2004; American Phytopathological Society, 2011; OMAFRA, 2011).
Pythium spp.
These soil-borne micro-organisms can cause damping-off, with young seedlings
wilting or not emerging at all (Agrios, 1997; American Phytopathological Society, 2011;
OMAFRA, 2011).
Sphaerotheca fuliginea
S. fuliginea causes a powdery mildew wherein whitish, talcum-like, powdery fungal
growth develops on both upper and lower leaf surfaces and on petioles and stems.
Symptoms usually develop first on older leaves, on shaded lower leaves and on the upper
leaf surface. Infected leaves usually die, and plants senesce prematurely reducing
photosynthesis, thereby reducing yield (Agrios, 1997; American Phytopathological
Society, 2011; OMAFRA, 2011).
Bacteria
Erwinia tracheiphila causes bacterial wilt. It is spread by the stripped cucumber
beetle, Diabrotica undecipunctata, and the spotted cucumber beetle, Acalymma vittata,
and controlled by eliminating cucumber beetles. The bacteria live in the digestive tract of
the striped and spotted cucumber beetles. The beetles defecate frass as they feed and
E. tracheiphila invades the plant through the wounds caused by the feeding beetles
(Sasu et al., 2010).
Insects
A number of insects can attack Cucurbita species. Some of the insects listed below
are cosmopolitan and have a worldwide distribution, e.g. Myzus persicae and
Aphis gossypii, while others are more limited in their distribution, e.g. Anasa tristis.
Aphididae
Aphid species most commonly found on Cucurbita include: Aphis gossypii, the melon
aphid; Myzus persicae, the peach aphid; Aphis fabae, the bean aphid; and
Aphis craccivora, the cowpea aphid.
Aphids extract sap from the terminal leaves and stems of plants. They may also feed
on developing pods causing them to shrink or become malformed. Their feeding can
result in deformation, wilting or death of the plant depending on populations and size of
the plant. Saliva injected during feeding can also cause deformation of plant tissue.
While aphids can cause significant damage on their own, they frequently present another
concern: the transmission of several plant viruses.
