122 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
approaches to addressing the economic losses associated with certain of the viruses
causing disease in Cucurbita.
Animals
Cultivated Cucurbita are bred to express only very low levels of cucurbitacins, and
are far more palatable to humans and other animals than wild Cucurbita. In many regions
of the world, for example, fruits of the cultivated Cucurbita are used as fodder. In tropical
regions, domesticated animals such as donkeys and horses will consume Cucurbita fruits
and vines when fodder is scarce at the end of the rainy season (Mariano and Dirzo, 2002).
Common pests and pathogens
This section lists some of the common pests and pathogens of Cucurbita. It is not
an exhaustive list.
Viruses
Although only a dozen problem viral variants have been identified, these variants are
serious problems for the crops due to the rate of disease spread, the severity of infection,
the potential for large economic losses and the difficulty in controlling the diseases.
These viral diseases are particularly important due to the susceptibility of the plants
to attacks by virus-transmitting insect vectors such as whiteflies, aphids and chrysomelid
beetles.
Cucumber mosaic virus (CMV)
This Cucumovirus has worldwide distribution and the widest host range of any plant
virus, including more than 1 200 species in over 100 families of dicotyledonous and
monocotyledonous angiosperms. The host range includes cereals, forages, woody and
herbaceous ornamental, vegetable and fruit crops such as squash, melons, peppers, beans,
tomatoes, carrots, celery, lettuce, spinach and beets, various weeds and many ornamentals
and bedding plants. Symptoms seen in infections of the virus include leaf mosaic or
mottling, yellowing, ringspots, stunting and leaf, flower and fruit distortion.
CMV can be vectored by 60-80 different aphid species in a non-persistent manner
from plant to plant in a stylet-borne fashion. The peach (Myzus persicae) and melon
(Aphis gossypii) aphids are the primary CMV vectors. CMV can also be transmitted in
seeds, and by the parasitic weeds, Cuscuta sp., as well as mechanically by humans
cultivating or touching healthy plants after touching infected plants. It can also be carried
by the striped and 12-spotted cucumber beetles but the transmission success rate under
field conditions makes these insects minor contributors to CMV infection. Many variants
of the virus occur, and it is difficult to identify CMV from symptoms alone.
CMV produces a systemic infection in most host plants. Older tissues and organs that
developed prior to infection usually are not affected by the virus, but newer cells and
tissues that develop after infection may be affected with varying severity. Leaves of
infected plants become mottled and vines are stunted. The concentration of the virus
increases for several days following inoculation, then decreases until it levels off or the
plant dies (Agrios, 1997). The virus can overwinter in perennial weeds, flower and food
crops by surviving in the roots.
