II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES) – 129
difficult to quantify. The mining activity of these insects may cause photosynthetic
reduction. High populations of leafminers can cause leaf distortion and premature leaf
abscission. Infestation may also predispose the plant to other foliar diseases.
Adult leafminers may be able to transmit viruses, because of their feeding habits.
Spider mite
Spider mites (Tetranychus urticae) are arachnids. Spider mites feed by sucking
the contents from individual leaf cells. The feeding of one mite is not damaging but mites
are usually present in huge numbers. Mite populations explode during hot, dry weather
as they reproduce very rapidly. A female lays an average of 100 eggs and most eggs hatch
within 3 days. Mites can complete a life cycle in 5 days when the temperature is 75°F
or above.
Biotechnological developments
Genetic modification
As noted earlier in this chapter, the cultivated Cucurbita species are important food
sources worldwide. Although some of the wild Cucurbita species have been reported
to display resistance to viral disease, the cultivated Cucurbita display far lower levels of
resistance. This is particularly true of the most economically important of the Cucurbita,
C. pepo, and diseases caused by viruses can result in large economic losses (Provvidenti,
1990). The presence of these viruses has been reported in nearly all countries and
territories where commercial crops of C. pepo are produced: Algeria, Australia, Brazil,
Bulgaria, Canada, China, Costa Rica, the Czech Republic, the Dominican Republic,
Egypt, England, France, Germany, Greece, Guadeloupe, Guam, Japan, Bailiwick of
Jersey, Jordan, Honduras, Islamic Republic of Iran, Israel, Italy, Lebanon, Madagascar,
Malaysia, Martinique, Mauritius, Mayotte, Mexico, Morocco, Nepal, the Netherlands,
New Caledonia, New Zealand, Nigeria, Pakistan, Portugal, Puerto Rico, Saudi Arabia,
Singapore, Spain, Sudan, Swaziland, Syrian Arab Republic, Chinese Taipei, Tunisia,
Turkey, the United States, Bolivarian Republic of Venezuela and Yemen (Desbiez and
Lecoq, 1997).
The primary means of controlling these diseases is control of the insect vectors,
a methodology presenting a less than perfect solution. Due to the importance of the
cultivated species affected by these viruses, and the difficulty in controlling spread of the
viruses, the use of biotechnological techniques to develop resistant varieties has offered
an alternative, successful approach. Cucurbita pepo cultivars containing the transgenes
ZW20 (OECD Unique Identifier SEM-0ZW20-7) and CZW3 (OECD Unique Identifier
SEM-0CZW3-2), have been commercially available since the mid-1990s in the
United States. The ZW20 transgene confers resistance to the zucchini yellow mosaic
virus (ZYMV) and the watermelon mosaic virus (WMV), both members of the potyvirus
group. The CZW3 transgene confers resistance to ZYMV and WMV and to the cucumber
mosaic virus (CMV), the type member of the cucumovirus group. Protection against these
viruses is provided by insertion of DNA sequences encoding the coat protein gene of the
various viruses into the C. pepo genome. Analysis shows that for both ZW20 and CZW3,
a single copy of the transgene has inserted at a single site in the C. pepo genome (USDA,
1994). Although it is now known that protection occurs through interfering RNA
mechanism (RNAi), expression of the transgenes was specifically engineered into the
C. pepo cultivars and is controlled by the 35S promoter of the cauliflower mosaic virus
(CaMV) to allow constitutive expression of the various coat proteins.
