Plant of the Week
Arizona Grape (Vitis arizonica)
By Walter Fertig
Grapes have played an important role in the diet and rituals of humans for millennia. The most widely grown species is the wine grape (Vitis vinifera), originally native to the Mediterranean but transplanted widely across the world in areas with similar climates. Unfortunately, wine grapes are prone to a number of diseases, especially when grown in monocultures. In the 19th Century, grape growers learned to graft flowering stems of Wine grapes onto the rootstalks of other grape species that were resistant to many diseases. Hybrids have also been introduced that have the sweet flavor of the wine grape and the hardiness of other native species.
Keeping domestic grapes secure from disease is an on-going process, however as disease organisms evolve resistance to herbicides and new threats emerge. In recent years, plant scientists have been mapping the genes of wine grape and related species and hybrids to identify where resistance genes are located and how this knowledge might be applied in breeding or genetically manipulating crop species. The Arizona grape (Vitis arizonica) is one of approximately 60 grape species that is being studied extensively to improve wine grapes. Recent research has shown Arizona grape is more resistant to Pierce's disease than cultivated forms. Caused by the bacterium Xylella fastidiosa (Xf) and spread by leaf hoppers, plants infected with Pierce's disease are prone to wilting and death as the water-transporting xylem tissue of woody stems becomes clogged by a gel-like substance produced by the Xf bacteria. Scientists hope that hybrids between Arizona and wine grape will reduce the incidence of the disease.
Arizona grape is native to streambanks and moist shady river canyons of the southwestern United States and northern Mexico. Like other grapes, Arizona grape is a perennial vine with woody branches sprawls over rocks, shrubs, and trees. The stems are able to stay secure because of forked tendrils at their tips. Grapes can be recognized by their broad, palmately-lobed maple-like leaves and their characteristic fruits. The grapes of Vitis arizonica are not particularly tasty to humans and have a leathery skin that does not readily slip free of the juicy pulp, but are relished by birds. Grape flowers are small, greenish, and borne in short panicles. Each has 5 petals that are fused at the tips into a cap that soon falls off to reveal a cluster of stamens (in male plants) or carpels (in female plants); grapes have separate male and female individuals.
A species of seed beetle (Amblycerus vitis) is closely tied to the Arizona grape. Female beetles lay their sticky eggs on the surface of mature grapes. When hatched, the larvae bore into the fruit and consume the contents of one of four seeds inside. Each larva pupates inside a seed before emerging as an adult. Because of its close relationship with the Arizona grape this beetle is only known from river canyons in central and southern Arizona.