The Colorado Commissioner of Agriculture enacted a quarantine in 1946 prohibiting the importation and planting of nectarines and white-flesh, clingstone and Rochester peaches in Mesa, Montrose and Delta counties. Known as Quarantine Order "G", this legislation has been updated several times to protect the peach industry in Western Colorado from Peach Mosaic Virus (PMV). The Montrose and Delta county quarantine was dropped in the early 70's. The Mesa county quarantine remains in effect. Mesa county is the major peach growing area in Colorado.

PMV first appeared in Mesa County Colorado in 1931. By 1934 this disease became serious enough to warrant an organized inspection and control program. This resulted in the removal of 6,000 trees. In 1935, '36, '37 and '38 respectively, 32,000, 10,457, 3,517 and 5,094 infected trees were located and destroyed. By 1992, 143,979 PMV infected peach trees had been destroyed. The Mesa County peach industry lost considerable income due to this virus.

PMV was identified in Texas in 1931. It also was identified in Arizona, Arkansas, southern California, New Mexico, Oklahoma, Utah and parts of Mexico. In California, 204,000 peach trees were destroyed in the first five years of control. In the 1980s only a few confirmed cases being reported. Eradication efforts have not been implemented in Mexico and some areas have high levels of infection.

Peach trees with large, solid pink blossoms, have unusual patterns on the petals when infected with PMV. Irregular white streaks, spots or lines create a mottled appearance. Leaf and twig growth is retarded and leaves may drop early. Diseased trees may be dwarfed.

Peach mosaic infection causes small, blotchy leaves, short stubby shoots, and small, bumpy, poorly flavored fruit. Chlorotic leaves develop during spring and summer on susceptible cultivars. Tolerant cultivars may show faint vein clearing or yellow flecks in the leaves. Fruit may ripen several days to a week later than fruit on non-infected trees. Infected trees are worthless and serve as sources of infection to non-infected trees.

Peach mosaic virus is spread by Eriophyes insidiosus, the peach bud mite. This Eriophyid mite feeds and reproduces in the bud on embryotic leaves. E. insidiosus is one-quarter the size of a spider mite and is carried from tree to tree by wind. Eliminating the mite is nearly impossible. Prebloom and petal-fall miticide applications would be needed on an area-wide basis to control the spread of the mite. Destroying infected trees which exhibit symptoms of the disease is critical to preventing spread of PMV.

Many new freestone peach and nectarine cultivars are available which exhibit symptoms of peach mosaic and have been declared legal to plant in Mesa County.

A survey of white-flesh, free-stone peaches and nectarines hs been conducted by Dr. Harold Larsen, Fruit Pathologist, Western Colorado Research Center, Orchard Mesa site. Some white-flesh, freestone peaches as well as yellow and white-flesh nectarines are legal in Mesa County. Not all white-flesh peaches and yellow-flesh nectarines exhibit symptoms of PMV. The cannot be planted in Mesa County. Erly-Red-Fre, a white-flesh, semi-freestone peach, and the yellow-flesh freestone Marigold and Panamint nectarines show inadequate symptoms and their planting is consequently prohibited in Mesa County. The planting of clingstone and ornamental peach trees are prohibited in Mesa County for this same reason.

Orchardists continue to inspect orchards for PMV and destroy trees exhibiting symptoms. Any new cultivars planted in commercial or home orchards should be inspected annually.

References:

Ogawa, J.M., E.I. Zehr, G.W. Bird, D.F. Ritchie, K. Urie, & J.K. Uyemoto (editors). 1995. Compendium of Stone Fruit Diseases. APS Press.
Anonymous. 1976. Virus diseases and noninfectious disorders of stone fruits in North America. USDA. Agricultural Handbook #437.

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