By Alfredo Flores
Agricultural Research Service (ARS) scientists in Florida have made
recommendations to help growers deal with several whitefly-transmitted
viruses that threaten cucurbits and other crops in that state.
In recent years, the number of whitefly-transmitted viruses in
cucurbit fields, home to crops like cucumbers, squashes, pumpkins, melons
and watermelons, has increased to almost epidemic proportions in Florida.

Researchers led by plant pathologists Scott Adkins and Bill Turechek
at the ARS Subtropical Plant Pathology Research Unit in Fort Pierce, Fla.,
are dealing with a "triple threat" to cucurbits: three major viruses, all
transmitted by silverleaf whitefly, Bemisia tabaci. The host range is
similar?mostly cucurbits?but the symptoms differ. And the researchers have
discovered a new host for the whiteflies: green beans.

In January 2008, Adkins and Turechek released a report that revealed
that the Cucurbit Leaf Crumple Virus (CuLCrV ) has been detected in green
beans in southwest Florida, the first report of CuLCrV infecting any host
other than cucurbits in Florida. Symptoms on green bean plants included leaf
deformation, especially of younger leaves, and a mild or chlorotic mosaic, a
blotching of the plant?s skin.

The discovery of CuLCrV on green bean plants in Hendry County in
southwest Florida in mid-December 2007 suggests that CuLCrV may be more
widely distributed than previously known in Florida, and that green beans,
and potentially other legumes, may become an important reservoir host of
CuLCrV in the state. Previous reports of CuLCrV affecting non-cucurbit
plants include beans and tobacco in the late 1990s in California, Arizona,
Texas and Mexico.

Another of the 'triple threats' is Squash Vein Yellowing Virus
(SqVYV), whose hosts include cucurbits, especially squash and watermelon.
SqVYV causes death in young watermelon plants, death of vines of older
plants and other problems. Trials of SqVYV resistance are currently being
conducted by grafting watermelon germplasm onto gourd rootstock and
evaluating the watermelon scions for symptoms.

The third of the threats is Cucurbit Yellow Stunting Disorder Virus
(CYSDV), which was first seen in Florida in 2007 and infects melons,
cucumbers, gourds and squash, with symptoms including yellowing of the leaf
veins and leaves.

The recommendations the ARS researchers made for controlling these
viruses include selecting the most vigorous and well-adapted varieties;
using whitefly- and pathogen-free transplants; using reflective mulches that
repel aphids; and maintaining a host-free period between spring and fall
crops.


[www.ars.usda.gov]