[Arid_gardener] watch your tomato plants!

[Clair Spackman]

This from pro-med.

A new plant virus identified in fall 2006 in Arizona threatens not only 
home gardens, but also the commercial tomato industry in the state. Tomato 
yellow leaf curl virus, also known as TYLCV, was found in 2 home gardens in 
the Phoenix area.

Tomato plants infected by the TYLCV develop severely curled, yellowing 
leaves, shattered nodes, and short stalks. The virus causes many of the 
flowers to abort, lowering fruit set and reducing yields. Symptoms are most 
apparent on the growing tips of plants, where the newest growth is taking 
place.

"You'll see very small, stunted plants that aren't going to be able to 
produce fruit -- not good enough for shipping and storage," says Judith 
Brown, a virologist and whitefly vector biologist in the department of 
plant sciences at the University of Arizona. "The virus stresses the plants 
beyond belief, and they simply stop growing."

Brown isolated DNA and identified the virus from plant samples submitted by 
Mary Ann Garewal, a UA-certified master gardener in Surprise, who observed 
symptoms and collected samples from her vegetable garden and those of 
another Phoenix gardener. At about the same time, Brown also identified a 
distinctly different isolate of the virus on infected commercial tomato 
plant samples from Sonora, Mexico, suggesting that the 2 isolates came from 
different sources.

The plant samples from both Arizona (USA) and Mexico were also infested 
with the "B" biotype of the sweet potato whitefly, _Bemicia tabaci_, which 
feeds on the leaves and can transmit the virus to other plants through its 
saliva.

TYLCV 1st infected tomatoes in the western hemisphere in the Caribbean and 
along the eastern coasts of both the United States and Mexico in the early 
90s. "It seemed to be contained in those areas until it was introduced in 
Texas last summer," Brown says. The source of that outbreak was traced to 
infected commercial transplants that were possibly infected by the virus in 
plants brought in from out of state.

TYLCV has also been tracked from the east coast states of Mexico to the 
Pacific coast production area in the state of Sinaloa, where the virus 
ravaged commercial tomato crops throughout the state during spring 2005 and 
again in fall and spring 2006, according to Brown. "In the summer of 2006 
we next found it in Sonora," she says. "Now it's becoming a giant epidemic 
across the Pacific coastal Mexican states, causing heavy yield losses in 
fresh-market tomatoes and peppers that are exported to the United States."

Brown notes that the TYLCV identified in Arizona did not come from Mexico. 
DNA testing matches it with isolates from Texas. When viruses pass through 
different hosts, particularly ornamentals or new hosts, they can mutate 
into different isolates and then be introduced back into a crop. "It's a 
very prolific virus," Brown says. "If TYLCV infects a variety of tomato 
that is not resistant, you'll have a huge problem. I don't think any of the 
varieties grown hydroponically in greenhouses are resistant, although some 
of the field processing varieties are. None of the current home garden 
varieties are resistant."

TYLCV infects not only tomatoes, but also beans and peppers in home 
gardens, and some ornamentals.

Brown advises against spraying whiteflies to control the virus because by 
the time the symptoms appear it is too late to stop the disease. She adds 
that there are currently no chemical or biological controls for TYLCV. "The 
best approach is to avoid introducing the virus in transplants. Virus-free 
seedlings are the way to go," she says. "You're safest if you buy 
virus-free plants or start your own from seed -- although that still may 
not help if your garden or crop is near an infected field or a neighbor's 
garden that harbors the virus."

For more information, contact Judith K Brown, at 520-621-1402, 
<jbrown at ag.arizona.edu>.

Clair Thunes, PhD.

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