Utah Pests News Summer 2009

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Curly Top Virus Beginning to Show Up in Southern Utah 

  Even a “resistant” tomato variety such as Rowpac, can become infected with curly top virus.
In the Washington County Extension office, we have already received reports of Curly Top Virus (CTV) from home gardeners.  Many call it “the blight” for lack of another term, which often causes confusion in diagnosis.  CTV is vectored by the beet leafhopper (Circulifer tenellus).  During their migration in early spring, leafhoppers feed on infected weeds such as lambsquarter, halogeton, Russian thistle, and four wing saltbrush.  They then transfer the disease with their piercing-sucking mouthparts to healthy garden plants such as tomato.

Past experience in southern Utah has shown that disease incidence increases as temperatures warm into late May and early June.  Several factors likely account for the increase.  As temperatures rise, the weed hosts begin to dry, making them less suitable for feeding.  This probably encourages leafhoppers to move onto more desirable species.  In addition, high winds in spring may blow leafhoppers long distances into commercial fields and residential home gardens.

Plants infected with CTV appear yellow and stunted.  Tomato leaves become thicker, and roll upward.  Veins on infected leaves may turn a purple color.  These symptoms are quite different from other common “blight” diseases of tomato such as early blight and late blight where dark spots with a yellow halo appear.  Early blight-infected plants commonly show lower leaf dieback, as the symptoms progress slowly toward the top of the plant.  Late blight-infected plants generally decline quickly and symptoms appear widespread on the plant.  In the case of these blights, overhead watering will increase the spread of disease.  With CTV, water is not as issue, and although overhead watering is not recommended on vegetables, it does not play a role in spreading this disease.

Things growers can do to decrease the chances of curly top virus infection:

  • As much as practical, locate vegetable plantings as far from weed hosts as possible.
  • Place row cover materials such as remay or similar lightweight cloth over plants to exclude beet leafhoppers.
  • Leafhoppers prefer sunny areas, so planting in shade may decrease the chance transmission from feeding.
  • “Double plant,” or increase plant density to lower the probability that every plant will be infected, allowing some plants to survive without decimating the entire field.
  • Use one or more resistant tomato varieties, including Rowpac, Roza, Salad Master, or Colombian.  In our USU trials some of these varieties did get CTV. Since there are several known strains, and viruses historically mutate quite easily, the use of resistant tomato varieties may lower incidence but will not likely be a silver bullet for this disease.

Since curly top virus is so sporadic it is a very difficult disease to study and predict. Dr. John Damicone with Oklahoma State University has written several bulletins on CTV.  He reported a couple of years ago that he had set up some research to evaluate several of the control methods mentioned above, only to have a season with almost no CTV in the region.  It is the same sporadic nature of the disease that makes it difficult to time insecticide sprays against leafhopper that can give effective control.  This is why insecticides are not recommended as a control option.

-Rick Heflebower, Washington County Extension
Horticulture Agent, extension.usu.edu/washington