Curly top virus: Why tomatoes get infected and how to prevent it

The tomatoes are starting to ripen in full force in eastern Idaho. Judging by the calls and emails I get into the office, the diseases that affect tomatoes has been fairly heavy this year.

Tomatoes are subject to a number of diseases in eastern Idaho, partly because they are a close relative of the potato. Next week I will cover late blight and early blight, but today I will discuss curly top virus (CTV) in tomatoes.

CTV is a bit perplexing because, unlike many diseases, it will infect one or two plants in your tomato patch, while the rest look and produce fine. The reason for this is the way it is transmitted. The vector for CTV is the beet leafhopper (BLH) that is native to the western U.S. It thrives on many of our introduced weed species, and then visits cultivated crops as it passes through.

The symptoms in tomatoes show up mostly in the newer leaves, but all parts of the plant are affected.

• Infected young plants will usually die.
• Plants will be stunted from the point of infection forward.
• The new leaves at the top of the plant (thus “curly top virus”) will be small.
• The leaf margin curls up while the petiole and midrib curls downward.
• Usually, but not always, the veins will be purple on the underside of the leaf. Sometimes tomatoes will have purple veins without the virus.
• Upper leaves will become thick and crisp.
• Infected tomato plants will be pale and unhealthy looking, especially the newer leaves.
• Plants can be infected after fruit starts to set.
• Fruit that sets after infection will ripen earlier than normal.
• Poor fruit quality and yield. Fruit that developed before infection will be normal looking.
• Infected plants are usually scattered.

The BLH feeds on over 300 plant species. Some are more susceptible than others, but we tend to notice tomatoes most often. Other common vegetable species that are commonly affect by CTV include peppers, potatoes and sugar beets. Other less-affected crops include watermelons, beans, spinach, and squash.

The main problem is the weeds that host the BLH and carry the virus. The BLH feeds on an infected plant and the virus incubates inside the BLH, then is transmitted to other plants when it feeds on them. It can continue to transmit the virus for the rest of its life. The virus cannot pass from plant to plant, it must be vectored by the BLH. Spraying your crop for insect control is not effective since the disease is already transmitted before the insect succumbs to the pesticide.

CTV is not carried in the seeds of the plants to the next crop. Utah State University Extension conducted trials and found that those cultivars reported to be resistant yielded no statistically significant difference to the infection rate of other cultivars.

So, what do you do if it shows up, or better yet, how do you prevent it? Plants that are infected will not recover, nor produce well.

• Pull up infected plants. They can be composted.
• Weed control is critical, especially Russian thistle, lambsquarters, kochia, nightshades, and mustards in the garden and areas surrounding the garden.
• Row covers, especially early in the season.
• While insect control on the crop is not effective, insect control on areas surrounding the garden can be beneficial.

If you see a tomato or pepper with curly top virus, there is no point in hoping it will recover and produce something. Pull it up and put together a plan to attack the problem early next year.