Homegrown: Nov. 5, 2011

The past few couple years I have noticed, half way through the growing season, some of my tomatoes begin to look variegated in color. They seem to have yellow stripes or discolorations throughout and then eventually the plant dies back.

What causes this? Is it contagious to the other plants? Are the tomatoes edible?

— John

The two most common viruses we see are curly top virus and tomato spotted wilt virus, which is sometimes also called tomato ring spot virus.

I tend to see more problems with curly top, but your description sounds like tomato spotted wilt virus.

We’ve had these viruses in western Colorado for years, and their severity seems to vary from year to year.

This past year was pretty bad for tomato problems, including viruses. It seemed I saw more virus samples this year than I have the past couple.

I have to admit that diagnosing virus problems can be difficult. Most virus diseases have a laundry list of possible symptoms and a particular plant may exhibit some but not all of them.

Another infected plant may not show all of the same symptoms of the first plant or some different symptoms.

And, to complicate it further, many different virus diseases share some of the same symptoms.

Many virus diseases are spread by insects. Most viruses need a living host cell to survive and what happens is that a sucking type insect, such as a leafhopper, aphid, or thrip, feeds on an infected plant, ingesting the virus. Then it hops over to an uninfected plant to feed some more.

These insects inject saliva into the plant before feeding and when they do that, they inject the virus into it — these little monsters are efficient syringes.

The last thing about viruses is that we have no cure for them now. Our only way to kill the virus ends up killing the plant. Not all virus diseases will end up killing their host plant but many do, including tomato spotted wilt and curly top.

The bottom line is that once a plant is infected, the plant is going to die and there’s not a darn thing that can be done about it.

Tomato spotted wilt usually begins with dark spots then bronzing of the leaves, tip die-back and stunting of the plant.

The most noticeable symptom of this virus are the yellow to orange spots or rings that appear on the fruit. Though this virus doesn’t affect us and the fruit is safe to eat, those spots turn hard and ruin the fruit.

Tomato spotted wilt is spread by thrips, a tiny insect that’s common in our landscapes.

Unfortunately, controlling thrips is difficult as they tend to nestle inside the little nooks and crannies of a plant, protecting themselves from any sprays.

In addition, they are pretty resistant to many of our current insecticides so even if we can get to them, control isn’t always very good.

However, if you want to try to cut their numbers down, you’ll need to do a series of sprays for them.

You should plan on spraying twice a week for three or four weeks, making sure you thoroughly cover the plant with the spray. You’ll also want to use a variety of different insecticides since any one product probably won’t kill them all.

I recommend using a rotation of Spinosad, Permethrin, Malathion, Bifenthrin and/or Season Long Spray Oil. You don’t have to work in all of these products, but I’d use at least three and use a different one every time you spray.

Another thing that can help reduce the chances of infection is to control any weeds in the area.

Weeds are often a host to thrips and keeping them out should help reduce thrip numbers. You also can remove and dispose of any infected plants as soon as you know they’re infected so they hopefully don’t help spread the virus to neighboring plants.

If you’re unsure if a plant is infected, bring a sample of it to us to look at or take one to the CSU Extension Office at the Mesa County Fairgrounds.

Dennis Hill is the nursery manager at Bookcliff Gardens, http://www.bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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