New insects enter the Valley each year, brought here in travelers’ suitcases, potted plants, cars, on pets, blowing wind, or some fly in. One that’s increasing in numbers is the potato-tomato psyllid, pronounced “sill-id,” the p silent. Commercial potato growers are in the forefront of watching out for this destructive insect, partly because some of them carry a bacterium that transmits a disease called “zebra chip.” Zebra chip results in brown streaks inside a potato, making it unsuitable for commercial sale. This disease was first found in Idaho in 2011.
Tomato-potato psyllids are keel-shaped, like upside-down boats, but are the size of aphids. Nymphs are green, and excrete tiny beads that look like grains of sugar on the undersides of the leaves. Adults have clear wings, held vertically, and their bodies are brown or black, with distinctive white lateral bands on the lower body. Nymphs and adults suck plant juices, secreting toxic saliva that may severely damage tomato or potato plants. At first the margins of the top leaves turn yellow or purple, and tend to curl upward, similar to that of curly top virus.
After the infection starts, the whole top of the plant changes color to yellowish green or reddish purple. The top leaves by that time tend to stand upright, giving the plant an unusual appearance. This stage of the disease is known as “psyllid yellows.”
If the attack occurs early on tomato plants, fruit set may be low or non-existent. The fruit that does set is usually small, soft, and generally poor in quality. If psyllids invade before tubers set on potatoes, it may result in numerous small, malformed tubers, or tubers sprouting prematurely. Unless the psyllid is carrying the Lso bacterium, it won’t transmit zebra chip infection, but its feeding can severely damage the plant and reduce yield. I don’t know what effect the Lso bacterium would have on tomatoes.
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Adult psyllids are difficult to see, since they and their nymphs inhabit the under sides of leaves. The “sugar” like grains excreted by nymphs are a way of detecting their presence, or laying a cloth and tapping plants so the insects fall to the cloth is another way. Once they reach adulthood, they may fly to nearby plants and infest them too. They lay their eggs on short stalks, usually on leaf margins so are best seen in silhouette.
They may not be able to survive our winters, but new waves of them arrive in late spring, especially in our area of Idaho. As one travels eastward in Idaho, there seem to be fewer infestations, especially beyond Twin Falls. They winter in northern Mexico, Texas and the American Southwest, then fly to our area, or are borne by winds.
Since they live and feed on the bottom sides of leaves, control is somewhat difficult. Permethrin or sulfur dust is the usual recommendation for control. Use a garden dust applicator for the sulfur and permethrin dust. Permethrin compounds are also available in other forms. Since destruction may be severe, some folks routinely dust their plants. Psyllids may be found, too, on eggplant and pepper plants, but they usually don’t cause much injury to those plants or their fruits.
If you use permethrin, although it may be plant-based, it is in the pyrethroid family. Just because a pesticide is plant-based, don’t rely on its being safe. Follow label instructions carefully. If you use a permethrin dust, keep pet cats away from it.
Send garden questions to firstname.lastname@example.org or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.