Margaret Lauterbach

Margaret Lauterbach: Viruses, leafcutter bees among drawbacks to Treasure Valley’s arid climate

The Treasure Valley’s climate usually keeps us from many plant diseases common in the rest of the U.S., but we have some diseases and plant problems that may be unfamiliar to newcomers. Specifically, curly top virus, Iris Yellow Spot Virus (IYSV) and the fact that leafcutter bees attack plants here.

Leafcutter bees do cosmetic damage to leaves such as lilac, redbud, green ash, Virginia creeper and roses, cutting perfect semicircles from the edges of the leaves. The holes are about a half-inch in diameter. Bees then take those segments back to their solitary nests to create nest cells, each containing an egg and food for the larva when it emerges.

Leafcutter bees resemble honey bees in appearance, but sting only when handled, and then the sting is very mild. They build nests in woody plant pith such as pruned large rose canes, in rotted wood, or in bee boards set up by farmers to house these workers that are invaluable in pollinating alfalfa (lucerne) to produce seeds. Odd narrow wooden “houses” in fields contain bee boards.

Since damage is only cosmetic and the pollinators are so valued, control is not an issue. Appearance of the cut leaves may be alarming to folks who haven’t seen them before, though.

Iris Yellow Spot virus (IYSV) was first found in Idaho in 1989, and has since spread to Oregon, Washington, Utah, California, Nevada, Colorado, Arizona, New Mexico and Georgia. It’s spread by onion thrips, and while not usually fatal to plants, it greatly reduces plant vigor, and in the case of onions, bulb size.

IYSV unchecked has resulted in 100 percent crop losses in a few onion fields.

The disease overwinters and attacks onions and other allium species, iris, lisianthus, redroot pigweed, jimsonweed (Datura) and Nicotiana species. Its presence weakens plants and increases their vulnerability to other diseases. Once plants are infected, there’s no cure. Chives, garlic and leeks may show its presence.

Prevention includes providing good growing conditions, crop rotation, control of thrips and weeds, and destruction of infected plants and cull onions.

Yellow to straw-colored lesions on leaves and/or stalks is evidence of the disease. Lesions may be roughly oval or diamond-shaped. Western Flower thrips don’t carry the disease; only onion thrips do. When the larvae of onion thrips catch the disease, they have it throughout their lives.

Curly top virus is spread by beet leafhoppers that can ride the winds for up to 200 miles. They may infect many vegetable garden and ornamental plants, but we usually first notice it on our tomato plants. Leaves become coarse to the touch and turn purplish, and the top leaves curl upward. Don’t get too upset if you see some tomato leaves curl, but watch for a lot of curling and dieback of parts of the plant.

It often strikes mature fruit-bearing tomato plants, so the death of that plant is tragic. Some gardeners leave the dead plant in the belief that the leafhopper has moved on; others think that the diseased plant can infect other beet leafhoppers and continue to spread the disease, so they pull up the plant and put it in landfill-bound trash.

Distorted leaves indicate some disease on other plants such as beans or squash. It may be this virus or another.

Curly top virus is present in the Intermountain West from Canada into Mexico, and in the eastern Bay Area of California and occasionally into southern Colorado, if the winds blow leafhoppers north from Texas. It’s also present in the arid Middle East.

It’s incurable, but may be prevented by planting resistant varieties, planting under row cover (barring bees and other pollinators too, so you must do their work) or shading vulnerable plants. Keep in mind, though, that most sun-loving plants require six hours of direct sun. Seeds for resistant tomato varieties are only commercially available from Sand Hill Preservation Center in Iowa. Those varieties include Columbia, Latah, Owyhee, Payette, Rowpac and Saladmaster.

Send garden questions to melauter@earthlink.net or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.

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