T is for tomato

  • Article by: NANCY ROSE , Special to the Star Tribune
  • Updated: March 8, 2005 - 10:00 PM

Tomato breeders seek to incorporate tolerance to major diseases when they develop new tomato cultivars, and these tolerances are indicated by certain letters.

A reader writes: What do the string of letters mean after tomato variety listings in seed catalogs? I thought I remembered that V and F had something to do with diseases but I've also seen other letters as well.

Tomatoes are the most popular homegrown vegetables, but they're susceptible to an array of diseases. Since natural resistance -- or at least tolerance -- to a disease is more preferable than using pesticides, gardeners often seek out disease-tolerant tomatoes. Tomato breeders seek to incorporate tolerance to major diseases when they develop new tomato cultivars, and these tolerances are indicated by certain letters.

Here are the primary letters listed in many seed catalogs:

V indicates tolerance to verticillium wilt, a soil-borne fungal disease that can affect many edible and ornamental plants.

F indicates tolerance to fusarium wilt. Since there are two main races of fusarium wilt, tolerance may be further indicated as F1 for Race 1, F2 for Race 2, or FF for tolerance to both. There are no pesticides to treat verticillium or fusarium; the solution is selecting tolerant tomato cultivars.

TMV indicates tolerance to tobacco mosaic virus. Viral diseases can't be treated with pesticides so natural tolerance is particularly important.

N indicates resistance to nematodes, types of microscopic worms that can damage roots and introduce diseases.

Additional letters you may see listed include: A (alternaria), C (cladosporium), and St (stemphylium or gray leaf spot).

There is a difference between resistant or tolerant. A resistant plant is affected by a given disease very little, while a tolerant plant may show more disease symptoms but is still able to produce a good crop.

Last year, the ends of the branches started turning brown on a small Black Hills spruce in my yard. The spruce is within the drip line of a black walnut tree. Could the walnut be affecting the spruce?

Certain plants, including black walnut (Juglans nigra), contain toxic chemicals that can inhibit the growth of other plants.

Black walnut contains the toxin juglone in all of its plant parts, including roots.

Not all plants are sensitive to juglone, but those that are may be damaged or killed when grown within the root zone of a black walnut. And the root zone can extend well beyond the drip line (outer edge of the branch canopy).

Unfortunately, there is no definitive list of plants that are either sensitive or resistant to juglone. Several lists do exist, but they are based primarily on observation rather than actual testing, so you may find discrepancies between the various lists.

There are some vegetable plants that are well known to be juglone-sensitive, including tomatoes, eggplants and peppers.

Some trees and shrubs that are often listed as being sensitive to black walnut toxicity include: azaleas, basswood, hackberry, lilacs, Scotch pine, white pine, silver maple, potentilla and Norway spruce.

While I did not find Black Hills spruce (Picea glauca var. densata) listed as being sensitive to juglone, it's certainly possible that the browning foliage on your spruce is being caused by black walnut toxicity.

Nancy Rose is a horticulturist with the University of Minnesota Extension Service. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.

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