Vegetable Disease Identification: Fruit Spots and Rot of Tomatoes

Alan McNab
Professor of Plant Pathology

Tomatoes: Fruit Spots and Rots

Virus-Like Disease are caused by several different viruses and mycoplasma; the diseases are characterized by symptoms common to several of the diseases. This has led to confusion in disease identification. Symptoms are helpful in the identification of a virus-like diseases; however, positive identification requires additional procedures conducted by virus-identification specialists.

Maize Dwarf Mosaic
Maize Chlorotic Dwarf
Corn Stunt
Wheat Streak Mosaic
Maize Streak

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Maize Dwarf Mosaic (MDM) symptoms are distinctive on plants in the pretasseling stage. Affected young plants have a fine stippling of dark green streaks on light-colored young leaves. Upper internodes are shortened. Older plants have yellowish leaves and are stunted; they produce excessive tillers and multiple ear shoots with poor seed set. MDM is caused by a virus that overwinters in grasses. Several virus strains exist. Johnsongrass may be a major overwintering host for most strains; strain B does not overwinter in this host. At least 12 kinds of aphids transmit the virus from infected grasses to corn.

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Maize Chlorotic Dwarf (MCD) symptoms appear first as a yellowing in the whorl. Infected plants then become stunted, usually with reddish leaves. MCD is caused by a virus that overwinters in Johnsongrass. The virus is spread by a leafhopper.

Corn Stunt is thought to be caused by a mycoplasma. Early symptoms are small circular to elongated yellowish spots at the base of leaves in young plants. These spots often coalesce and become olongated stripes that may be discrete or diffuse. As the plant develops, general yellowing, leaf reddening, ear shoot and sucker proliferation, and relatively short internodes become distinctive features o f the disease. Corn stunt is widely distributed in southern areas of the United States. The mycoplasma is transmitted by at least five kinds of leafhoppers; it cannot be spread mechanically and is not seed-borne.

Wheat Streak Mosaic, caused by a virus, is widespread but the little economic importance on corn. Early symptoms are small yellowish spots or broken streaks at the tips of young leaves. The streaks may elongate. Older leaves may become yellowish near the tips. Ears may be poorly developed. The virus is transmitted in the field by the wheat curl mite, and can be mechanically transmitted. The virus overwinters in some cultivated and wild grasses; wheat, oats, barley, rye, stinkgrass, and species of foxtail and panicum are susceptible.

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Maize Streak, caused by a virus, is characterized by narrow yellow streaks distrivuted uniformly over leaf surfaces; some of these streaks may grow together as they expand. Symptoms are most severe on plants that are infected early. Stalk internodes and leaf size are greatly reduced; ears may be partially filled.
As of 1982 the maize streak virus has not been reported in the United States.

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Last modified Tuesday, March 6, 2007