Primary symptoms

Viral diseases or those caused by mollicutes (spiroplasmas and phytoplasmas) frequently result in symptoms that resemble nutrient deficiencies.

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Summary

Vectored diseases (including viral diseases and those caused by mollicutes (spiroplasma and phytoplasma)) frequently result in symptoms that resemble nutrient deficiencies. The most common diseases are shown in Table 11 below, along with the nutrient problems with which they might be confused. All of these diseases are spread by an insect vector. The symptoms may appear only on upper leaves, or over the entire plant, depending on when infection occurs.

In order to distinguish vectored diseases from nutrient problems, ask the following questions:

1. Are the affected plants found in patches where most of the plants have symptoms, or are they scattered around the field? You would expect the disease to affect scattered plants, though the plants may be concentrated along the margin of a field where the vector entered. Mineral deficiency will tend to affect all of the plants in an area of the field. Look for a mottle or mosaic pattern on the leaves which is characteristic of some viral diseases. Generally, streaks or stripes caused by a disease will be centered on the veins, while chlorotic stripes caused by nutrient deficiencies are often interveinal.

2. Is the development of the plant affected? Vectored diseases, if they strike early in the season, will often cause the plants to be stunted, with distorted leaves and undeveloped ears. Mineral deficiencies may affect growth, but the development of the plant will usually be fairly normal.

The control of vectored diseases usually relies upon planting resistant varieties, attempting to control the insect vector, and/or eliminating alternate hosts for the disease.

Table 11. Diseases caused by viruses or mollicutes which might be confused with nutrient deficiencies.