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Symptoms and Signs

Soil-borne wheat mosaic (SBWM) affects autumn-sown small grains. The first visual evidence of Soil-borne wheat mosaic virus (SBWMV) infection in a growing season usually occurs in the spring, after the crop begins to green up. However, in some years SBWM symptoms are expressed in the late autumn or early winter, especially in warmer climates. Irregular, chlorotic patches in the field suggest soil-borne viral infection, although similar symptoms may be caused by a number of other viruses or other biotic or abiotic factors. In dry environments, patches of symptomatic SBWMV-infected plants usually occur in low-lying, wet regions of the field (Figure 2) that are conducive for infection by the swimming zoospores of its protozoan viral vector, Polymyxa graminis. In wetter or more humid climates, these patches may occur anywhere in the field (Figure 3).

Figure 2

Figure 3

Upon closer inspection, SBWMV-infected plants are often stunted (Figure 4) and the leaves may have a general chlorotic mosaic or irregular mottling and streaking (Figure 5). A few strains of the virus cause rosetting in highly susceptible wheat cultivars; leaves and tillers remain short, growth is bunchy or compact, and tillering is excessive (Figure 6). Perhaps the most diagnostic characteristic of SBWMV infection is that symptoms are not expressed on leaves that emerge after the average temperature rises above 20 °C (68 °F). Thus, SBWM chlorotic patches tend to disappear in late spring. SBWM is most often confused with a disease caused by another soil-borne virus, Wheat spindle streak mosaic virus (WSSMV), which can also induce stunting and chlorosis and is also temperature sensitive. A good diagnostic feature to distinguish between these two viruses is that WSSMV induces chlorotic streaks that are elongated and spindle-shaped and often have a dark green island in their center (Figure 7).

Figure 4	Figure 5
Figure 6	Figure 7

However, accurate diagnosis of SBWM is complicated by the fact that symptom expression varies by host genotype and, as previously mentioned, similar symptoms may be caused by a range of biotic and abiotic factors. Therefore, additional diagnostic tests are often conducted to verify the presence of SBWMV. The most commonly used tests are detection of viral coat protein by enzyme-linked immunosorbent assay (ELISA) or detection of a portion of the RNA genome by reverse-transcription polymerase chain reaction (RT-PCR).

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