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Disease Management

Resistance to the virus
The most practical disease control strategy against SBWM is cultivar resistance, first described by H.H. McKinney in 1925 (Figure 14). The mechanism of cultivar resistance has yet to be fully characterized, although preliminary evidence suggests that resistance prevents systemic movement from roots to foliage, but does not prevent replication and cell-to-cell movement in roots. At temperatures above 20 °C (68 °F), resistance to foliar infection breaks down, but symptoms do not result. McKinney rightly noted that even genotypes with field resistance were susceptible to mechanical foliar inoculations. Resistance is conferred by 1 to 3 genes. Given the strong environmental influence on transmission and symptom expression, the broad sense heritability estimate of 43-55% reflects the high heritability of resistance. This means that about 50% of the variation in the number of symptomatic plants is due to genetically inherited host resistance.

Figure 14

Resistance to the vector
Several studies in barley suggest that all cultivars tested are susceptible to P. graminis infection, but also reveal a cultivar-dependent, quantitative variation in the number of zoospores released. Furthermore, isolates of P. graminis show host species specificity, which may limit the ability of P. graminis isolates to transmit the virus.

Varietal mixtures
Based on what is known about the SBWMV pathosystem, mixtures of resistant and susceptible host genotypes could theoretically provide protection against pathogens through reduced density of susceptible genotypes and barrier effects (Figure 15). However, because secondary zoospores are limited in their dispersal distance, it is more likely that a secondary zoospore would infect the plant from which it is derived (autoinfection) than another plant (alloinfection). Typically, there would be a maximum of one generation of secondary zoospores in the autumn, when the most damaging SBWMV transmission occurs. Therefore, one would not expect to observe significant benefits from varietal mixtures. However, Hariri et al. (2001) reported intriguing results that 1:1 and 1:3 (susceptible:resistant) mixtures developed significantly less disease than predicted by pure stand comparison. For more information on host mixtures, visit http://www.apsnet.org/education/AdvancedPlantPath/Topics/cultivarmixtures/top.htm.

Figure 15

Chemical control
The only chemicals that have been reported to provide reproducible control against P. graminis in the field are soil fumigants, which are not economically feasible in small grains systems.

Crop rotation
Continuous wheat amplifies P. graminis inoculum, and SBWM-susceptible cultivars increase the proportion of viruliferous zoospores. However, because of the long-term survival of the virus in P. graminis resting spores, crop rotation is not an effective management strategy for SBWM.

Sanitation
Any equipment or mechanism that transports soil has the potential to transport SBWMV-infested inoculum. In growers’ fields in New York State, where SBWMV was recently introduced, we observe SBWMV-infected patches most often associated with areas where machinery first entered the field. This observation stresses the importance of sanitation (e.g., cleaning machinery) to avoid the introduction of new soil-borne viruses (Figure 16).

Figure 16

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