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Disease Cycle and Epidemiology

Polymyxa graminis (Figure 10), the vector of SBWMV, belongs to the endoparasitic slime molds (Plasmodiophoromycota). This organism produces resting spores that harbor viral RNA and movement protein. These resting spores can remain dormant in soil for up to 30 years, waiting to infect the next graminaceous host germinating in an environment conducive for infection, at which time the virus-containing zoospores germinate. Soil water is critical for the swimming zoospore to reach a host root. The threshold matric potential is between –20 and –40 kPa. This means that soil pores between 7.4 and 14.7 μm must be filled with water in order for significant transmission to occur.

On reaching the host root, a P. graminis zoospore encysts on the surface of a cortical root cell, and develops a spear-like "stachel," which when mature will shoot through the adjoining zoospore and host walls (Figure 12). Along with the stachel, the zoospore contents and presumably the contaminating SBWMV particles are emptied into the host cortical cell. Thus begin the life cycles of the vector and of the virus. How the virus is attached to or carried by the zoospore, and exactly how the virus is transferred from the zoospore to the plant root are not fully understood, although SBWMV RNA and movement protein, but not capsid protein, have been detected within P. graminis sporosori.

Figure 12

Optimal temperatures for P. graminis infection vary among virus isolates and often reflect temperatures where the isolate is commonly found. Thus, isolates from India are active at warmer temperatures (27-30 °C/80-86 °F) than isolates from Belgium, Canada, Japan and France (15-18 °C/ 59-64 °F). In New York state, the optimal temperature for transmission of SBWMV is 15 °C (59 °F), with significant transmission occurring in the growth chamber in less than 24 hours, given conducive soil moisture. Importantly, no significant transmission occurs at 6.5 °C (44 °F), suggesting that SBWMV is either transmitted in the autumn or in the spring in temperate climates, but not during the winter months. Given the period of time required for SBWMV systemic movement and symptom expression and the brevity of spring temperatures conducive to SBWM symptom expression, spring infections by P. graminis are not likely to contribute to viral symptom expression.

After the stachel-mediated infection, one of two types of plasmodia (multinucleate non-motile masses of protoplasm) of P. graminis may form in a single host cortical root cell (Figure 13). The plasmodia differentiate to give rise either to secondary zoospores or resting spores.

Figure 13

Infection of plant roots by P. graminis is the first step in the poorly defined SBWMV infection process. Viral replication within the initially infected cell and movement to adjacent host cortical cells does not appear to be inhibited in wheat genotypes resistant to systemic foliar SBWMV infection. There is some evidence that SBWMV moves systemically via xylem. Since viruses move within the living cell contents of plant tissues, this suggests that SBWMV must be inoculated into young tissues before the living protoxylem cells have differentiated to become mature, dead xylem cells. Protoxylem infection is most likely to occur at root tips and where lateral roots initiate, which coincides with the region of the rhizosphere that is most likely to attract P. graminis zoospores.

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