Infection of sugar beet by Polymyxa betae in relation to soil temperature

The effects of soil temperature on infection of sugar-beet roots by the soil-borne fungus Polymyxa betae were investigated in controlled environments. Pre-germinated seeds were sown in pots of naturally infested soil and seedlings sampled at frequent intervals over a period of several weeks. Within the range 10-30-degrees-C, the optimum soil temperature for infection was c. 25-degrees-C; the time between sowing and the first detectable infection was shortest and the subsequent rate of infection most rapid at this temperature. No infection was observed over 80 days at 10-degrees-C. Both root and shoot dry weight were reduced on plants growing in infested soil at 15, 20 and 25-degrees-C compared with those growing in uninfested soil. In general, root growth was more severely affected than shoot growth and the effects were most pronounced at 20-degrees-C. These results were confirmed in a subsequent experiment in which P. betae-infected root material was used as the inoculum. In addition to its role as the vector of beet necrotic yellow vein virus (the cause of Rhizomania disease), the significance of P. betae as a plant pathogen in its own right is discussed.