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The effects of soil inoculum level and three environmental factors (soil type, soil moisture regime and temperature) on the incidence and severity of powdery scab caused by Spongospora subterranea were investigated in potato plants grown under controlled environmental conditions. Symptoms of powdery scab on tubers were assessed visually, after which DNA was extracted from tuber peelings and quantified in a real-time polymerase chain reaction assay using primers and a TaqMan® probe specific to S. subterranea to establish tuber infection levels. Soil inoculum concentration of S. subterranea did not significantly affect the incidence and severity of either tuber infection or powdery scab symptoms at maturity. No significant differences in disease incidence and severity were found between sandy, loamy and clay soils, although the two lighter soils yielded more powdery scab than clay soil. Constant dampness of the soil resulted in significantly more disease than a fluctuating moisture regime. Infection and disease levels were high at all three temperatures tested (9, 12 and 17 ° C), but symptoms were most severe at 12 ° C. The percentage of plants with infected tubers did not increase after tuber initiation, although the amount of S. subterranea DNA detected in tubers and the severity of powdery scab symptoms increased in mature plants. Latent tuber infections were found to be common, especially under conditions suboptimal for disease development. This new information may be important for the prevention of powdery scab in potatogrowing areas around the world.

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The incidence and severity of root infection and root galling caused by Spongospora subterranea were assessed in potato plants (cv. Estima) grown under controlled environmental conditions. The effects of temperature, soil type, soil moisture regime and soil inoculum level on infection and root gall development were determined by molecular and visual methods at two plant growth stages. Root gall severity was scored at harvest, after which DNA was extracted from the roots and quantified in a real-time polymerase chain reaction (PCR) assay specific for S. subterranea . Root galling was severe at 17 ° C, with a disease score of 3·1 on a 0 -4 scale, low (0·6) at 12 ° C, and did not occur at 9 ° C. The level of inoculum in soil, in the form of artificially added sporosori, had no effect on the incidence and severity of visual symptoms, with 21%, 41% and 33% incidence observed at 5, 15 and 50 sporosori g -1 soil, respectively. Incidence of infection, as detected by the real-time PCR assay, was greater with increasing soil inoculum concentrations, ranging from 48% at 5 sporosori g ) of plants infected at maturity, but this effect was not statistically significant. No correlation was found between the occurrence of galls on roots and powdery scab on tubers of the same plants.

dSutton Bridge Experimental Unit, Sutton Bridge, Lincolnshire, PE12 9YD, UK Controlled-environment and field experiments were done to quantify the individual contribution of seed-tuber and soilborne inoculum of Colletotrichum coccodes in causing black dot disease of potato tubers. Seed-tuber and soilborne inocula of C. coccodes were quantified using an existing real-time PCR assay and related to subsequent incidence and severity of disease. In four field trials, a controlled-environment experiment and through the monitoring of 122 commercial crops, seed-tuber inoculum was found to be relatively less important than soilborne inoculum in causing black dot, and the level of seed-tuber inoculum did not significantly affect either the incidence or severity of disease or the percentage of progeny tubers deemed unmarketable. By contrast, soilborne inoculum had the potential to result in high levels of disease and the level of C. coccodes soil infestation (pg DNA g)1 soil) was found to have a significant effect. At soil infestation levels below 100 pg DNA C. coccodes g )1 soil, 7% of commercial crops had an incidence of black dot greater than 20%, increasing to 40% and 57% of crops at levels of 100-1000 pg g )1 and >1000 pg g )1 soil, respectively. These arbitrary threshold levels for soilborne inoculum related to disease risk are discussed. Interpretation of disease risk based on inoculum levels must, in the future, be informed by agronomic variables and potential control strategies.