Two dry summers may have helped reduce the effects of powdery scab, but the threat remains for seed growers. Shirlan, applied as a soil drench at the time of planting seed crops, is the only control option available.
Seed potato growers should not be lulled into a false sense of security after two dry summers reduced the incidence of powdery scab, warns Stuart Wale of SAC. The pathogen, Spongospora subterranea, is still the most significant threat to seed growers as control is difficult, and many soils are contaminated.
Although cool wet conditions at tuber initiation are the normal indicators of a bad year for powdery scab infection, Dr Wale reports even after a dry summer powdery scab can still be found in fields where soil contamination levels are high. Last year SAC was successful in gaining a SOLA for the use of Shirlan as a soil treatment to counter the disease by seed crop growers.
"Soil conditions in early- to mid-June influence the level of infection," he advises. "But if control using a soil treatment is being considered, the decision to apply has to be made at planting; this will depend on evaluating the risk of powdery scab."
Three-quarters of varieties are deemed to be susceptible or highly-susceptible to powdery scab. SAC calculates seed growers' losses in Scotland alone could amount to over £7m a year in a favourable season for the disease.
Dr Wale believes the use of Shirlan for powdery scab has been widely adopted by Scottish seed growers, but other growers in high risk situations could also look to take advantage. Monitoring of diseases in seed stocks across England has revealed the incidence of powdery scab was highest in seed to be planted in the south east and west midlands; 50% of stocks in the south east were affected.
The SAC gained SOLA permits the soil application of Shirlan at 3 l/ha and incorporated into the soil immediately prior to planting seed crops. Dr Wale reports that whilst powdery scab control has never been complete, significant reductions in the incidence and severity of infection have been obtained in the majority of trials.
Further information can be obtained from the SAC web site: http://www.sac.ac.uk/news/newsarchives/powderyscab
A copy of the SOLA certificate 0893/2006, which is required before growers make an application, can be downloaded from the PSD website SOLA database: https://secure.pesticides.gov.uk/offlabels/search.asp
