Speaking at the East Anglian Potato Event last week he told growers that 20% of the blight outbreaks tested in recent years contained both A1 and A2 blight, and where both types occur there is the potential for sexual reproduction and the creation of oospores with new blight strains.
"We know that oospores can survive in the soil over several seasons, which creates the risk for early infection from the soil-borne sources," he warned. However, Dr Cooke acknowledged that no incidence had been identified in the field in the UK, and that the general blight population showed no real change in the number of strains that may be associated with sexual reproduction. He believes growers must remain vigilant to the threat of a blight shift that could compromise the natural blight resistance of varieties such as Lady Balfour and Setanta, along with the efficacy of some fungicide options.
The significant change in recent years has been the rapid rise in the proportion of A2 Type 13 blight, which has increased from less than 10% to over 70% of blight samples in the past three seasons. Initial testing of the 2008 samples from the Potato Council's Fight against Blight campaign samples indicates that populations this season are similar to last year, but further analysis is being undertaken, he added.
"For growers, the key issue with Type 13 is that it completes its life cycle in the crop faster and becomes active again more rapidly than predicted in the past, which has a serious implication for spray intervals when conditions are conducive to blight.
"Reports this season have been that in difficult periods a spray interval of seven days has not been good enough, and that keeping tighter spray intervals will be a continuing trend in the future," he advised.
The East Anglian Potato Event was organised by Greenvale AP, Frontier Agriculture and Syngenta Crop Protection UK Ltd, and supported by The Potato Council. The event was hosted by Stratton Streles Estates.