Trends in UK potato production appear likely to increase the risk of alternaria, or early blight, becoming a serious problem in the UK, SAC‘s Ruairidh Bain said.
The factors that have increased alternaria in continental Europe in the past 10 years are all trends happening in the UK, he said.
Those included changes to more susceptible varieties, such as Markies and Saturna, warmer periods in the growing season, more crops being irrigated – alternaria likes alternating wet and dry, reduced N, and reduced use of mancozeb, maneb and fentin products in blight programmes, he explained.
Alternaria symptoms were distinctive from late blight infections in the early stages, looking a little like a target spot, but as the disease developed it became more difficult to distinguish.
Control relied on fungicides and varietal resistance mainly, he continued. “There are differences in varietal resistance, but unfortunately there isn’t much information on UK varieties in the European Cultivated Potato database.”
A list of 15 varieties with recorded alternaria infections in the past two seasons had been produced by Syngenta, however, he noted. Varieties included Maris Piper, Estima, Hermes, Vivaldi as well as Markies and Saturna.
The strongest fungicides currently appeared to be the strobilurins. Azoxystrobin was available in mix with chlorothalonil through a specific off label approval as Olympus, while Signum (pyraclostrobin + boscalid) had also performed well in trials, although no SOLA was available yet.
“Unfortunately there is potential for resistance to the strobilurins. In USA there was insensitivity within two years.”
Blight fungicides Tanos and Consento also offered some control, as did mancozeb. “But you really need at least 1500g/ha of mancozeb for good control, which very few UK blight products contain.”