Potato Review

28 POTATO REVIEW SEPTEMBER/OCTOBER 2020 ALTERNARIA particularly those for processing – with lower sugars for a light fry colour, could also be an important factor in making crops more susceptible. “A. solani usually comes into the crop mid- season, just when growers are trying to keep the nitrogen (N) levels down in the plant, so this is the time the crop can become stressed.” Sometimes there is confusion between magnesium de ciency – which causes interveinal yellowing and centralised brown legions on the leaf – and the A. solani pathogen. However, as de ciency causes a halo around the lesion on the centre of the leaf, close inspection makes it possible to distinguish between the two. A. solani symptoms tend to be more sporadic and random across the leaf with legions showing classic contouring/ circling, says Matt. e healthier the crop, the lower the risk, so he suggests considering a foliar application of low-level N early in the growing season. Susceptible varieties Some varieties – such as Markies – are particularly susceptible to Alternaria, which is partly because of the genetic susceptibility of the variety and partly because of the requirement to reduce nitrogen levels to improve fry quality. ere are currently no UK ratings for varietal susceptibility but many varieties express poor natural resistance, including King Edward, Vivaldi, Melody and Agria. Because of the 6-8 day latent period between infection and symptoms appearing, he recommends a prophylactic approach to protect the crop, he advises. “You see the e ects after a long, hot spell, but the disease takes hold a long time before that.” Most growers have relied on products from their normal late blight ( Phytophthora infestans ) protection programme during canopy development to also protect against Alternaria.” Many late blight programmes contain only infrequent applications of mancozeb, an active which o ers useful control of Alternaria. In addition, it will become more di cult to control if we lose mancozeb from our armoury, he adds. “If a high risk stage is reached, putting straight difenoconazole into the blight programme may help prevent infection.” Matt says: “Alternaria is widespread in the USA where yield losses of up to 30% have been “You see the e ects after a long, hot spell, but the disease takes hold a long time before that.” Mold Alternaria alternata, allergic fungus, 3D illustration and photo of colony on agar plate. reported, although such a high loss seems unlikely in the UK. It could become more of a problem in the future, but growers are currently able to control this pathogen. “Nevertheless, with a high value crop, the risk of losing yield – and pro t - is greater than the expense of applying what is really a relatively small input cost. If you notice crop is under stress, particularly at the rapid canopy growth phase, protection is a no-brainer.” Norfolk-based Farmacy agronomist Matt Hardy