Nutrient deficiency could be just one of the causes of stress that can increase a potato crop’s risk of Alternaria attack, warned Syngenta Field Technical Manager, Stephen Williams. Other stress factors including drought, heat, or pest damage, which could all make plants more susceptible to infection and losses from early die-back, he added. “Plant stress appears to play a greater part in the incidence of the alternata strain of Alternaria, which has certainly been the more prevalent infection in recent years, compared to the more classic disease infection of A solani,” he said.
Mr Williams advised growers and agronomists at Syngenta Potato Science Live that risk assessment of situations likely to result in Alternaria infection would be important to achieve the most effective application timing for the new Syngenta combined blight and Alternaria fungicide, Amphore Plus.
“Farms with a history of Alternaria have a significantly increased risk, with varietal susceptibility also a major factor. The risk for some varieties is particularly well documented – but we have identified cases in a wide range of crops,” he warned. “Hot, humid weather conditions will be conducive to infection, and is also likely to coincide with increased irrigation that raises the spectre of blight.
“The full dose of the best blight fungicide, mandipropamid, along with difenoconazole that is active on all strains of Alternaria, makes Amphore Plus the most effective and easy-to-use option.” Mr Williams reported trials had shown the crucial importance of early application to protect the crop from Alternaria infection, which had proven more effective than curative treatment once infection had occurred.
“Trials have also confirmed the need for a high dose of difenoconazole to maximise control of the more challenging A. alternata strain, which makes the Amphore Plus mix a more appropriate option than the straight fungicide, which has a lower rate of active use,” he added.
Mr Williams believed that Alternaria treatments would typically start in mid-June at canopy complete stage, with growers and agronomists using seasonal risk factors and pressure to decide on a block of consecutive applications targeting the disease, or alternating with straight blight products. “The good news is that with Amphore Plus there is no need for growers to compromise on the all-important blight protection when looking to control Alternaria too.”