Results of last season’s alternaria monitoring in potato crops has further reinforced the pattern of earliest infection from A alternata, with A solani typically coming into crops later in the season.

The monitoring, sponsored by Syngenta and undertaken by independent and industry potato agronomists sampling, suspected cases throughout the season, with laboratory analysis by NIAB specialists to determine the species of alternaria (early blight) present.

Until a sampling date of July 23, 85% of the positive alternaria infections identified were only alternata, with 15% solani and only at low levels. From July 24, however, 81% of samples contained solani and just 9% solely alternata. Around 20% of samples tested had mixed infection and no alternata was identified in the sampling after August 9.

Whilst AHDB advice pointed to solani being the dominant early alternaria species, annual in-field monitoring has again shown it has been the alternata coming in first.

Syngenta's technical manager, Michael Tait, said the information was particularly valuable to highlight the timing of onset for alternaria infections and assessing agronomy options.

“Alternaria leaf spots can result in rapid loss of green leaf area. The control options have to be applied preventatively, which typically starts in early July,” he said, recommending that two sprays of Amphore Plus (difenoconazole + mandipropamid) could help manage the risk of early blight (alternaria), whilst still maintaining a strong late blight programme. This product can also be worked into any late blight spray plan.

Research has shown that difenoconazole is active on alternaria spores at an early stage. Using Amphore Plus can provide a higher rate of 150 g/ha of the active ingredient, compared to the maximum 125 g/ha permitted for straight difenoconazole that could be included in a mix.

“This season’s weather conditions have also seen a very high risk of late blight developing,” added Mr Tait. “It’s essential for growers and agronomists to maintain a high level of protection against infection.

"Amphore Plus also contains mandipropamid (as in Revus) that delivers more effective foliar blight control, compared to mancozeb that may have previously been considered to prevent alternaria.”

The monitoring identified positive infection on 11 different varieties, indicating more widespread risk than previously considered. Later sampling dates showed the greatest diversity of varieties infected.

In 70% of instances, alternaria had been identified before the crop had reached 100 days after planting, where the planting date was known. “That could prove especially important, since early infections and leaf loss would tend to have the greatest implications for crop yield and tuber size,” warned Mr Tait.