Stress factors set to hit potato crops over the coming weeks could increase the risk of alternaria (early blight) outbreaks, Syngenta's technical manager, Andy Cunningham, has warned.

Many crops are still suffering the adverse effects of exceptionally low rainfall this spring, while the hugely inflated cost of fertiliser has seen potato growers cut back nutritional inputs to the bare minimum.

“Any agronomic factor that increases stress on the crop can act as a precursor for Alternaria infection, as well as the severity of the outbreak,” he pointed out. “Early season infection and subsequent loss of leaf area inevitably has a greater impact on yield.”

Read more: Heat stress alert warns of local crop risks for Quantis timing

With the onset of 'stress', he said growers should be aiming to consider alternaria protection earlier in their blight programme.“Alternaria treatments have historically been focussed to start around the end of June or beginning of July. Where growers have identified an increased risk of stress, or have historically suffered from alternaria losses, however, that may need to be brought forward this season,” he added.

To counter alternaria threat, he advised growers and agronomists to utilise Amphore Plus through the primary risk period. This will incorporate the strong late blight protection of mandipropamid with the alternaria activity of difenoconazole, said Mr Cunningham..

“Using the pre-formulated combination delivers 20% more difenoconazole with each application to boost alternaria protection, compared to the maximum permissible rate of the active when applied as a straight product,” he said. Independent research by Eurofins in 2021 identified Amphore Plus as particularly effective for blight control and giving control of all blight strains, including EU_36 inoculated in the trials.

Furthermore, where growers are using Revus in their blight control programme as the most effective straight CAA group fungicide, the addition of mancozeb in the tank mix would be a valuable resistance management strategy and adds potential alternaria protection.

Mr Cunningham also highlighted the cumulative effects that multiple stresses could have on potatoes, even at relatively low levels. “Plants that are already performing sub-optimally under stress from moisture or nutrient deficits, for example, can then be impacted badly by further pressures.

“That has been especially evident with the increasing impact of heat stress on potatoes. Enabling plants to be better prepared to cope with the adverse effects of heat stress with applications of the biostimulant, Quantis, could help mitigate the effects of other stress factors and reduce the crops’ susceptibility to Alternaria,” he added. Further Syngenta trials into these stress mitigating effects are planned for the 2022 season.

Recent alternaria research in Denmark, has suggested that while all cultivars in the trials were susceptible to the disease, the rate at which the infection developed in the foliage could indicate some varietal resistance. Later maturing varieties were typically, but not exclusively, more resistant to disease spread.

Successive seasons reporting of UK alternaria monitoring by NIAB, sponsored by Syngenta, had highlighted no significant varietal trends. It haa, however, repeatedly shown that A alternata has been the first early blight pathogen to hit potato crops, followed by later infections of A solani.

“That’s important because alternata can create multiple leaf lesions and damage, which could prove to be entry points for the more aggressive solani,” advised Mr Cunningham. “It also validates why agronomists would choose to use difenoconazole first as an active against alternata, followed by Amistar later in the season for solani and green leaf area protection.”