Blight zoospores pose the greatest threat of infecting potato crops early in the season, according to recent findings by Syngenta.

The crop protection specialist advises that where weather conditions at emergence are conducive to blight, growers must focus early blight applications on the control of zoospores before they can infect healthy new growth. This must be followed up with the powerful systemic and protectant fungicides at the first conventional spray timing, to protect vigorous new growth through the most vulnerable early stages of rapid growth.

Potato technical manager Michael Tait commented: 'Until temperatures really warm up, the vast majority of blight reproduction is in the form of asexual zoospores. When zoospores land on the soil surface, they use their whiplash and tinsel flagellum to propel themselves towards new growth through soil moisture.

'In cases where early sources of blight zoospores create a risk, this will mean starting control programmes earlier than the traditional timing of plants meeting down the row. Growers should be ready to star soon after emergence if necessary.

'Shirlan is inherently highly active against zoospores. It works to protect against early infection – paralysing the flagella, stopping zoospore movement and preventing spore germination. The aim must be to prevent zoospores moving down from the soil surface to attack new potato stolons and tubers.' Planting infected seed tubers could also be creating the early sources of infection. Testing of seed sources by the Scottish Crop Research Institute revealed three out of seven commercial stocks contained at least some evidence of blight infection.

Dr Ruairidh Bain of SAC believes that blight in seed tubers is not a new issue. He said: 'It's clearly important that growers who suspect they have blight in seed stocks start their control programmes earlier than conventional practice suggests.'