Although there are not yet any ‘Fight against Blight’ confirmed outbreaks in Scotland, the risk of outbreaks has increased considerably in the last few weeks.

In all parts of the country there have been high-risk weather periods. Fortunately, for this year, the elevated blight threat looks as if it will occur when crops have already been well protected by multiple applications of fungicide.

That means that the aim from now until harvest is to ensure that the risk of tuber infection is zero, or very low – in addition to keeping the haulm blight-free, of course.

This can be achieved by a combination of the following:

n Ensure adequate fungicide protection until the crop is completely desiccated.

n Make sure that tuber blight-active fungicides continue from stable canopy until the final fungicide application.

n Lower the risk of tuber blight by boosting the control of foliar blight during the stable canopy phase.

This can be attained in various ways, for instance using more effective fungicides, by including anti-sporulant fungicides, using angled nozzles and/or optimising fungicide timing in relation to high-risk periods (Hutton or Smith).

The risk of tuber infection increases considerably if there is foliar or stem blight combined with substantial rainfall (or irrigation) after high-risk weather for blight and/or only a limited depth of soil cover protecting progeny tubers.

In the past two years, insensitivity to the fungicide active ingredient, fluazinam, in p infestans has emerged as a highly significant issue and a higher priority needs to be given to minimise the threat of insensitivity developing to other fungicide modes of action.

In the absence of fluazinam, the most effective control of tuber blight by fungicides will depend on only two FRAC modes of action – the active fluopicolide (in Infinito) is one, and cyazofamid and amisulbrom (in Ranman Top and Shinkon, respectively) the other.

To protect these modes of action so they are available for years, these should be mixed or alternated to limit selection pressure. Also make use of multi-sites, ie mancozeb, in co-formulations or tank mixes.

Tuber infection can also occur during harvest, essentially undoing what was a good tuber blight control programme. Two common scenarios for this are 1), when haulm regrowth becomes blighted and is harvested along with the tubers; and 2), when a crop is harvested too early in relation to desiccation.

The desiccation programme should be sufficiently robust to avoid regrowth. Blighted regrowth is a potent source of inoculum for tuber infection.

Crops shouldn’t be harvested until a minimum of 14 days after the haulm is dead to reduce the risk of viable spores of p infestans infecting tubers on the harvester. Experience has shown that varieties with low 1 to 9 scores for tuber blight resistance can require even longer between desiccation and harvest to allow more p infestans sporangia to die prior to harvest.

The role of varietal resistance to tuber blight in integrated control of potato blight is currently being researched at SRUC, with Scottish Government funding. But one clear way is to avoid damage that breaks the skin of tubers to reduce the number of entry points for tuber infection.