RECENT weather conditions in Scotland have been highly conducive to blight, according to newly developed risk criteria – so blight fungicide programmes need to be tuned accordingly by growers.

The blight risk varies hugely between seasons, driven by prevailing weather and there are several tools to help growers judge the risk in their region.

At its simplest, knowledge of blight in the region flags that there is a present danger of blight and spray programmes must tightened immediately. The ‘Fight against Blight’ website marks outbreaks as they occur through its system of local scouts.

Despite conducive weather conditions, there have been no confirmed outbreaks in Scotland up to June 24 – but checking the outbreak maps regularly for the most up to date information is essential.

Preventative use of blight fungicides is essential for best efficacy, which is why blight forecasting systems are so important. The Smith period has been an established means of judging when weather conditions were conducive to blight have occurred, but this season has seen the launch of an updated model, called the Hutton Period.

It was validated by looking back at an extensive database of previous outbreaks in the UK, where it is clear that it predicted them more accurately.

How do they compare? The Smith period works on a minimum temperature of 10°C and a relative humidity of 90% for at least 11 hours a day. The Hutton period is triggered on a much reduced number of hours of 90% humidity of just six hours a day.

A third model (BlightCAST) is in use too. This combines the two by using the 11 hour humidity threshold of the Smith Period, but a lower temperature threshold of 8°C.

Because the Smith period needs a period of prolonged humidity, it could sometimes miss conditions that didn’t quite meet the Smith period thresholds but still allowed blight to infect. This is where the more sensitive Hutton period has advantages.

Many growers are using information from all of the above to judge the risk until they are comfortable with the new system. Broadly, a Hutton Period indicates high risk of blight, whereas if a Smith Period occurs, then the risk is even higher. The Hutton Period used by AHDB on its BlightWatch website predicts high risk periods a day in advance. Fungicide actives differ in their characteristics, so features to consider include their strength for leaf blight control, their curative activity(useful if sprays are delayed) and their rain fastness.

EuroBlight tuber blight ratings pool expert opinion and trial results from blight experts all over Europe and provide a robust source of reference for assessing relative strengths and weaknesses in actives.

Recent seasons have seen the welcome introduction of a wider choice of active groups for blight control, so it is important these are mixed and alternated where possible and that multi-sites are include to reduce pressure on higher risk actives.

All other integrated measures of control should be used to reduce reliance on fungicides and the inevitable selection pressure for resistance that this creates on blight populations. Integrated measures include the use of resistant varieties, extended rotations to reduce the risk of sexual resting spores, the use of certified seed tubers and, of course, the management of blight sources such as volunteers and outgrade piles, which remain a perennial source of blight each season.