Foiling the tuber blight risk.

Byline: By Michael Woods

Desiccating potatoes whilst maintaining tuber blight protection right to the end of harvest has become more complicated with the trend to split dose desiccation treatments and the need for adding organosilicone adjuvants to certain blight fungicides.

Whatever haulm destruction method is chosen, it is essential to maintain thorough protection against tuber blight right up to the time the haulm has died and lifting has started, advises Dow AgroSciences Ltd. "The mature potato foliage and stems can easily act as dangerous sources of zoospores, which will drop off or are washed off on to the soil through which the tubers will be lifted. An effective blight programme should have prevented zoospore production in the first place, but this protection needs to extend right through the desiccation process so that the risk of blight is fully covered," advises Andy Leader, principal biologist for Dow AgroSciences.

The trend towards split applications of desiccants, where the first spray opens the canopy up and the second spray desiccates more thoroughly, has complicated this process.

Using split treatments can lead to the crop being exposed to blight infection for a longer period of time and could increase the risk of tuber blight.

"The choice of blight fungicides late in the programme makes a significant difference in the overall fight against tuber blight. This is because not all products have good activity on zoospores and not all fungicides can be mixed with desiccants," adds Andy. "The zoxium in Electis adversely affects zoospore release, resulting in the production of non-motile spores, incapable of infecting tubers. When used repeatedly in a suitable programme, Electis protects against tuber blight and can be tank mixed with diquat, glufosinate-ammonium and carfentrazone-based desiccants, protecting tubers right through to lifting. Because of its action, sulphuric acid cannot be mixed with any blight fungicides or adjuvants.

"Diquat and glufosinate are not recommended with adjuvants either, as this may jeopardise crop safety and efficacy, but they can and should be mixed with blight fungicides with known zoospore activity, especially when used in a split dose sequence," concludes Andy Leader.

Electis contains 8.3% w/w zoxium and 66.7%w/w mancozeb, formulated as a WDG. It is recommended on all varieties of potatoes at spray intervals of 7,10 or 14 days. Electis has a dose rate of 1.8 kg/ha in 200-600 litres of water per hectare. It has a maximum number of treatments of 10 per crop and a harvest interval of 7 days. It is packed in a 3-hectare pack (5.4 kg) and marketed exclusively by Dow AgroSciences.