Time to move over to blight fungicides with known tuber blight activity

As the main crop grows and moves towards tuber initiation, this is the time to start integrating fungicides with proven foliar and tuber blight activity into the programme.

"It is just not feasible to rely on foliar blight control alone to prevent tuber blight. You need a "belt and braces" approach from emergence through to lifting. Just one zoospore falling from the foliage and entering the developing tuber can result in tuber blight and each infected leaf can produce millions of zoospores when conditions are right," explains Dr. Dominic Lamb, UK and Ireland Business Manager for Gowan.

He advises that the potato crop is protected by a robust foliar blight programme using fungicides with known zoospore activity, as these are the causal agents for tuber blight. "This protection needs to start at tuber initiation onwards, normally around three weeks after crop emergence, and continue right through to harvest. The actual definition of tuber initiation is when the tuber is twice the diameter of the attached stolon and for many crops in the ground we are not far off this stage."

"The reason we need to be thinking about tuber blight from this early stage is that tubers are at risk from zoospores as soon as they are formed. Zoospores can produced any time in the crop’s development when the temperature is around 10-15° and they are released or are washed down by rain splash from the foliage into the soil. Crops with a thin coverage of soil or in cracked soils are more at risk. The motile zoospores cause infection by germinating close to the tuber and entering via easy access points such as buds, lenticels and wounds. The blight fungus then spreads within the tuber and starts to breaks down cellular tissue. You only need a few zoospores to reach potato tubers to suffer significant and damaging blight infections," adds John Edmonds, European Technical Manager for Gowan Comércio.

Symptoms are usually seen one month after lifting, but the disease must be prevented in the field well before it gets into store and as soon as the crop is vulnerable to attack.

"Not all blight fungicides have proven zoospore activity, but those that do need to be used in a specific way according to how they work. Zoxium® in Electis® and Roxam®, for example, stops zoospore formation and release, resulting in non-viable spores, which are incapable of infecting tubers. By reducing the zoospore loading in the crop as early as possible, Electis® will optimize the activity of other fungicides that act on motile zoospores," says John.

He explains that other blight fungicides such as fluazinam, cyazofamid or fluopicolide + propamocarb act on zoospores once they are released, preventing them from reaching tubers. Such products would be beneficial once Electis has been applied, he says.

John Edmonds also warns growers to look out for the signs of Alternaria or early blight in the field and be prepared to integrate fungicides with activity against this disease into the programme. "Electis has one of the highest ratings for the control of Alternaria, as well as one of the best ratings for protectant activity against late blight plus a very acceptable rating for tuber blight. So there is no need to go to the trouble or expense of a separate treatment."

"Even with lower blight pressure as is the case this season so far, it is important to keep spray intervals tight right up to desiccation. Electis can be used up to 10 times in the season, important when regular applications are being made. It also gives growers much better flexibility as to how they integrate this fungicide into their programmes for effective foliar and tuber blight control, additional Alternaria activity and as a resistance management strategy."