Earlier tuber initiation in potato crops this season plus increasing blight pressure overall has put focus on integrating fungicides with proven foliar and tuber blight activity and no resistance issues into the programme from now on.

“It’s not been an easy season for potato growers with extended plantings plus a dry March which hampered emergence and a very wet April, May and June hampered growth. This has meant that crops have been struggling and in Scotland crops are still not meeting across the rows. This has lead to much earlier tuber initiation and means that the blight programme must now include fungicides that have proven foliar and tuber blight control,” explains Dr. Dominic Lamb, UK and Ireland Business Manager for Gowan.

“A “belt and braces” approach from emergence through to lifting is needed to prevent tuber blight infection. You can’t just rely on good foliar blight control alone. All you need is just one zoospore, the causal agents of tuber blight, falling from the foliage and entering the developing tuber. Each infected leaf can produce millions of zoospores when conditions are right,” says Dominic.

He also warns that growers need to be aware of the importance of mixing and matching fungicides with different modes of action to reduce resistance risk. “This season we are seeing more concern about Green 33 strain. A leading blight specialist has recently advised growers to tank mix fluazinam with another tuber-blight active or, if it is used alone, be integrated with fungicides with a different mode of action. We know that when you drill down into the FRAC information, there are only 4 actual sites of action for blight fungicides. So this too needs to be taken into account when selecting the right tuber blight product,” he says.

A robust foliar blight programme using fungicides with known zoospore activity and alternating chemistry is required.  “This protection needs to start at tuber initiation as tubers are at risk from zoospores as soon as they are formed. Zoospores are produced any time in the crop’s development when the temperature is around 10-15°C and they are released or are washed down by rain splash from the foliage into the soil. Motile zoospores cause infection by germinating near the tuber and entering via easy access points such as buds, lenticels and wounds,” he warns.

Tuber blight 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 should be used according to how they work. For example, zoxium^® in Electis^® and Roxam^®, stops zoospore formation and release, resulting in non-viable spores, incapable of infecting tubers. By reducing the zoospore loading in the crop early on, Electis or Roxam will optimize the activity of other fungicides that act on motile zoospores. Other blight fungicides such as fluazinam, cyazofamid or fluopicolide + propamocarb act on zoospores once they are released, preventing them from reaching tubers. These products are more beneficial in the programme once Electis/Roxam has been applied,” says Dominic.

“With high blight pressure this season, it is even more important to keep spray intervals tight right through to desiccation and to protect against tuber blight right through to harvest. Electis/Roxam can be used up to 10 times in the season, important when regular applications are being made and can be mixed with desiccants if need be. This gives growers greater flexibility as to how they integrate this fungicide into their programmes not just for effective foliar and tuber blight control, but also for additional Alternaria control and for resistance management strategy.”