The current period of cool and wet weather not only favours the production of foliar blight but also offers ideal conditions for the production of zoospores, the causal agent of tuber blight.
"The BPC have been reporting significant early blight pressure this season, with a high number of blight outbreaks plus a run of Smith periods being reported in their Fight Against Blight and BlightWatch services. But the cool and wet conditions we are experiencing are also going to result in higher levels of zoospore production and so higher tuber blight risk. Plant pathologists have found that zoospores are produced at lower temperatures of between 10 to 15°C and this is why they are normally more common in the early part of the season and towards the latter part of the growing season. This year they could be produced right now," says Andy Leader, Principal Biologist for Dow AgroSciences.
Andy explains that tuber blight is caused by motile zoospores. "The twin-tailed spores are produced in sporangia, which develop on the leaves from foliar infections. Each individual sporangium produces 8 to 10 zoospores but each infected plant can release millions of zoospores when the conditions are right. Once released or washed down by rainsplash, the zoospores fall to the ground and then propel themselves in the soil water towards the tubers. They cause infection by germinating near to the tuber and entering access points such as buds, lenticels and wounds. Once in, the fungus spreads and starts to breaks down cellular tissue. Symptoms are usually seen within one month of lifting, but the disease needs to be prevented in the field before it gets to this stage," advises Andy.
"Tuber blight can be prevented indirectly by keeping foliar and stem blight out of the crop and directly by preventing zoospores reaching the tubers. Realistically it has to be a combination of both approaches. It's a numbers game and there is no fungicide strong enough to completely prevent tuber blight through controlling foliar blight control alone. This is why you need to introduce robust fungicides with known zoospore activity into the programme from tuber initiation onwards," says Mr. Leader.
"There are a number of zoospore active fungicides that can be used in a programme to prevent tuber blight, but they need to be used in a specific way. Zoxium in Electis is unique in that it stops zoospore formation and release, resulting in the production of non-motile spores, incapable of infecting tubers. By reducing the zoospore loading as early as possible, Electis can optimize the activity of other fungicides that have more direct zoospore activity. Other fungicides such as Shirlan (fluazinam), Ranman (cyazofamid) and Infinito (fluopicolide + propamocarb) act directly on zoospores once they are released, preventing them from reaching tubers. Electis is flexible in use and can be used up to 10 times in the season, which is important in a season like this when more applications are being made at closer intervals."
According to Andy, growers cannot rely entirely on foliar blight control to prevent tuber blight, particularly this year when blight conditions are so high. "The crop foliage must be thoroughly protected by robust blight fungicides that also have proven zoospore activity. Spray intervals need to be kept tight and the programme should continue right up to and beyond desiccation. Once tubers start to form they are at risk from tuber blight. You only need a few zoospores to reach potato tubers to suffer significant and damaging blight infections."
