Belt and braces needed to prevent Tuber Blight

UK - July 20, 2005

Preventing tuber blight in potato crops requires a belt and braces approach from emergence through to lifting.  Effective control relies on keeping blight out of the crop through the use of a robust and effective foliar blight programme, followed by the planned and integrated use of fungicides with confirmed zoospore activity, such as Electis, cyazofamid and fluazinam, from tuber initiation onwards, advises Dow AgroSciences.

"Tuber blight can cause up to 40% yield loss plus down grading or rejection of the sample.  There are also the hidden costs of this disease; for example grading costs £15 per tonne.  It is important therefore that the entire blight spray programme, from start to finish, contributes to the prevention of this damaging and costly disease.  Action should not be left to the end of the programme.  The best policy is to adopt an integrated or a "belt and braces" approach against tuber blight.  Firstly it is important to plant uninfected seed and to remove any nearby sources of infection such as dumps and volunteers.  We are approaching the optimum time to apply Fazor (maleic hydrazide) in the growing crop, which will reduce volunteers in any following crop - so forward planning is important, too.  Then you need to maintain a robust foliar blight programme, applying treatments regularly, shortening the spray interval in high risk conditions such as we are experiencing now in many areas of the UK.  In your blight programme it is vital that you include repeat sprays of products with known zoospore activity, such as Electis, from the mid to the late part of the programme when the risk of tuber blight is at its highest," advises John Sellars, Marketing Specialist for Dow AgroSciences.

The motile zoospores that are produced in the sporangia on the leaves are responsible for causing Tuber blight.  They tend to be produced any time in the crop's development when the temperature is around 10-15°C, lower than the temperature conditions, which favour the spread of foliar blight, which is around 25°C.  Each infected leaf can produce millions of zoospores when the conditions are right.  Zoospores drop or are washed down from the leaves and stems to the soil where they move in soil water.  They infect potato tubers by germinating close to the tubers and entering at easy access points such as buds, lenticels or wounds.  Crops that have a thin shallow or cracked ridge, or are brought into contact with zoospores during the lifting operation, are liable to infection.  Once inside the tuber, the fungal mycelium spreads and causes breakdown of cellular tissue.  Symptoms are normally seen within one month of lifting, explains Andy Leader, Principal Biologist for Dow AgroSciences.

"Zoxium (as in Electis) prevents zoospore release, resulting in the production of non-motile spores, incapable of infecting tubers.  Other fungicides such as cyazofamid and fluazinam will kill zoospores once they are released and so the alternation of Electis with such complementary products in a programme will provide a "belt and braces" approach and will give excellent protection against tuber blight.  This approach was confirmed last year in British Potato Council trials.  The Belchim/Dow AgroSciences manufacturers' programme, consisting of two treatments of Epok (mefenoxam plus fluazinam), followed by Electis (zoxium + mancozeb) alternating with Ranman TP (cyazofamid plus adjuvant) up until desiccation, gave the best control of both foliar and tuber blight." 

"Moreover, in 2003 trials at the Scottish Agricultural College, a centre of excellence for potato blight management, various tuber blight programmes were compared.  All programmes started with four straight mancozeb (Dithane) treatments before infestation reached 10%, followed by six other sprays.  The programme that gave the best results was Dithane followed by three Electis treatments and then three Ranman," advises Andy.

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.  It 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.