Caution advised over extra early spraying for blight
Caution advised over extra early spraying for blight
By Andrew Blake
JUST how early do you need to go in with your first potato blight spray? An East Anglian agronomist urges an earlier start than normal, but ADAS and SAC are more cautious.
Martin Beaton of Banks Cargill believes poor harvesting in the past two years has led to more groundkeepers and potentially more blight-infected seed. So he advocates extra early treatment to nip the disease in the bud.
"Stem blight has been appearing earlier and earlier recently. Its certainly more noticeable. A lot more systemic products have been used early, but I havent been comfortable that they are doing a good enough job."
Instead he recommends Tanos (cymoxanil + famoxadone), at the early rosette stage or up to two weeks after emergence, to cure any infection and provide 10-14 days protection.
"The days of waiting until the crop meets in the rows have gone," says Mr Beeton. "You need to get something on – its too high a value crop to try to save just £20-£25/ha."
The products kickback action offers flexibility in the timing of the next treatment, he adds.
DuPonts Neil Beadle believes growers who wait too long could be underestimating the role of blight carried into crops from blight-infected tubers.
Although certified seed may contain no more than 1% soft rots, and not all those may be due to the disease, any latent infection is a clear early threat should conditions favour its development, he explains.
"Its all a numbers game." Most rot-ridden tubers die before emergence. But even if only 1-2% emerge in a typical main crop, that can still leave 4-8 infected plants/ha ready to act as blight foci in warm, moist weather, he calculates.
"Remember each infected plant can produce up to 300,000 spores/day. Cymoxanil has the ability to move into the plants tissue and stop sporulation."
ADAS plant pathologist Nick Bradshaw says growers should keep seed-borne infection in perspective.
"We have never dismissed the significance of it. Even a low incidence of seed infection can start an epidemic in the right conditions."
The problem of seed-borne inoculum is likely to be greater where growers use home-saved seed from ware crops, he adds. "Seed crops are usually burnt off earlier which clearly helps reduce the risk of blight infection towards the end of the season."
However, by far the greatest threat remains uncontrolled dumps harbouring the disease. And the need to treat crops early should mainly be determined by Smith Periods or information from blight prediction programmes, together with local knowledge of blight outbreaks, he stresses.
Spraying earlier than usual might be justified where other farm activities could prevent timely treatments, he continues. "Its also important to make sure that all areas of a field can be sprayed effectively. My advice – if you cant spray it, dont plant it."
Much still needs to be known about blight, adds the SACs Ruairidh Bain who hopes a new PCR genetic investigation may help improve understanding of the threat from seed-borne infection.
"I have seen no evidence that stem blight is occurring any earlier. But there is increasing evidence that the pathogen is changing and becoming more aggressive."
If that is proven, then there could be a case for spraying earlier, he acknowledges. *
BEATING BLIGHT
Seed-borne disease threat.
Changing aggressive pathogen.
Extra early spraying debate.
New PCR project may help.
BEATINGBLIGHT
• Seed-borne disease threat.
• Changing aggressive pathogen.
• Extra early spraying debate.
• New PCR project may help.