Times ripe for blight

24 April 1999

Times ripe for blight

Late emerging potatoes were just in time to catch serious blight infection last year. David Millar asks what can be done to prevent repeat epidemics in 1999.

DELAYS to planting are far less severe this year but the mild winter and continuing frost-free weather could spell trouble ahead for growers in blight-susceptible areas.

It may be far too early to predict how the blight fungus will behave this year but, regardless of the weather, there is now standard advice for growers to follow at the start of each potato season.

It may be boring to say so again, admits ADAS plant pathologist Nick Bradshaw, but it is vitally necessary to get out into fields and identify dumps and other sites where waste tubers from last years crop have been left to become a potential source of early blight infection.

"I dont know what the level of tuber blight has been in store this winter but I havent heard of any serious breakdowns. I suspect this is because last years epidemic didnt progress as expected due to intensive spraying and drier conditions in August.

"Nevertheless, as a plant pathologist and knowing this disease only too well, we should assume there will be higher levels of inoculum carried over on volunteers or dumps, particularly in view of the mild winter.

"If we have the right conditions in the spring with more of this primary inoculum around then the risk is high."

Mr Bradshaw accepts that potato growers may be busy planting or engaged in other farm spring work but there is every good reason to be clearing or spraying off dumps in farm nooks and crannies.

Its advice echoed by the Scottish Agricultural College which warns that blight risk and infection can be very severe from early in the growing season, and advises early ordering of fungicides so the appropriate sprays are available to suit the eventual situation. Maintaining fungicide intervals, especially during rapid growth of the haulm, will be critical, says the SAC.

That view is shared by Mr Bradshaw who points out that ADAS trials some years ago showed that mancozeb applied at weekly intervals was as effective at blight control as more expensive two- and three-way mixes. "However, as the intervals were extended, it was necessary to use the more potent mixtures," he adds. "The closer the interval, the better the blight control."

UK pesticide regulations stipulate minimum intervals for application of most blight fungicides and these are clearly stated on the label and in product information. While these intervals apply only to individual products, it is possible to alternate with other blight fungicides containing different materials should this be necessary in extreme situations.

A typical ADAS-recommended spray programme would be to start with two or three applications of sprays containing a systemic material – either a phenylamide or propamocarb – followed by a translaminar-acting product such as cymoxanil or dimethomorph or protectant materials such as fluazinam or mancozeb.

The systemic fungicide component is particularly useful in protecting new growth during the early rapid canopy development phase. Non-systemics may rely on regular, short spray intervals to maintain blight protection in such situations.

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