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Protecting against Alternaria

29 July 2011

Cost-effective fungicide regimes to keep Alternaria at bay will come under the spotlight at the East Midlands Potato Day, organised by AHDB Potatoes, QV Foods and McCain Foods, which takes place in early July.

The disease, also known as early blight, is becoming increasingly important as a result of the susceptibility of varieties currently being grown, the tendency to hotter spells of weather in early summer and the reduction in mancozeb use in late blight protection programmes, say Dr Reuben Morris, technical support manager for high value crops at Frontier Agriculture.

“The disease is widespread in the USA and on the continent, and we seem to be catching up. Markies, a widely grown variety that is still increasing in popularity, seems particularly susceptible to Alternaria, so effective control is important. Other popular varieties like Saturna, Maris Piper, Hermes and Estima are also susceptible and need managing carefully,” he says.

Hot weather encourages the disease, initiating spore release and predisposing plants to infection during the next humid period, such as after irrigation. Left unchecked, dark brown or black target spots that appear on leaves can coalesce into lesions and spread, defoliating the canopy. Yield loss can rise to 30 per cent in severe cases.

Fungicides have to be applied before the disease takes hold, as all products are protectant only, he warns. “You see the effects after a long, hot spell, but the disease takes hold a long time before that. The aim is to ensure the disease is controlled along with the normal late blight protection programme during canopy development.”

However, modern late blight programmes contain only infrequent applications of mancozeb, an active which offers useful control of Alternaria. Similarly chlorothalonil, which is less effective but gave some reduction in disease pressure, has largely been withdrawn from use, he adds.

Three plots of Markies at the East Midlands Potato Day will illustrate different control programmes for potato blights, to stimulate discussion about how growers might best control Alternaria on their own farms, says Dr Morris. 

One block will be treated with a Ranman (cyazofamid) / Revus (mandipropamid) late blight protection programme only, while the other two will receive similar programmes plus strobilurin (QoI) fungicides such as famoxadone (Tanos) and pyraclostrobin (Signum), or mancozeb for added Alternaria control.

“QoI fungicides are limited to six applications per crop, so these have to be spaced out to ensure control starts in time without leaving the crop exposed later in the season, especially with indeterminate varieties like Markies,” he notes. Trials so far indicate the aim is to start the QoI programme during the rapid canopy growth phase.

The mancozeb-based programme will receive Ranman initially followed by Revus + mancozeb applications that will then be alternated with Electis (mancozeb + zoxamide) to stay within CAA fungicide anti-resistance rules.

“We hope to see a step change in disease levels across the plots,” says Dr Morris. “We can then look at the cost of each programme and discuss suitable levels of control to help growers devise the most cost-effective programmes for crops on their own farms.”

The East Midlands Potato Day takes place on July 5 at Worth Farms, Holbeach Hurn, Lincolnshire.

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Alternaria update
The ‘potato team’ within UAP now has the fundamentals of an IPM approach for Alternaria in place for 2011. This will be reported in the next edition of Grower Gateway.

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