17 May 2002

Careful planning needed to beat tough pathogen

Minimising the amount of

early-season inoculum and

improving the targeting of

fungicides is key to the

successful control of potato

blight this year, say the

experts. Our latest baseline

advice article reports

WITH four new fungicide products to choose from, growers might be fooled into thinking blight control will be easier this year, says blight control expert Ruairidh Bain of SAC.

"Unfortunately that is not the case, because the blight pathogen is becoming more aggressive and difficult to control. Growers need to consider how to get more from each fungicide application as well as how to use other control measures."

Sources of inoculum do not change from year to year, he says. "It is the same annual message – deal with dumps and groundkeepers. But it is more important than ever, because fewer sporangia of the new population will cause the same amount of disease."

Haulm growth on dumps should be treated with herbicide or the dump should be dampened and covered with black plastic, he advises. "Groundkeepers wont be such an important source of blight as they were for the past two years, because of the wet harvest conditions."

Fungicide timing

Make sure the blight fungicide programme is started early enough, says Dr Bain.

"For high risk crops, susceptible varieties and high risk locations, the first spray should be applied either when there is a warning of high risk weather or when there is sufficient crop to intercept the spray, whichever is sooner."

For lower risk situations the first fungicide should be applied at the high-risk warning, or when the leaves meet along the rows, he adds.

"Most blight fungicides work best if they are applied shortly before the high risk period. So use of a system which flags up high risk periods in advance has an advantage."

There are three main approaches to blight control in the UK: Routine spraying, use of Smith periods, or computerised warnings.

"Each has its advantages and disadvantages. Routine applications, say every seven days, has an ease-of-management benefit. But it wont give the best control, because fungicide application timings wont necessarily coincide with high-risk periods. And it can be wasteful."

Use of Smith Periods is helpful in deciding when to start the programme and when to shorten intervals. "But a major disadvantage is that growers can only be informed when it has occurred, rather than forecasting it. It is not crop specific, but should be used as a general warning."

Computerised warning systems, like Plant Plus, provide a forecast of blight risk conditions, so timings can be optimised. "It is crop specific or farm specific, and gives recommendations for the type of fungicide and the spray timing.

"It also takes varietal resistance, rate of crop growth and fungicide persistence into account. The downside is cost."

Product choice

John Keer of Agrovista agrees that an early start to spraying is required and points out that the weather is by far the greatest factor influencing blight incidence.

"Spray interval is the most important aspect of the programme, but it cant be established until we are into the growing season."

Systemic fungicides are best during periods of fast growth to protect new foliage, which is not present at the time of application, he says. They should be used for the first few sprays.

"The phenylamide-based products (as in Fubol Gold, Galben M) are the most systemic, but their use should be limited to prevent build up of resistant strains."

Locally systemic fungicides with kick-back curative activity are useful where weather has caused a delay, he adds.

For the settled canopy phase there is a whole range of products to choose from. "Contact-only products have a place mid-season and are cheaper. Otherwise there are translaminar materials, such as Invader, which are useful when the programme slips, as they are more persistent, or products such as Shirlan.

"The aim at this stage is to maintain enough protection to prevent infection of the haulm."

Tuber blight

Tuber blight control must be considered throughout the season, says Dr Keer. "Growers have got to learn how to control it without placing all their reliance on tin-based fungicides. These are not likely to be with us much longer."

Tubers are infected when zoospores find their way through the soil and on to the tuber. "So use of a product which controls zoospore production, such as Ranman Twinpack, will reduce the risk."

Stay focused

Rob Clayton of the British Potato Council adds that growers must consider the protection of new growth, the persistency of different chemistry and canopy density when choosing the most appropriate product.

"We need more information about the new fungicides before we can be certain about where they will be most useful in a programme," he says. "Do not relax your guard – keep up the protection until the bitter end."

New this season

Four new products are widely available for blight control this year, but none of them are aimed at the important early rapid growth stage.

Most offer better control than older chemistry.

Electis (zoxium+mancozeb) is a mid-late season product, which gives tuber blight protection. Ranman Twinpack from BASF contains cyazofamid and is also used for the settled canopy phase and just before the final tin sprays. Tanos (famoxate+cymoxanil) is another mid-season blight fungicide.

C50 (cymoxanil) is a flexible mixer product, which can be used to give kick-back. A Shirlan (fluazinam) plus C50 tank-mix has been approved and shown to have very good activity against tuber blight in Cambridge University Farms trials. &#42

Potato blight is getting tougher to control, despite the arrival of new fungicide products, so plan control carefully, agree blight experts.

Make the most of cultural control measures as well as fungicides, urges Ruaridh Bain of the SAC.

1 Sources of inoculum Deal with dumps and groundkeepers to remove early inoculum.

2 Start early Get an early start with blight programmes.

3 High risk crops Spray when there is a high risk weather warning or if there is sufficient crop to intercept the spray.

4 Lower risk situations Apply first spray at the high risk warning or when the leaves meet along the rows.

5 Fungicide efficacy Most blight fungicides work best if applied just before the high risk period.

6 Spray interval Will be determined by the weather. Change products where intervals slip.

7 Systemic products Use for the first few sprays, in the rapid growth stage. Restrict phenylamides to 3 applications.

8 Settled canopy Wide choice of products. Use kick-back properties of cymoxanil where intervals slip.

9 Canopy senescence Do not over rely on tin-based fungicides. Opt for products which control zoospore production.

10 Tuber blight Consider tuber blight control throughout the season.

An early start to blight control is now a must, says John Keer of Agrovista, with product choice then matched carefully to canopy growth stage.