New weapons in disease battle

24 April 1999

New weapons in disease battle

Genetic modification or better spray timing are the ways to tackle potato disease, according to scientists at Dundee Crop Protection.

DONT assume that tuber blight neednt be tackled until the end of the growing season approaches.

Dr Ruaridh Bain, potato specialist with the Scottish Agricultural College in Edinburgh, said that applying fluazinam or tin in the last two or three sprays in a blight programme might be leaving it too late. In 1997, tuber blight was already infecting trial crops three or four weeks before the first tin application, he pointed out.

High risk periods for tuber blight infection mostly occurred in mid- to late August but again last year the weather was suitable at the end of July. This called into question the value of a late-season application of fungicide particularly effective at minimising late tuber infection.

He suggested it might be necessary to treat tuber blight separately from foliar blight. The risk of tuber infection was not related to the number of Smith Periods – weather suitable for blight development – but rather to the timing of a Smith Period followed by substantial rainfall (at least 5mm daily) on foliar blight.

In the high blight year of 1995 in Scotland, there was just one Smith Period from 25-29 August to encourage sporulation, but it was followed shortly afterwards by sufficient rain to wash spores from foliage down into the soil. However, despite there being more Smith Periods in 1996, that year there was very little blight because of a lower incidence of foliar blight and lack of rainfall at the critical timings.

The SAC has looked at the effect of fungicide programmes on tuber blight and Dr Bain reported on trials in 1995, 1996 and 1997. Phenyl-amide-based programmes and fluazinam treatment produced significantly less tuber infection in 1995 than did a programme based on mancozeb. In 1997, however, the high level of foliar blight up to 10% infection may have been responsible for the lack of strong benefit from any of the treatments.

Variety resistance was also examined and Dr Bain found that Pentland Squire, which is moderately resistant to leaf blight, was unexpectedly susceptible to stem blight. It and King Edward were the most susceptible of the six varieties assessed.

Brodick was the most resistant to all forms of blight with Desiree, Maris Piper and Russet Burbank in between the two extremes.

NEW symptomless strains of the yield- and quality-sapping virus disease spraing will eventually arrive in the UK from the Continent, the Dundee Conference heard.

These new strains can overcome the few resistant varieties, including Bintje, said Finlay Dale of the Scottish Crop Research Institute. "We are shipping thousands of tonnes of potatoes in from the Continent," he said. "The odds are that eventually we shall bring in strains of the tobacco rattle virus we dont wish to see. We shall need good diagnostics to spot them."

The tobacco rattle virus causing spraing is seedborne in a number of weeds, including field pansy, knotgrass, shepherds purse and groundsel. It can survive for several years and is transmitted to potatoes by free-living nematodes in the soil. As well as affecting the yield of some varieties, spraing causes brown rings or spotting in the tuber flesh which will lead to instant rejection by chippers and other processors once levels rise above 3-5%.

Some plants show foliage symptoms but many do not. King Edward, Home Guard, Wilja and Romano can become systemically infected. Arran Pilot, Bintje, Record and Saturna show resistance.

IT is perfectly natural to die of smallpox, but it is unnatural and a threat to the worlds biodiversity to try to eradicate the disease. Thats the enigmatic statement Dr Phil Dale, of the John Innes Centre, Norwich, put to the Dundee Conference before outlining the case for genetic modification of potatoes.

It was argued by opponents that genetic modification was unnatural and therefore undesirable, he said. However, much public concern stemmed from a lack of knowledge about conventional plant breeding and how genetic manipulation could introduce greater precision into variety improvement. Several thousand genes might be transferred into new potato varieties by conventional hybridisation techniques when only one was required.

Donor species used in potato breeding might carry undesirable traits such as high glycoalkaloid levels which could be introduced into crosses. Mutation breeding by radiation or chemicals has also been widely used in traditional plant breeding, and is an entirely random process – Golden Promise spring barley is the best known example of radiation being used to produce a mutation.

However, Dr Dale pointed out that in traditional breeding, as with GM breeding, plant scientists discard crosses which are undesirable. Referring to the controversy over the lectin gene placed in potatoes at the Rowett Institute, he said the development of one plant with undesirable characteristics could not be used to condemn a whole technology.

Internationally-agreed risk assessment procedures were in place to ask questions about potential impacts from GM plants on food and feed safety. These included: the effect of a gene in the donor organism; the way in which the gene modifies a plant; any evidence of allergenicity or toxicity; effects on non-target organisms; changes in invasiveness or persistence; and the consequences of transgenes moving into other plants.

Asking these questions about GM potatoes would mean, for example, careful consideration of the introduction of cold tolerance when it was known that the susceptibility of potatoes to winter temperatures is an important means of controlling groundkeepers and volunteer seedlings.

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