16 August 2002

Spuds to be subject of DNA testing technology

Growers had a glimpse of the future for potato disease

diagnosis and prevention at last weeks BPC-sponsored

Potatoes in Practice event. Andrew Swallow reports

POTATO agronomy is on the verge of a new era thanks to DNA testing technology, say scientists.

Realtime PCR testing techniques, developed for medical research, are being deployed by researchers at the Scottish Crop Research Institute to detect and quantify the presence of potato diseases at previously invisible infestation levels.

The findings seem set to revolutionise the precision with which growers can target when and where to grow potatoes and how they select seed, says the BPCs Rob Clayton.

"It could change which fields you use to grow potatoes in, cha-nge rotations, change what seed and what quality of seed you buy."

However, more work is needed to develop practical guidelines so the diagnostics deliver reliable field benefits. For example, the new PCR tests can identify precisely how much powdery scab, blackleg, or potato blight a seed tuber is carrying. But what that quantity of infection means for seed or ware crop production needs to be determined.

Similarly, the tests can be applied to soils, allowing scientists to quantify the infestation level of diseases such as powdery scab or rhizoctonia, the causes of black scurf.

"On your own farm that could help determine decline of the disease during the rotation and hence decisions about rotation length, which variety to plant, what market to target and how you manage the crop. If you are renting land it could lead you to reject certain fields."

Store loading risks should be reduced, too, because growers and packers will be able to quickly identify which and how many fungal spores tubers are carrying at intake. "If there are a lot of blight spores you would need to think hard about whether it is worth putting that crop into a long-term store," suggests Dr Clayton.

The technology could also be used to monitor fungal build up in store by testing dust residues trapped on fan filters.

"You could be able to determine that by the end of March a stock will be covered in silver scurf, hence it had better be sold in February."

Nationally, the extra speed and accuracy of the tests should help the UK maintain its high health status by detecting potential sources of brown rot infection faster, says Dr Clayton

His BPC colleage, Ewen Brierley, adds: "The new test is so sensitive it will detect the disease even in the presence of millions of other bacteria and compounds found in washings."

The work to develop the tests for potato pathogens was carried out at SCRI and CSL with funding from BPC, DEFRA, SEERAD and industry LINK funding.

NEWDNADISEASEID

&#8226 Fast and precise.

&#8226 Quantifies all main pathogens.

&#8226 Soil tests too.

&#8226 Will take potato agronomy to new level.

Seed bearing blight

An illustration of the potential of the new rapid realtime PCR tests comes from some surprising findings in a random sample of seed stocks, says SCRIs David Cooke. "In the survey, 20-30% of seed stocks tested positive for Phytophthora infestans using molecular diagnostics. Most had no visual symptoms." Dr Cooke says that figure is not necessarily representative, but says it does illustrate the potential problem of blight on seed and the potential of the science to detect such unseen risks.

Soils in Scotland heavily infected

IN developing the PCR tests, the Scottish Crop Research Institute sampled many soils across Scotland and found all to be infected with the powdery scab-causing fungus Sporangospora subterranea.

"We havent found a soil without sporangospora," says SCRIs Alison Lee. "We even took a soil sample from a golf course and found infection."

However, whether or not soil infestation causes crop disease will depend on environmental conditions when the crop is growing.

"It is one thing to identify the fungus in the soil, it is another to say that a crop planted in that soil will develop the disease."

Learn more about varieties

BREEDERS and growers could get more from new varieties if they did more work to determine how they grow, says SACs Stuart Wale.

"To get the best out of a variety you need to know it and some of the key characters are not mentioned in the potato variety book."

For example, growers need to know how a variety will respond to plant and row spacing, depth of planting and nitrogen regime. Such work cannot be done in comparative variety trials. It needs to done by breeders, variety maintainers or even grower groups, he says.

"If you want to get the agronomy of a variety right quickly you need to be running variety profiling trials, preferably at the same time as the comparative variety work is being carried out."

Monitoring development of a variety is also fundamental, recording how it covers the ground, how long it takes from emergence to tuber initiation and tuber initiation to dormancy break of the daughter tubers.

"You could just assess the yield at harvest, but there is an opportunity to learn a lot more," says Dr Wale.

For example, harvest management could be improved with more information on early yields, depth and spread of tubers, strength of stolon attachment and rate of skin set. Similarly, noting berry formation could identify the risk of volunteers from true seed.

A profile trial should not be over-complicated with a maximum of five or six factors examined in a step-wise fashion in a trial, he suggests (see example).

"It is possible to change several factors simultaneously and this could indicate how separate changes interact. But interpretation may be more difficult."

Profiling work should ideally start with the variety breeder or maintainer, who would pass the information to growers by way of product support, says Dr Wale. However, it is the grower taking the risk with the commercial crop.

"If you are told to plant a variety at 14in and it turns out that a 9in spacing is what is needed for your market it is going to make a big difference to your returns," he concludes.

Possible protocol for profile trials

Six treatments to four replicates in randomised block design.

1 Standard Osprey planted at 38cm spacing in 154cm bed at 15cm deep. 175kg/ha N applied, 125kg/ha in seed-bed plus Calcium Nitrate top-dressing to total.

2 Lower seed spacing As 1, but 34cm spacing.

3 Higher seed spacing As 1, but 42cm spacing.

4 Lower N As 1, but 150kg/ha total.

5 Higher N As 1, but 194kg/ha total.

6 Deeper planting As 1, but 20cm depth.

VARIETYPROFILING

&#8226 Essential to target agronomy.

&#8226 Keep work simple.

&#8226 Monitor as much as possible.

&#8226 Gain competitive edge?