9 July 1999

Calcium limits rust spots

By John Allan

INTERNAL rust spot in pre-pack potatoes can be offset to a large extent by foliar applied calcium, even though its role is not fully understood.

"When potato prices are poor it makes the quality issue even more important," says Gidon Bahiri of Omex, manufacturer of calcium foliar feed Calmax. "Rubbish wont sell at all."

Independent work carried out by CSC Potato Care in Scotland shows foliar calcium can improve skin finish and cut IRS. The replicated trials showed a useful reduction in common scab, powdery scab, silver scurf and other skin blemishes The mechanism is believed to involve calciums strengthening of tuber cell walls.

Among other factors, plants stressed by water logging, stop-start growth, under irrigation or fast growth following irrigation tend to exacerbate IRS, says Greenvale AP technical director Simon Bowen.

"We have had some good results within our group trials when foliar calcium has reduced the incidence of IRS. But I dont think it is quite the panacea that some people are painting it and we need more work on understanding the original causes of the problem. We must remember it is also a varietal problem and that we need to encourage an even growth pattern in the crop."

Maris Piper, Russet Burbank and Lady Rossetta all suffer more from IRS than Cara and King Edward, says Mr Bahiri.

"The calcium must be applied early because it cannot erase damage that has been done," he says. "The latest thinking is that you treat from early tuber initiation, when tubers are the size of a pea. Starting with the first or second blight spray, use three applications of 2.5 litres/ha Calmax at fortnightly intervals. By the time untreated tubers are one or two inches across you will either have an IRS problem or will not." American work also shows there is no point applying calcium too late in the season, he adds.

In last years CSC trial, three foliar calcium products were evaluated alongside liquid and solid calcium nitrate. All treatments reduced the severity and incidence of IRS in the variety Cultra, reports Mark Ballingall. Calmax foliar applications at 2 litres/ha were started at tuber initiation and continued for six applications at two-weekly intervals. "Last years encouraging results have led to a follow up trial to validate results under different stress factors this year," he says.

Where susceptible varieties are being grown on a farm with a history of IRS, Mr Ballingall suggests foliar calcium should be applied, especially to crops which are likely to be stressed. &#42

CALCIUMFORQUALITY

&#8226 Adequate tuber calcium can limit internal rust spot.

&#8226 Skin blemish diseases may also be reduced.

&#8226 Start foliar applications at tuber initiation.

&#8226 Limited mobility of calcium in plant.

&#8226 Use repeat applications.