Airborne Spores Spread Swede Dry Rot

Press, Volume XCIX, Issue 29246, 2 July 1960, Page 9

Airborne Spores Spread Swede Dry Rot

Three years' research by Dr. Harvey Smith and Mr 3. D. Allen, plant pathologists of the Crop Research Division at Lincoln, has shown that infected seed is of little importance in the spread of dry rot disease in brassica crops, and that the major primary source is from airborne spores from the residues of the previous year’s crops. By sowing seed with known levels of infection and noting the development of the disease in the crop they have reached the conclusion that 0.5 per cent, of infected seed is of some importance in the • transmission of the disease if the weather is wet after sowing, but in a dry season a seed infection as high as 3 per cent, gave no transmisaion. Low Infection As most of the commercial seed sown in New Zealand rarely has an infection higher than 0.1 per cent, they conclude that the annual recurrence of the disease is not caused by infected seed. This is regarded as strong circumstantial evidence that airborne spores, which were first discovered by Dr. Smith in 1956, originating from old brassica residues, are the chief agents of primary infection in the majority of swede crops in New Zealand. At one time it was thought that controlling seed borne infection would eventually enable the complete elimination of dry rot in brassicas, and this led to the first

f. large scale production of twede >. end turnip seed in New Zeae land. i- By producing diaeaae-free aeed d it was generally thought that a n 90 per cent, control of dry rot e could, be achieved but, in fact, e effective control was, at the maxi- *- mum, up to 10 per cent. >f “The emphasis must now be placed on control by hygiene n until disease-resistant varieties e are available,” Mr Allen said, e Dr. Smith said that one variety i- which was available was highly i- resistant to dry rot ( but was only e of minor commercial importance e because of a high susceptibility to >t other diseases such as club root n and mosaic. ;r “It is now found that dry rot is closely tied up with the other diseases. Some of the varieties now available are resistant to j one disease, or occasionally two, but when grown on the same j ground in successive years can . be wiped out by one of the other diseases carried on in the resiig dues remaining in the ground . from the previous crop,” Dr. e Smith said. ,Z Resistance y About half a dozen lines have been developed Independently by the Department of Agriculture it and the Crop Research Division n which have increased resistance to aphids, club root, mosaic and a dry rot, and some of these lines, it as new varieties, should be available commercially to farmers by 1963.. • “Until the new varieties being developed have been released, two new overseas varieties called Doon Major and Doon Spartan are being increased by the De- , partment of Agriculture because ; of their dry rot and mosaic ret sistance. Doon Spartan, in par- , ticular, has shown up well ' throughout Canterbury in field , trials, and should be a useful adt dition to the Calder and Sensa- . tion varieties which are at pres- , sent recommended,” Dr. Smith f said. - ,