NATURE NOTES

Press, Volume LXXIV, Issue 22371, 7 April 1938, Page 6 (Supplement)

NATURE NOTES

! DRY-ROT QF SWEDES AND TURNIPS

■ Joint Gray, of Tormore School, North Canterbury, forwarded a turnip which was seriously affected by a disease known as dry-rot. The iscaused by a fungus, that is a plant without chlorophyll or leaf-green. As it cannot make its own starch it is dependent on some other plant for Its food; in obtaining -fins food it causes the other plant to become diseased. Since 1905 when - it was' first noticed in New Zealand, dry-rot has become so widespread that it is now fixe most serious disease of swedes. On turnips its effects are not usually so severe, but even in the case of this crop, ofie out of fhwn hnihc mav be destroyed. It appears that the fungus spends its resting or over-wintering stage hxtfae seed-coat of turnip or swedeseed. When the seed germinates the fungus begins to grow, sending its threads into the two cotyledons or

seed-leaves of the turnip. As these threads feed on the sap in the cells a scar or lesion is produced. Having killed a portion of the leaf, the fungus prepares itself for dispersal. • 2d the dead parts of the leaf are produced small flask-shaped struc- - tores in which large numbers of - small, colourless spores are produced. These spores are not able ' to escape until the “flask” absorbs . water, swells, and pushes them out. . Thus we find spore-discharge < possible only in wet or muggy . weather. « The fungus spreads by means of these spores to other leaves of the same plant or to other plants close by- The scars or lesions on the leaves are usually so small in number that little damage is done by the disease at this stage. The real trouble commences when the plants are six to eight weeks old and are commencing to form their bulbs. Dry-rot, if prevalent, becomes very noticeable owing to the death of plants here and there. Examination shows the upper parts of toe bulbs to be quite rotten. The spores had been washed from toe leaves.

(By L. W. McCaskill)

landed on the neck and rapidly caused it to rot. Very soon these decayed areas produce spores every time the,weather is moist and the infection may thus be spread over the whole field. The necessity for moisture to be present for spore

scattering explains why dry-rot Js most prevalent in a wet season and in the wet or low-lying portions of the paddocks. If the bulbs are attacked when small, the death of the plant always follows. This is because the diameter of the bulb is small and

the fungus threads quickly destroy all the tissues. In a large bulb, on the other hand, the number of scars may be .much, but as the diameter of the bulb is now large each scar penetrates only a little distance. Such bulbs may remain sound through the winter. If left on the ground these infected but sound bulbs will act as a source of infection to another crop in the next season.

Turnips and swedes are biennials. As we grow them for the food stored up in the bulb in the first year we rarely see them flower. If we did leave infected bulbs in the ground to flower 1 , we would probably find that the leaves would develop the spots' where the fungus had attacked them. We might also notice the spots on the stalk and again on the seed pods. From these spots on the pods the fungus can grbw into the seeds and there take up its resting stage in the seed coat.

Except on infected bulbs dry-rot does not seem to be able to overwinter in the soil; nor does it appear to be carried by stock. As it is carried in the seed it would appear that killing it in the seed would be the best method of dealing with it. The problem is to find something which will kill the fungus in the seed but which will be harmless to the seed itself. The treatment consists of soaking the seed in a chemical at a temperature of 115 deg. F. for one hour. It has been suggested that if all the seed imported into New Zealand could be so treated, it would soon be possible to reduce the disease to very small proportions. If such seed were used farmers would have to sow it on land which had not been growing turnips and swedes for at least a year. EXCHANGE George' Gilling, 2 Aotea terrace, Huntsbury Hill, St. Martins, Christchurch, has the following Cadbury’s stamps of No. 2 album. Nos, 23, 39, 40. 42, 93, 97, 98, 103, and 109 to exchange for any of these Nestles stamps; Nos. 22, 30, 32, 33, 49, 54, 77, 80, 111, 112, 115, 116, 117, and 125 (No. 3 album)..