CORTICIUM-DISEASE OF POTATOES

New Zealand Journal of Agriculture, Volume XXX, Issue 2, 20 February 1925, Page 93

CORTICIUM-DISEASE OF POTATOES

EXPERIMENTS IN CONTROL.

( Concluded.)

G. H. CUNNINGHAM,

Mycologist, Biological Laboratory, Wellington.

In a previous article {Journal for last month) it was shown that the standard treatments recommended for the control of corticium-disease, Corticium vagum var. Solani Burt, were not successful because they did not kill more than about 80 per cent, of the sclerotia present on the tubers treated. Numerous experiments were undertaken with a view to discovering some cheap and efficient means of killing these sclerotia, with the result that mercuric chloride, when acidulated with hydrochloric acid, was found completely to kill all sclerotia even at strengths much less than usually recommended. These experiments solved the problem in so far as the steep was concerned, so that there then remained the problem of cheapening the process in both material and labour. In regard to the latter factor, it is evident that a more practical method than the two-hour steep is necessary in dealing with the large quantity of seed tubers used by commercial growers. Therefore experiments were undertaken with a view to modifying the period of immersion. For the smaller growers with a small quantity of seed to treat, it might be more practicable to place the tubers overnight in some suitable solution, to be ready for planting the following morning. Graph 7 shows that a solution of 1 part of mercuric chloride in 10,000 parts of water, when acidified with as little as o-i per cent, of hydrochloric acid, gives complete killing of all sclerotia, a result not obtained with an acid-free solution of this strength.

Further experiments were undertaken with a view to reducing the time of immersion to such an extent that treatment could be made a continuous process, thus making machine treatment feasible. The results finally obtained are shown in Graph 8. In this experiment tubers were immersed for five minutes in acidulated mercuric-chloride solutions of the following strengths : Mercuric —l-1,000, 1-1,500, 1-2,000, 1-2,500, 1-3,000 ; hydrochloric acid— per cent., 1 per cent.,

I’s per cent., 2-5 per cent. After treatment the tubers were placed in a heap and covered with a sack wetted with 1-3,000 mercuric chloride. They were left overnight and the sclerotia plated out in the morning. Graph 8 shows that this treatment resulted in the death of all sclerotia taken from those tubers which had been immersed' in 1-1,000 and 1-1,500 solutions to which had been added 1, 1-5, and 2-5 per cent, of hydrochloric acid. This reduction in time required for treatment allows of the treatment of tubers in large quantities by -the use of a mechanical device whereby tubers may be carried on a conveyer through the solution and transported to the drying-floor. ; ■ DIRECTIONS FOR TREATMENT. Overnight Treatment.- —Procure a wooden or concrete (not ■ metal) tub or trough and fill with Solution A (see below).- Place the tubers in this solution, being careful to have sufficient liquid present to cover all to a depth of at least 3 in. Leave the tubers in overnight,' and in the morning remove and either immediately plant or dry and store until required. The time of immersion (sixteen hours) is merely an arbitrary one, as equal results will be obtained whether the tubers are left in the solution for twelve or twenty-four hours. ’ Five-minute Treatment. — Prepare solution in the same way as in the previous treatment, but use instead Solution B (see below). Leave in for at least five minutes (longer periods than 'this will have no detrimental effect upon the tubers) and remove to a floor, where the tubers should, be piled in heaps and covered with sacking dipped in the same solution. Leave the heap covered for from sixteen -to twenty-four hours, and either’ plant immediately or dry and store until required. SOLUTIONS. Solution A, Sixteen-hour Treatment. Stock solution: Mercuric chloride, 50 grams (if oz. approx.) ; hydrochloric acid (cone.), | litre (•§ pint approx.). This quantity of stock solution contains sufficient to make no gallons of steep. For smaller quantities add 1 fluid ounce of the stock solution to 12 gallons of water. Solution B, Five-minute Treatment. Stock solution: Mercuric chloride, 200 grams (7 oz.) ; hydrochloric acid (cone.), 2 litres (3I pints). This quantity of stock solution is sufficient to make 66 gallons of steep. For smaller quantities add I pint of stock solution to 19 gallons of water. After treatment with A the solution should be discarded; Solution B may be used three or four times before being discarded.’ As these solutions are. corrosive' and very poisonous they should be handled with care and kept away from children or stock. The use of metal implements should be avoided, as they will become corroded if allowed to come in contact with these solutions. The mercuric chloride used should be of good quality, and the hydrochloric acid of commercial concentrated standard (31 per cent.). The cost of these substances is comparatively low, mercuric chloride : being 6s. per pound, and hydrochloric acid 7s. 6d. per “ winchester ” of 10 lb. Thus the cost of the material required to prepare 66 gallons of Solution B is 7s. Bd., and that of Solution A is. 2d.

FUTURE WORK.

Further work is required to determine (i) the effects of treatments upon the tubers, and (2) the detrimental effects of the disease upon the yield. With,regard to (1), it may be stated that all tubers were kept after treatment, and the production of shoots noted. In all cases where the tubers had well-developed shoots these were killed back to the tuber ; but. in the course of a few days further shoots developed in abundance, save in those cases where the solutions used had been acidified with 2-5 per cent, hydrochloric acid. With these, delay of a fortnight was evident, and with tubers which had been cut prior to immersion death was not infrequent. It is hoped next season to be able to carry out an extensive series of field experiments, with a view to solving these two last problems. The writer is indebted to Mr. J. C. Neill, of this Laboratory, for assistance rendered during the course of the experiments.