CORTICIUM-DISEASE OF POTATOES.

New Zealand Journal of Agriculture, Volume XXX, Issue 1, 20 January 1925, Page 14

CORTICIUM-DISEASE OF POTATOES.

EXPERIMENTS IN CONTROL.

G. H. CUNNINGHAM,

Mycologist, Biological Laboratory, Wellington.

Corticium-disease, caused by the fungus Corticium vagum Berk, et Curt., var. Solani Burt, is a widespread disease of the potato, for it has been recorded as occurring wherever the host is grown. During a recent plant-disease survey made by Mr. J. C. Neill, of this Laboratory, through the potato-growing regions of New Zealand it was invariably found to be present ; and in the South Island it was found to be abundant in every line cf potatoes inspected. Although so widespread, it is, a disease that is commonly overlooked, due no doubt to the fact that its effects upon the host arc not marked. When the crop is being lifted the disease may be seen in the form of small black sclerotia scattered over the surface of the tuber (Fig. i). These sclerotia are resting bodies of the causal organism, capable of remaining in a quiescent condition for an indefinite period. They are firmly attached to the tubers by means of hyphae, and are not readily removed, consequently they accompany the tubers when the latter are planted. In the presence of moisture, such as is present in the soil, the sclerotia produce hyphae which ramify through the soil in the vicinity of the tubers, and spread to the potato-shoots as they develop. These hyphae would,’ appear frequently to damage the growing points of the main shoots,' with the result that secondary shoots are produced from below the injured portions. In this manner affected tubers may give rise to bunches of small and spindly shoots, which, being weakened, produce small and stunted plants ; these in turn produce few and small tubers. Thus infection, when severe, may tend greatly to reduce the yield. In the literature dealing with this disease the standard treatment recommended (i, 3, 4, 5, 6, 8),* which has been claimed to give complete control (100 per cent, killing of sclerotia), is the immersion of infected tubers in a solution of mercuric chloride (corrosive sublimate, HgCl 2 ) in water—1 part in 1,000 parts of water,- or 1 part in 2,000 — for one and a half or two hours. More recently it has been claimed (2)* that immersion for half an hour is sufficient to ensure death of all sclerotia. A potato-grower in Otago this season treated the whole of his seedtubers before sowing by immersion for one hour and a half in 1-1,000 mercuric chloride. After treatment he forwarded samples to this Laboratory to ascertain whether the treatment had been successful. Sclerotia from these tubers were plated out on suitable media, - and 30 per cent, were found to be viable. This led to preliminary experiments being carried out in the Laboratory with a view to ascertaining whether the standard solutions recommended were at fault. These experiments were unsatisfactory in that a proportion of sclerotia treated were found to be viable. Therefore an elaborate series of experiments was undertaken with a view to definitely ascertaining the. most

* References at end of this article.

efficient steep to be used, its strength, and the most suitable time of immersion (one hour and a half being considered much too long for farm praclici). In the following experiments tubers showing well-developed sclerotia were chosen. They were washed free from adhering earth and allowed to dry. The solutions were prepared in large glass vessels, and the tubers immersed for definite periods of time, then removed, labelled, and air-dried. Three tubers were considered sufficient for each phase of the experiment, and from each five sclerotia were removed and transferred to petri dishes containing certain culturemedia— the most suitable being potato-dextrose-agar, which gave ioo per cent, of growth in controls. Each, petri dish, by means of grease-pencil markings on the bottom, was divided into three sectors, and in each sector five sclerotia were placed. In this manner economy of media and petri dishes was obtained.

During the course of each experiment cultures were examined every twenty-four hours for five days, the final examination being made under the dissecting microscope. The cultures were then discarded, for it was found that if treated sclerotia made no growth before the third day they would not grow at all. MERCURIC CHLORIDE (HgCl 2 ). The first series of laboratory experiments was undertaken with a view to determining whether the mercuric solutions recommended would really kill sclerotia, and with the additional object of cutting down the time of immersion. The following. strengths of solutions and times of immersion were used :—■ . Mercuric chloride — 1-500, 1-750, [-1,000, 1-1,250, 1-2,000, 1-2,500. Times of immersion— 15, 30, 45, 60, and 90 minutes.

The accompanying Graph i shows the results obtained from this experiment. In this and successive graphs the ordinate (vertical lines) show growth of sclerotia, the cipher O representing no growth or total killing, indicating that complete control has been attained ; whereas the abscissae (horizontal lines) represent strengths of the solutions used (unless otherwise stated). The series of curves given represent different times (in minutes) of .immersion, acidity of. solution,', or strength of solutions. From Graph I it will be seen that the usual strengths of mercuric chloride recommended (1-1,000 or 1-2,000) do not give favourable results, for total killing. is not obtained. ; It was considered possible that the erratic nature of the curves obtained resulted from the frequent ■ presence upon the sclerotia of air-bubbles, for it was noted at the time that these often persisted throughout the duration of the experiment. Therefore a second, series of experiments was undertaken, in which the same strengths of solutions were used, and the tubers immersed for the same periods of time ; but in addition the tubers were first presoaked for sixteen hours in tap water, it being considered that long immersion would tend to dispel the air-bubbles, and, permitting of the'ready penetration of the mercuric-chloride solutions, tend , to flatten the resultant curves.. The time (sixteen hours) was merely an arbitrary one, being chosen for the reason that it would be most convenient to the farmer, who would require merely. to place his tubers in water in the evening and have them ready for treatment in the morning. In Graph 2 are shown the results obtained —results which are unsatisfactory both in point of time and killing, for it will be noted that, even after immersion for ninety minutes, solutions of a greater dilution than 1-750 do not give complete killing. , ■ The latter experiment showed that lack of control was not due to imperfect penetration through interference by air-bubbles, so' it was considered that greater concentration of solution might give satisfactory results. Therefore a further series of experiments was undertaken, in which tubers were immersed in. various solutions for different times as follows :—- Third, experiment—No presoak. Fourth experiment—Presoak sixteen hours. . Solutions—Mercuric chloride, 1-200, 1-300, 1-400, 1-500, 1-750, 1-1,000. Times of immersion—15, 30, 45, 60, 90, 130 minutes. Results obtained showed that in Experiment 4 complete killing was obtained with solutions 1-200 to 1-500 (or 1-200 to 1-400 in Experiment 3) after immersion for two hours, but solutions weaker than 'this failed to give complete control. As the cost factor was considered to be such that solutions of 1-200 to 1-500 would be too expensive to use, a further series of experiments was undertaken in which solutions . of Uspulun and copper sulphate were tried. USPULUN. ' - This is a mercury-chloro-phenol compound, widely used to-day for the treatment of seed-wheat as a preventive of stinking-smut. In the experiment the following strengths of solution - and times of immersion were used :— , Solution — 1-400 (standard), 1-500, 1-600, 1-750, 1-1,000. Times of immersion— 15, 30, 45, 60, 90, 120 minutes. ■" Sill .

Graph 3 (which differs from the preceding two in that the abscissae give times of immersion in minutes and the curves strength of solution) shows that even after two hours’ immersion 1-400 failed to give satisfactory control. Solutions 1-750 and 1-1,000 had no effect, for after two hours’ immersion 100 per cent, growth was obtained from treated sclerotia. . COPPER SULPHATE (CuSOJ. Preliminary experiments with this fungicide gave every promise that it would . prove a satisfactory if somewhat slower reagent than

mercuric chloride. Therefore a series of experiments was undertaken, in? which tubers were immersed for sixteen hours in the following solutions : i-ioo, 1-200, 1-300, 1-400, 1-500, 1-600, 1-750, 1-1,000, 1-1,500, 1-2,000. Graph 4 shows that even after this long immersion 1-100, the strongest solution used, would not completely kill all the sclerotia. In this last experiment nine tubers were used for each phase, instead of three as formerly. As these two compounds gave such unsatisfactory results, further experiments were undertaken with mercuric chloride.

MERCURIC CHLORIDE (FURTHER SERIES).

In the next experiment (No. 17) tubers were immersed for sixteen hours (as in the previous experiment) at the following strengths: Mercuric chloride —1-500, 1-750, 1-1,000, 1-1,250, 1-5,000, 1-1,750, 1-2,000, 1-2,500, 1-3,000, 1-3,500, 1-4,000. Graph 5 gives a curve of the results obtained. It will be noted that even after sixteen hours' immersion solutions of a weaker concentration than 1,-1,250 did not give complete killing, thus demonstrating forcibly the fact that one of the standard treatments widely recommended — namely, immersion for two hours in 1-2,000is ineffective.

As the result of a chemical and physical study of mercuric-chloride solutions it was decided to determine the effect of acidity upon their killing properties. Preliminary experiments -gave such promising results that a wide series was. planned. In the next experiment (No. 21) strengths of mercuric-chloride solutions were used as follows : 1-1,000, 1-1,500, 1-2,000, 1-2,500, 1-3,000, 1-3,500, 1-4,000, 1-5,000, 1- . 1-7,500, 1-10,000. - Each solution was made up in seven -vessels, and to ‘ each was added hydrochloric acid (commercial concentrated, 31-5 per cent.) at the following percentage strengths : None, o-i per cent., 0-25 per cent., 0-5 per cent., i-o per cent., 1-5 per cent., 2- per cent. Concurrently 'with this was run another experiment (No. 20) to determine the effect of hydrochloric acid alone on the growth of ..sclerotia. Tubers, in lots of three, were immersed for two hours in vessels containing water to which the following percentages of acid had been added :— - . HC1 a, none ; b, o-i ; c, 0-25 ; d, 0-5 ; e, i-o ; f, 1-5 ; g, 2-5. When the cultures were, examined twenty-four hours later it was seen that all sclerotia from tubers immersed in solutions a to & respectively were alive, while ten (out of fifteen) in /, and none in g, had germinated. In forty-eight hours all the sclerotia in / and g . had germinated. Thus even a 2-5 per cent, solution of hydrochloric acid has little effect upon sclerotia other than to retard their germination for twenty-four hours. Therefore it may be asserted that the acid alone has no effect upon the'germination of sclerotia. In Graph 6 are given the results of Experiment 21. Here it will be noted that although unacidified solutions do not kill at any of the •strengths used, when as little as o-i per cent, of hydrochloric acid is added killing is obtained even at a strength of 1-3,000. It will be noted that there is a definite relationship between the amount of acid added and the killing of sclerotia. Further experiments have been carried out with regard to. this work. These will appear in a subsequent paper, in which also will be given the cheapest and most economic method of control of corticiumdisease.

LITERATURE CITED. (1.) Bisby, G. R.,.and Tolaas, A. G., 1920. Potato-diseases in Minnesota. Univ. Minn. Agr. Exp. Sin. Bull. 190, p. 20. {2.) Coons, G. H., and Kotila, J. E., 1923. Michigan Potato-diseases. Mich. Agr. Exp. Sin. Bull. 123, p. 50. .(3.) Gloyer, W. O., 1913. The Efficiency of Formaldehyde in the Treatment of Seed Potatoes for Rhizoctonia. New York Agr. Exp. Sin. Bull. 370, pp. 417-431. . • (4.) Gussow, H. T., 1912. Report of the Botanist on Plant-diseases. Canada Exp. Farms Rep., p. 200. .(5.) McAlpine, D., 1911.. Rhizoctonia Rot, or Potato Collar Fungus. Potatodiseases in Australia, p. 65. .(6.) Noble, R. J., 1924. Rhizoctonia Scab in Potatoes. Ag. Gaz., N .S.W., vol. 35, p. 632. • (7.) Rolfs, F. M., 1903. Corticium vagum B. et C. var. Solani Burt, a Fruiting Stage of Rhizoctonia Solani. Science, vol. 2, p. 729. <(8.) Potato Failures : a Second Report. Colorado Agr. Exp. Stn. Bull. 91, p. 33, 1904. , . (To be continued.)