254 



THE AGRICULTURAL NEWS. 



August 11, 1917. 



PLANT DISEASES. 



THE CONTROL OF DAMPING-OFF. 



Damping-ofF is the term applied to the failure of seedlings 

 due to their infection while in a tender state by certain 

 soil-inhabiting fungi. The reason for the name is the 

 association of the trouble with conditions of more or less 

 excessive moisture, whirh favours the development of the 

 parasite at the same time that it increases or prolongs the 

 tenderness of the plants. Though not necessarily confined to 

 seedlings crowded in boxes or seed-beds, it is amongst such 

 that the afTection usually appears and, by progressive infection, 

 is able to cause extensive losses Merely reducing the 

 density with which the seed is sown is often sufficient, by 

 permitting increased ventilation, preventing the drawing up 

 of the seedlings, and making the spread of infection more 

 difficult, to avoid < r reduce the damage. 



The longer the soil of the seed-bed has been in use and the 

 more decaying animal or vegetable material it contains, the 

 moie likely it is to harbour fungi capable of causing damping- 

 off. Heavy water-retaining soils are more favourable to the 

 afl'ectiou than those which are light and porous, and provision 

 for rapid drainage is one of the most important precautions 

 against it. Over-shac'ing and close shelter, by maintain- 

 ing humidity, increase the tendency to it; in these respects, 

 as in the supplying of water, the conditions which favour 

 the .seedlings favour the disease, and a mean has to be struck 

 between slow growth and loss. 



The trouble occasioced by damping-off in these islamls 

 is not so great as might be expected by an agriculturist 

 accustomed to temperate countries, where warm, humid 

 weather, such as is asssociated with ideas of the tropics, is 

 greatly feared in this connexion. 



In the first place nearly all the staple food plants, sugar- 

 caue, bananas, tannias, dasheens, sweet potatoes, yams, cassava, 

 are raised from cuttings of one .sort or another. Cotton, corn and 

 pulses, which are raised from .seed, are planted a few seeds 

 together, in their permanent positions in the open ground. 

 Of agricultural as distinct from garden crops tobacco, onions, 

 and limes, which are raised in seed-beds, have been the 

 plants to suffer most in the West Indies from the affection 

 under notice. 



Xotes on the subject, embodying the results of experi- 

 ments conducted in the United States, were published in 

 Volume .\I1I, of this Journal (p. 380). A bulletin recently 

 received (United Statex Department of Agriculture Bulletin 

 No. 4oJ) by Messrs. Carl Hartley and Koy G Pierce, states 

 the conclusions derived from further studies, made on conifer- 

 ous seedlings. 



The authors point out that the methods of prevention 

 commonly adopted by nurserymen, such as the use of sandy 

 soil, the u.se of .sand or gravel for surfacing the beds, the 

 provision of good drainage and ventilation, while often 

 succes.sful, do not avail to prevent heavy lo.sses under 

 unfavourable circurastancos, while the withholding of water 

 often does as much direct harm as the affection which it is 

 sought to avoid. 



The use of imperfectly rotted manure, of lime, of wood 

 ashes, and of a mixture of eoal and wood ashes are indicated 

 as having had bad effects. - 



The one means so far discovered which can be relied 

 upon to give satisfactory results under any reasonable con- 

 ditions is the disinfection of the soil. The question remains 

 as to which of the various methods available for this purpose 

 it is most convenient to adopt. The use of steam or of 

 formaldehyde has been widely recommended, but these 

 methods are so expensive as to be impracticable except for 

 relatively small quantities of valuable material, as in green- 

 houses or market ^lardens. The method of their application 

 has been already outlined in this .Journal (Vol. .\III, pp. 7S 9). 



For the broader requirements of nurserymen the treat- 

 ment finally adopted by the authors of the bulletin under 

 review consists in the application in standard soils of three- 

 sixteenths of a fluid ounce of commercial s'llphuric acid to each 

 square foot of seed bed, applied in solution in water immedi- 

 ately after the seed is sown and covered. This has proved 

 more reliable than the more expensive methods mentioned 

 above. 



The amount of water used to carry the disinfectant does 

 not appear to be a mHtter of importance, provided that the 

 necessary amount of the acid is applied to each unit of area. 

 The quantity used by the authors varies from 1 pint 

 per square foot when the soil is wet to 2 pints when the 

 soil is dry. There is a possibility, especially in light soils, 

 of a concentration of the acid by evaporation to a strength 

 injurious to the root-tips, which in practice has been found 

 to be completely avoidable by watering the beds frequently 

 during the period of germination. When the root-tips have 

 penetrated to a depth of half an inch this is no longer 

 necessary. 



There are differences in the amount of acid required for 

 succe.ssful results in different soils. In sandy soil which was 

 probably somewhat alkaline, a heavier application, one-fourth 

 to three-eighths of an ounce, was indicated. In a fine sandy 

 soil which was piobably already acid, chemical injury to 

 seedlings was more difficult to avoid, and reduction of the 

 acid to one-eighth of an ounce was advisable. On heavier 

 soils the use of five eighths of an ounce produced no injury, 

 and reduced losses by damping-off to less than 1 per cent. 



On a soil with a high carbonate content, evidenced by 

 a vigorous effervescense when the acid was applied, the 

 method was found to be ineffective. On this soil the use of 

 copper sulphate, one-fourth ounce per square foot, gave 

 good results This substance was applied in the same way 

 as the acid, and the same precaution to avoid chemical injury 

 was found necessiry 



An interesting indication was given by the experi- 

 ments of the effectiveness of cane sugar, 2^ oz. to the 

 square foot, in tlie control of damping-off. "The authors 

 point out that if some unrefined sugar-bearing substance were 

 available, it is possible that for certain soils the application of 

 sugar would become an economically satisfactory treatment. 

 Experiments on this subject might well be carried out in the 

 West Indies. 



There are secondary advantages to be expected from 

 sulphuric acid disinfection which may be of considerable im- 

 portance in some oases. Under appropriate circumstances 

 a. larger germinati.in percentage is secured, the number of 

 parasites in the soil is reduced, and the well-kno^vn effect of 

 disinfection on fertility results in increased growth. Another 

 valuable effect has proved to be the reduction of weeds owinf 

 to the greater susce[)tibilily of their seeds. 



In considering the application of the methods to local 

 conditions, it is necessary to emphasize the fact that the 

 results stated havL- been obtained with the seedlings of 

 a definite group cf plants, the conifers. It will be necessary 

 to find by experiment how far they are transferable to the 



