Annals of Applied Biology, 7: 161-162 (December, 1920)
“Damping off,” etc., of Tomato Seedlings
W. F. Bewley

Experiments were set up to test the effect of different methods of making up the seed boxes upon the incidence of the disease. The standard method was to weigh out 5 lbs. of soil into each box. Then the soil was compacted by means of a builder's board, soaked with water, 100 seeds per box were sown and covered with a thin layer of sterile soil. In certain instances, however, sand, lime or charcoal was used in place of the sterilised soil. In other cases a layer of sand, lime or charcoal was put on over the sterile soil, and in still other cases the soil was mixed with different proportions of sand, lime or charcoal. A parallel set of experiments was set up at the same time, using sterilised soil. Diseased seedlings were removed from the boxes as soon as they were attacked in order to eliminate the factor of superficial spreading of the disease. The results of these experiments are shown in Table II.

In the above experiments the soil sample, its weight, the sterility of the seed-boxes, the seeds, the temperature, the barometric pressure, the quality and quantity of the light, and the quantity of sterile water given to each box were constant factors. The limiting factors were solely those indicated in the above table. An examination of the percentage germination columns shows that the difference between the lowest percentage germination and the highest is fairly constant and the remainder of the results are symmetrically placed about the mean. The percentage diseased seedlings showed a similar arrangement. This further indicates that the average percentage results can be taken as accurate measure of the experiment in question.

A covering of sand, charcoal or lime either alone or above a covering of sterile soil produces only a small increase or decrease in the percentage of diseased seedlings per box. Charcoal has no effect, when put on as a covering to the seeds. Sand reduced the percentage diseased by 20 per cent., while lime increased it by 25 per cent. Five per cent. of charcoal added to the soil has a distinctly beneficial effect, for, besides reducing the percentage of diseased seedlings by 25 per cent., it produced a fine crop of sturdy dark green seedlings. A further increase in the amount of charcoal added is not advisable, for it only increased the difficulty of keeping the soil at an even degree of moisture. Certainly a decrease in the percentage of diseased seedlings is induced but this is obviously caused by the increased proportion of sterile particles in the soil. In the case of lime, whether it is added as a covering to the seeds or mixed with the soil itself it increased the percentage of diseased seedlings by nearly 50 per cent., and appears actually harmful. This is in agreement with the fact that the parasitic organisms grow best in a neutral medium.