Wire Stem of Cabbage 55 



five moistures, not counting the control plants. Results similar to those 

 in the previous experiment were obtained. Permanent wilting resulted 

 in bottom-watered soil with a moisture content of 18 per cent. Incipient 

 wilting only resulted at that moisture when the water was added to the 

 surface of the soil. The rate as well as the total number of diseased 

 plants was greatly increased by surface watering (table 18 and figure 14). 

 This is a bit of experimental evidence, little of which is found in literature 

 in support of the recommendations mentioned above. The fact that little 

 such evidence is found is probably because it concerns a fact apparently 

 too obvious to require experimental proof. 



Effect of surface layer of sand 



The question as to the most practicable method of maintaining such a 

 dry surface still remains. Probably a current recommendation is a cover- 

 ing of sand. Johnson (1914) and Sherbakoff (1917 b), however, give 

 rather conflicting opinions. Johnson thinks that such a practice is of 

 doubtful value, while Sherbakoff states: 



It is often advantageous to have on the surface of the seed bed a half-inch layer of pure, 

 coarse sand. Damping off fungi grow on the surface or in upper layers of the soil and thrive 

 best when the soil is well supplied with organic matter. This layer of pure sand will hinder 

 the growth of the fungi. 



To obtain some data on this phase of the subject, the following experi- 

 ments were performed: 



1. Naturally infected soil was thoroughly mixed and placed in flats. 

 Seed was sown in the ordinary way and covered very lightly with soil, 

 and a |-inch layer of clean, coarse sand was placed over the surface. Only 

 50 per cent of the seedlings pushed through the surface. On investigation 

 it was found that all the seed germinated, but half of the seedlings were 

 destroyed before they reached the surface. All the plants which actually 

 started to grow damped-off later. Apparently the sand, instead of 

 permitting rapid surface evaporation, prevented it and formed an ideal 

 environment at the soil-and-sand juncture for damping-off. 



2. Another flat was sown to seed where the soil consisted of a mixture 

 of 50 per cent of infected soil and 50 per cent of sand. Although practically 

 100 per cent germination occurred, only 8 per cent of the seedlings reached 

 the surface of the soil. All of these damped-off later. 



3. Seed was sown on top of the infected soil in another flat, and covered 

 with sand only. Here ail germinated and grew, but later damped-off. 



4. As a control for the above-mentioned experiments, seed was sown 

 in infected soil containing no sand. Twenty-eight per cent of the seedlings 

 emerged, and all damped-off. All the seed in a flat of sterilized soil germi- 

 nated and remained disease-free. 



5. At a later date, seed was again sown in several flats of infected soil 

 and the surface was covered with coarse sand. These flats were all given 



