W. F. Bewley 171 



Cultural Methods of Control. 



The control measures first described, namely soil sterilisation, are 

 those of prevention and cannot be applied after the seed is sown. 



It is clear that a certain amount of infection comes from the use 

 of contaminated water, and in nurseries where this has been proved to 

 be the case, it is highly important to obtain a clean water supply. 



If the disease has started when the plants are growing, the application 

 of a mixture of 10 parts of lime and 1 part of copper sulphate, put on the 

 soil at the rate of f oz. per square foot, has been found to be useful in 

 keeping down the disease. The use of hexamethylene tetramine shows 

 promising results, and further work on this compound is in progress. 



The Moisture Factor. 



A relatively high percentage of moisture in the soil and the air favours 

 the rapid spread of the "damping off" organism. Careful regulation of 

 the watering, so as to keep the seed-boxes uniformly moist and good 

 ventilation of the propagating houses to dry out the surface soil, will 

 produce the best moisture conditions for checking the disease. 



Temperature. 



The optimum temperature for growth of PA. terrestria = Ph. parasitica 

 is about 30° C. (86° F.), and that of Ph. crijptogea and Rhizoctonia about 

 25° C. (77° P.). Below 12° C. (54° F.) the growth of all three is very slow. 

 When the disease has started among the plants, the grower should there- 

 fore endeavour to keep the temperature as low as possible without 

 impairing the health of his crop. 



Further work is in progress upon the physiological relations of the 

 disease organisms to their environment, and upon their reaction to 

 certain chemical compounds; also upon the method of cleansing con- 

 taminated water. 



The author takes pleasure in thanking Mr W. B. Brierley of the 

 Rothamsted Experimental Station for the many helpful suggestions and 

 criticisms he has so kindly given him. 



Conclusions 



ft 

 1. "Damping off" of tomato seedlings is a communicable disease 



due to a group of pathogenic organisms, particularly species of the genus 



Phytophthora. 



