672 " Damping off " of Tomato Seedlings. [Oct., 



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 check- 

 ing the disease. The optimum temperature for growth of 

 P. parasitica is about 30° C. (86° F.) , and that of 

 P. cryptogea and Rhizoctonia about 25° C. (77° F.). Below 

 12° C. (54° F. ) the growth of all three is very slow. If the 

 disease starts, the grower should endeavour to keep the tem- 

 perature down as far as possible, without impairing the health 

 of his plants. 



Sterilisation of the soil by heat or formaldehyde has proved 

 the most effective method of controlling " Damping off." 

 All strengths of formaldehyde solutions from two per cent, 

 upwards are effective for soil sterilisation, but weaker solutions 

 are not sufficiently strong to sterilise the soil completely. The 

 method employed for sterilisation by formaldehyde is as 

 follows: — 



A two per cent, formaldehyde solution is made by adding 

 one gal. of commercial formaldehyde (40 per cent, formal- 

 dehyde) to 50 gal. of water. Pots and boxes are sterilised by 

 soaking them in the formaldehyde solution for ten minutes and 

 then placing them in a heap and covering with sacking for 

 48 hours in order to allow the vapours of the formaldehyde to 

 act. After this period the sacking is removed and the boxes 

 are allowed to dry. They may be used as soon as the smell of 

 formaldehyde Has vanished. The soil may be sterilised, either 

 in the boxes or in a heap. In the first case the boxes are 

 filled with soil and the formaldehyde solution is poured on until 

 the former is saturated. About a pint of solution is necessary 

 for 5 lb. of soil of average wetness. The boxes are then covered 

 with glass for 48 hours to keep in the vapours. At the end 

 of this period the covers are removed and the soil is allowed to 

 dry. When the formaldehyde vapours have evaporated, usually 

 a week or ten days after removing the covers, it is safe to sow. 

 Soil may be sterilised in a heap by saturating it with the 

 formaldehyde solution, covering with sacking for 48 hours, and 

 allowing it to dry before using. 



Summary of Methods of Control. — The following points 

 should be observed: — 



1. A pure w T ater supply should be used. 



2. All seed boxes, pots, and propagating soil should be steril- 

 ised by heat or formaldehyde. 



