18 Fufifjfts Flora of Water Siq^plies 



killing the mycelium and " summer " spores which it contains. Only the 

 thick-walled resting spores remain and these have not been found to any 

 great extent in the water samples tested. 



By raising the temperature of the water to 70° C, for one minute, the 

 mycelial and "summer" spore forms of the fungi tested were killed. 

 Complete sterilisation of the water is obtained by raising the temperature 

 to boiling point for one minute. 



Control. 



Investigation of the cause and source of any plant disease is not 

 complete unless the water supply has been carefully examined as a 

 possible source of infection and in cases where it is probable that this 

 is contaminated, steps must be taken to purify it. 



The first process should be the thorough cleansing of the well, by 

 removing as much algae and decaying plant material as possible, for a 

 surface scum of the former assists the fungi mechanically and the latter 

 makes the water a good medium for fungal growth. Copper sulphate 

 used at the rate of 10 lb. to 1,000,000 gallons of water will keep down the 

 growth of algae, which are so objectionable in wells, tanks, etc. When 

 the water is very thick with particles of decaying plant material it may 

 be cleared by precipitation with alum and sodium carbonate, added at 

 the rate of 18 lb. potash alum and 5 lb. sodium carbonate for every 

 25,000 gallons of water. Perchloride of iron at the rate of 9 lb. per 25,000 

 gallons of water is also a useful clarifying agent. Of the chemical com- 

 pounds tested, mercuric chloride and "chloros" are the most toxic to 

 fungi. Mercuric chloride, however, is highly poisonous to man and it is 

 not advisable to use it unless very special precautions are taken to pre- 

 vent the workers drinking the treated water. No such restrictions apply 

 to "chloros" in such a highly diluted condition. It should be added at 

 the rate of 50 gallons to 100,000 of water, but half that concentration 

 will give protection against the "summer" spore forms of Phytojjhtliora 

 spp. Complete sterilisation of the water is only obtained by boiling, but 

 beneficial results have been obtained by arranging the pipes leading to 

 the tank in such a manner that they are heated by passing through the 

 boiler. While it may not be so necessary to supply the more mature 

 plants with disease-free water, it is advisable to sterilise in some way all 

 the water used in the early stages of propagation, when the plants so 

 readily succumb to " damping-off " and "foot rot." 



In choosing the source of water supply the following points should be 

 considered. Stagnant brooks are really natural sewers and may be 



