64 



In considering methods for preventing this disease it 

 IS necessary to make use of the facts which are known of 

 the means of spreading of the fungus causing the disease. 

 We have seen that once a seed-bed is infected the fungus 

 rapidly spreads, first by an abundant growth of filaments 

 over the soil from seedling to seedling, and secondly by 

 means of spores which swim in the water in the soil. Any 

 treatment to be successful must check the vegetative 

 growth of fungus in the soil and also the production of 

 spores. It has been shown that the fungus only grows m 

 this way in warm, very moist, badly ventilated seed-beds. 

 Moreover, the eftect of excessive moisture and lack of 

 sufficient air is two-fold. Xot only do such conditions 

 favour the growth of Pythiiiiii^ but these verv conditions 

 tend "to produce weak drawn seedlings which of course 

 are more susceptible to attack. Very careful attention to 

 cultural conditions will therefore make it possible to ward 

 off" and. 111 most cases, entirely prevent the disease. The 

 seedlings should never be overcrowded, and they should 

 be given as much air and as little water as possible. If 

 through inattention to these conditions a few of the seed- 

 lings m a bed should show signs of disease they should 

 be immediately removed and iDurned, and more air and 

 less water given to those that remain. In any treatment 

 it is also necessar;\' to prevent the infection of the seed-bed 

 at the beginning of the season. This infection invariably 

 arises from the presence of resting spores in the soil. 

 Naturally, then, soil known to have previously produced 

 the disease should not be used, but since the resting spores 

 are present m most garden soils it is wisest to endeavour 

 to get rid of them. They can be killed by partially 

 sterilising the soil by heating with steam or with boiling- 

 water. It has been shown in an earlier lecture that the 

 partial sterilisation of soil is valuable for other reasons. 

 By adopting the practice it is possible to eliminate the 

 resting cells note only of PytJiiinw but also of other 

 harmful organisms. 



