176 DISEASES OF GLASSHOUSE PLANTS 
grower to a better understanding of the effects of spraying 
and perhaps prevent undue disappointment. This pro- 
tection is obtained by covering the entire plant surface 
with a thin, poisonous coating able to destroy any fungus 
spores which may settle on the plant. Spraying efficiéney 
is governed by a number of different factors, each of 
which must be taken into account if success is to follow. 
In the first instance it is necessary to determine 
correctly the cause of the disease. This is important 
because different organisms are more sensitive to one 
poison than to another, and what is a certain cure for 
one disease may not affect another in the slightest degree. 
Thus spores of Botrytis are extremely sensitive to calcium 
bisulphite, being rapidly killed by a two per cent solution, 
which is quite innocuous to spores of Cercospora melonis. 
Having, however, determined what organism is 
causing the trouble, a suitable spray liquid may be 
chosen; but it is necessary to remember that complete 
protection of the plant tissues depends upon every part 
of the surface being covered by the poisonous film. 
Some plants are easily covered by watery solutions, but 
others, such as those possessing waxy or hairy surfaces, 
offer considerable resistance. This is overcome by 
adding a “‘ wetting agent’ or “ spreader ”’ to the liquid, 
which assists it to wet the entire surface. Further, any 
new growth which develops after spraying has been 
carried out is of course unprotected, and so at critical 
periods the spraying operation must be repeated at 
frequent intervals. 
Because the work of the spray is to protect the plant 
by destroying any fungus spore which happens to fall 
on its surface it is necessary to spray only immediately 
prior to and at the time when the spores are being shed 
and blown about in the air. Spraying at other times, 
when spores are not being produced and therefore do not 
exist in the air, is merely wasted time, money, and 
energy. ‘This being so, a knowledge of the life-history 
and methods of growth of the disease organisms is 
