DISEASES AND THEIR CONTROL 131 
Crop rotation is an important means of avoiding 
troublesome diseases of vegetable forcing crops. If a 
three or more crop system of rotation can be adopted, 
the chances of serious losses from diseases are much less 
than if but one crop is grown. 
Resistant varieties or strains——Some progress has been 
made in the development of varieties and strains of vege- 
tables for outdoor culture which are largely resistant to 
diseases. Very little progress, however, has been made in 
this direction with vegetables which are profitable for 
forcing purposes. There is no reason why strains or even 
varieties should not be found or produced which would 
be highly or quite resistant to fungous and bacterial in- 
fections. 
Steam sterilization—This is one of the most im- 
portant means of preventing numerous fungous diseases 
of greenhouse crops. See Chapter VI. 
Formalin sterilization is effective as a preventive meas- 
ure, where it is impracticable to use steam. See Chapter 
VI and page 98. 
Summer mulch.—It has been found that mulches of 
manure or other vegetable matter, applied during the 
summer and watered often enough to keep the soil moist, 
are effective in destroying disease germs of greenhouse 
forcing crops. See page 78. 
Spraying to control diseases affecting greenhouse vege- 
tables is just as unpopular as spraying to check the ray- 
ages of insect pests. It is a slow, tedious operation, that 
should be avoided if possible. But however thorough has 
been the work of sterilization and fumigation, and the 
observance of the various precautions previously dis- 
cussed, the grower sometimes finds it an advantage to 
employ fungicidal sprays. Their effectiveness depends 
upon the selection of the proper mixture for each dis- 
ease and applications that will be both timely and 
thorough. 
Bordeaux mixture is unquestionably the most im- 
