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 rav- 

 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- 



