182 PRINCIPLES OF PLANT CULTURE 



made when the trees are in full leaf. A modified lime 

 sulfur known as self-boiled lime sulfur is used on tender 

 plants which are injured by the boiled product. For the 

 preparation of this material students are advised to con- 

 sult the bulletins of the United States- Department of 

 Agriculture. 



333. Other forms of sulfur are also used, especially 

 for the powdery mildews of the hop, rose and of various 

 greenhouse plants. The simplest method is to dust the 

 flowers of sulfur freely over the plants. Another simple 

 sulfur compound used for mildews and the like is potassium 

 sulfide (livers of sulfur), 1 ounce in 2 to 4 gallons of 

 water. 



334. Non-parasitic diseases also occur and one should 

 not make the mistake of concluding that when a plant 

 looks sickly this is always due to a fungous or insect 

 parasite. Some of the commonest maladies, like tip- 

 burn of potato, are directly due to dry heat, or other 

 unfavorable environmental condition. 



335. Resistance to disease. — Plants vary in suscepti- 

 bility to disease, some varieties and some individuals in 

 the variety excelling in hardiness or disease-resisting qual- 

 ity. Thus with fire blight the Bartlett pear and the Yel- 

 low Transparent apple are highly susceptible, while the 

 Kiefl^er pear and the Mcintosh are more resistant. The 

 selection or breeding of plants for disease-resistance is, 

 therefore, one of the important ways of lessening loss 

 from diseases which the plant cultivator should always 

 bear in mind. 



It should also be noted that crop rotation, pruning, 

 thinning, watering and various other cultural practices 

 may have an important influence on the development or 

 control of diseases. 



