146 PLANT DISEASES 



The control of plant diseases depends on the cause and char- 

 acter of the disease in question. In some cases the diseases are 

 carried on the seed and can be controlled by seed selection and 

 seed treatment ; some diseases are carried on nursery stock which 

 should be subjected to careful insjDection for the elimination of 

 both diseases and insect pests ; some diseases persist in the soil 

 and can be controlled by crop rotation and soil sterilization ; and 

 other diseases are controlled by spraying. Advice concerning 

 the character and treatment of diseases of plants can be obtained 

 by writing directly to the Agricultural Experiment Station of 

 your state. 



EXERCISES WITH PLANT DISEASES. 



1. Collections Showing Diseases. — Examine a large number of 

 plants and make oollcotions of diseased plants or parts of plants. 



2. Inoculating Soiind Apples. — Select a few sound apples, wash 

 in 5 per cent formaldehyde, then in sterilized water, and put them in 

 two or more closed glass dishes that have been treated in the same 

 manner. Set one or more aside for comparison. Puncture the apples in 

 the other with a sterilized needle or sharp pointed knife and insert a 

 minute fragment of the rotten part of another apple. Observe from day 

 to day and report results. 



3. Examine apples kept in this manner for gigng of formation of 

 spores. It may be necessary to keep them under observation for a month 

 or more. 



4. Potato Diseases. — These experiments may be duplicated with 

 potatoes and other plants. 



5. Make a list of diseases as a result of your own observation. 



QUESTIONS 



1. What do you understand by "plant diseases" ? 



2. Enumerate the common symptoms of disease. 



3. To what causes are these due? 



4. Explain a common cause of wilt. 



5. Descrilje the crown or root gall and its effect. 



6. What are the chief forms of leaf diseases? 



7. Name some common plant diseases which you know. 



