260 PLANT DISEASES 



tion of methods of control will be somewhat simplified, for if the 

 term " blight " is restricted to bacterial diseases of the nature of pear 

 blight, it will be understood that certain control measures, such as 

 spraying, will not be effective. At present, each case must be con- 

 sidered separately, and in the following pages the popular names are 

 used. These names are followed by the technical botanical name 

 of the organism causing the disease, in italics, and this by a brief 

 description of the disease, the most prominent symptom being men- 

 tioned first. 



Certain General or Unclassified Diseases 



Damping-off. A term applied to the decay of young seedlings or cut- 

 tings at or near the surface of the ground. The trouble is due to 

 the action of various organisms, especially Pythium deBaryanum, 

 Phytophthora cactorum, Rhizoctonia sp., etc. Wet soil, confined 

 atmosphere, and crowded plants are conducive to damping-off. 



Control. Steam-sterilize seed or cutting beds. Sterilize nursery 

 seed beds with formalin, using 1 gallon of 1 per cent solution to 

 the square foot, i.e. 1 pint of formalin in 12-15 gallons of water. 



(Edema or Dropsy. A disorder of various plants under glass, as 

 tomatoes, violets, geraniums, which have insufficient sunlight, 

 stimulating temperature and soil, and too much moisture. It has 

 also been observed on twigs of the apple. It is usually indicated 

 by elevated corky or spongy points or masses, much resembling 

 fungous injury. The leaves curl. The only remedy is to improve 

 conditions under which the plants are grown. 



Smuts of cereals. Practically every cereal is attacked by a specific 

 smut fungus, and most of them by two perfectly distinct species. 

 These smuts are confined to a single species of cereal, and never 

 cross from one to another. Some of the smuts produce a loose 

 black spore-mass (loose smuts), while in others (covered smuts) 

 the seed coat of the grain is not affected, so that the smut is not 

 detected until the grain is broken open. The most important 

 difference to be noted, however, is the method of wintering. In 

 some the spores adhere to the surface of the seed and infect the 

 young seedling plant at the time of germination, while in the other 

 case the spores fall upon the blossoms and grow down into the seed 

 directly, there lying dormant until the seed is planted. 



