GENERAL CONSIDERATION OF PLANT DISEASES 279 



lates in the case of the crops grown upon such fields the development 

 of the vegetative organs and, therefore, delays the formation of flowers 

 and fruit and the ripening of seeds. Such delay may mean the attack 

 of parasitic fungi. For example, a large field of winter wheat which 

 had been sown about the end of October was much attacked by stink- 

 ing smut (60 per cent.), while the adjacent fields belonging to the same 

 farmer, under the same variety of wheat and treated in a similar 

 manner, but sown early in October showed no sign of infection. With 

 fruit trees, one notices greater frost susceptibility in those plants which 

 have received an excessive nitrogen supply. Lipman (Science, new 

 ser. xxxix: 728-730, May 15, 1914) has suggested that the poor nitri- 

 fying power of soils is a possible cause of "die-back" (exanthema) in 

 lemons. It has been a serious disease with the citrus growers of 

 Florida and California. 



Physical Character of the Soil.- — The physical character of the soil 

 is of great importance as a determining cause of disease. When we 

 speak of the physical character of the soil, we refer to the size of its 

 particles, the relation of these particles to each other, the presence 

 of colloidal bodies, the presence of air spaces associated with the air 

 content, the distribution of the water through the soil, the presence or 

 absence of organic matter, or humus, the color and temperature of the 

 soil. Of greatest importance to the life of the plant is the water 

 which is available for the needs of the plant. ^ A too plentiful sup- 

 ply of water causes the formation of a wet ball of roots with the 

 formation of alcohol. Frequently gardeners fearing that the soil is 

 dry, water potted plants with more water than the plants actually 

 need, so that the lower part of the soil is continuously saturated with 

 water. Alcohol is formed and decay of the roots sets in, because they 

 are gradually suffocated. Too little water on the other hand causes 

 a drooping or wilting of the plant, and if water is not supplied in 

 time permanent wilting and death of the foliage results. But a 

 diminished water supply may be decidedly beneficial to plants, as it 

 has been found that the formation of flower buds is best initiated by 

 preserving a period of rest following a diminished water supply. 

 Different plants have different water requirements and these require- 

 ments vary with the season of the year and the development of 



* Cf. SoRAUER, Paul, Lindau, G. and Reh, L., transl. by Dorrance, Frances: 

 Manual of Plant Diseases, vol. i, parts i and 2. 



