J A >l AICA 



BULTjETIN 



OF THK 



DEPARTMENT OF AGRICULTURE. 



Vol. I. FEBRUARY, 1903. Part X, 



HEALTH AND DISEASE IN PLANTS.* 



By Prof. F. S. Earle. 



A plant is in health when all its organs or parts are doing their pro- 

 per work, and the processes of growth and reproduction are going forward 

 in the natural and regular manner A diseased condition results when for 

 any reason an organ fails to thus perform its usual normal function. The 

 causes that induce disease are very numerous and are often obscure. For 

 convenience diseases may be grouped under three headings: (1) environ- 

 mental, (2) functional and (3^ parasitic. 



Under environmental diseases are classed those disturbances of normal 

 growth caused by uncongenial surroundings, such as unfavourable soil con- 

 ditions, too much or too little water, the absence or over-abundance of 

 some of the food elements, or unsuitable soil t"mperatures ; unfavourable 

 atmospheric conditions caused by the pollution of the air with smoke or 

 gases ; or unfavourable position as to sunlight. Such unfavourable sur- 

 roundings often cause a slow and feeble, though perfectly normal, grcwth 

 that should not be confused with disease. It is starvation or semi-starva- 

 tion and not sickness. The so-called " scalding" of plants after unusually 

 heavy and protracted rains, the " tip burn" of lettuce and potatoes due to 

 exposure to bright sunshine and dry winds after periods of wet cloudy 

 weather, and the chlorosis or yellowing of the foliage of fruit trees on 

 alkali soils in the West may be mentioned as examples of this class of 

 diseases. 



Functional diseases are due to abnormal activities within the plant 

 itself. These may be the excessive or insufficient formation of enzymes or 

 acids or other secretions, or the disturbance of nutritive or other chemical 

 processes. The dreaded peach yellows and the now destructive disease 

 known as " little peach " probably both belong here, though their true 

 nature is not yet fully understood. The " mosaic disease" of tobacco, and 

 the " yellow disease" of the china aster are examples of the abnormal pro- 

 duction of an enzyme or ferment. Diseases of this class are usually very 

 obscure, and few of them are as yet fully understood. Tn the case of the 

 " yellow disease" of the aster the trouble is caused by the failure 

 of the leaf to secrete sufficient diastase, the enzyme or ferment that 

 converts starch into soluble sugars. Starch is being constantly termed in 

 green leaves when they are exposed to sunlight, but it is onl)' after being 



* Lecture given in the Autumn Course at the Museum, New York Botanical 

 Garden, ;;ept. 11, 1902. From Journal of The Neto Ycyrk Botanical Garden, Not. 

 1902, 



