NON-PARASITIC, OR PHYSIOLOGIC PLANT DISEASES 575 



The next year young trees-may be set in the vacant places, care being 

 taken to obtain trees for resetting that are free from yellows. 



Tip-burn of Potato. — This disease is also called leaf burn or scald. It 

 occurs in many parts of the country and is often confused with early 

 bhght. The tips and edges of the leaves turn brown and these dis- 

 colored areas soon become hard and brittle. The burning or scalding 

 may occur at any time and as a rule is the result of unfavorable con- 

 ditions surrounding the plant. Long continued cloudy and damp 

 weather followed by several hot bright days are very apt to result in the 

 burning of the foliage. This is especially the case on soils carrying a 

 comparatively small percentage of moisture. When the weather is 

 cloudy and damp the tissues of the potato become gorged with water and 

 this has a tendency to weaken them. If the sun appears bright and hot 

 when the leaves are in this condition there is a rapid evaporation of the 

 moisture stored up in their cells. The evaporation may be more rapid 

 than the supply absorbed by the roots, and if this continues for any 

 length of time the weaker and more tender parts first coUapse, then 

 die, and finally turn brown and dry up. Tip burn may also occur as the 

 result of protracted dry weather.^ 



Little of a specific nature can be said as to the treatment of this 

 trouble. The plants should be kept as vigorous as possible by good 

 cultivation, with plenty of available food. 



Leaf-casting. — The fall of leaves at the end of the growing season, at 

 the approach of winter, or periodically in the tropics is a normal result 

 of the formation of an abscission layer. The premature dropping of 

 leaves, the leaf-fall in house plants, the dropping of flowers and twig 

 abscission are all manifestation of abnormal, even diseased conditions. 



The premature dropping of leaves owing to the sudden weakening of 

 functional activities concerns the plant pathologist and is known as 

 "leaf-casting." The dropping of pine needles is only one phase of the 

 general phenomenon. I may be allowed to quote here from the EngUsh 

 translation of the third edition of Sorauer's "Manual of Plant Diseases " 

 (1:349) by Frances Dorrance, concerning the leaf -fall in house plants. 

 "Among the most delicate of the house plants belong the Azaleas, 

 because, as a rule, they suddenly drop their leaves in summer, or in the 

 autumn; the broom-like little tree then at best develops only a few piti- 



' Galloway, B. T.: Potato Diseases and their Treatment. U. S. Farmers' 

 Bulletin 91, 1899. 



