NATURE AND SYMPTOMS 7 



so weaken a plant as to make it possible for one of the 

 specific organisms of the first group to attack it. 



In the study of a disease it is desirable to gain as 

 thorough a knowledge as possible of (l) its causes; 

 (2) the effects upon the plant ; and (3) the methods for 

 prevention or remedy. The practical grower is more 

 especially interested in this last phase of the subject, 

 but in many cases it is impossible to treat a plant 

 successfully for a disease without a thorough knowledge 

 of the cause. However, in some instances, methods 

 have be.en devised for keeping diseases under control 

 without a complete knowledge of the organisms causing 

 them. 



A disease may be serious on one species or variety 

 of plant, and of little importance or not present on one 

 closely related ; or it may be serious under certain 

 climatic conditions, and harmless under other conditions. 

 For these reasons a disease-producing organism, when 

 introduced from one part of the world to another, may 

 become more or less virulent than in its original home. 

 It may find new host plants which may be more or less 

 susceptible than its previous hosts, or it may find the 

 climatic conditions more or less favourable to its growth. 

 It is also true that when a plant is introduced into 

 another part of the world that it is liable to meet 

 enemies which will endanger its existence. 



Diseases make themselves manifest by certain 

 peculiar characters or symptoms which readily dis- 

 tinguish a sick plant from a normal or healthy one. 

 The most common characters or indications of disease 

 are : (a) discoloration of the foliage and of new growths ; 

 (6) wilting ; (c) dropping of foliage ; (d) spotting of 

 foliage ; (e) perforation of foliage (shot-hole) ; (/) 

 variegation of foliage (mosaic) ; (g) wilting or " damping 

 off" of seedlings ; (h) death of leaves, twigs, stems, etc. 

 (necrosis) ; (i) reduction in size of parts (dwarfing or 

 atrophy) ; (j) increase in size of parts (hypertrophy) ; 

 (k) formation of excrescences, namely galls, pustules, 

 corky outgrowths, etc. ; (I) cankers ; (m) malformation 



