DISEASES CAUSED BY FUNGI 13 



lesions in the form of spots, streaks or patches. The cause of 

 complete discoloration, wilting or shedding of leaves is more 

 frequently to be found in an infestation of twigs, branches, 

 stems, or roots, according to the distribution of the symptoms. 



Leaves are most susceptible in the early and late stages of 

 their development. The tenderness of young leaves, including 

 the absence of cuticle, exposes them to infection by fungi which 

 are unable to attack mature leaves ; the anthracnose of limes 

 is an example of this. The fungus may later disappear, but the 

 effects of its presence remain in the shape of spots or distortions 

 which are difficult to account for if the early stages are not seen. 

 Senescent leaves show waning powers of resistance and are 

 frequently infested by parasites too weak to have much if any 

 effect until this condition is reached. Very many leaf spots are of 

 this nature. Influences which depress the vigour of the plant 

 may bring on the susceptible condition prematurely ; thus when 

 a coconut palm is infested with nematodes Diplodia attacks 

 the leaves at an earlier stage. Many secondary infestations are 

 brought about in a similar way. 



Leaves are affected differently by parasites which establish 

 a relationship with the living cells, like the rusts and mildews, 

 as compared with those which live at the direct expense of the tis- 

 sues. In the former case the leaf though more or less discoloured 

 remains alive, it may be for its full period or even longer ; in the 

 latter case dead spots are quickly produced. A common type of 

 leaf spot consists of a dead central circle or streak, surrounded 

 by a discoloured margin of cells recently invaded or affected 

 by the secretions of the mycelium. Such an area may become 

 isolated from the surrounding tissue through the production 

 by the host of a ring of corky cells which prevents further ex- 

 tension, and if the dead tissue later drops out the type of injury 

 known as shot-holes is produced, in which the leaf is more or less 

 heavily perforated with roundish holes. In other types of 

 spotting the lines of the veins are followed, or the spot is bounded 

 by veins (angular spot) or quite irregular perforations may be 

 produced (Hevea leaf spot). 



It is often difficult to distinguish between leaf spots caused 

 by fungi and those of insect origin, especially as the latter are 

 soon invaded by saprophytic fungi. 



The infestation of leaves with fungi reduces their efficiency 

 to a proportionate degree and may lead to partial or entire 

 defoliation. This causes a severe check to growth, and, if re- 

 peated frequently, brings about the death of shoots or twigs 

 and sometimes of the whole plant. 



Flower and Fruit Diseases. 



The infestation of flowers is not common, but examples of it 

 are found in the blossom bUghts, due to Glceosporium spp., of 

 mango and of lime, in which buds, open flowers, and newly set 



