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Diseases originated by insects form a very large field of inquiry 

 coming within the scope of the Entomologist. 



Fungi are the plant parasites which most often cause disease. 

 Infection is usually by transmission from plant to plant of the seeds 

 (spores) by the wind, but insects, birds, agricultural implements, 

 and various other agencies co-operate. When the spore lights 

 on the host-plant, if the moisture, wound or other condition which 

 it requires for germination is present, it sends microscopically thin 

 threads for a longer or shorter distance into the host, which is some- 

 times stimulated to excessive growth, as witches' brooms on 

 branches or huge excrescences on stems, at others atrophy or 

 imperfect development of parts, such as blasted fruits or distorted 

 leaves, may result. The parasite may destroy the leaves and as a 

 consequence the host, as a leafless plant can no longer elaborate 

 food for itself. Without causing actual death a parasite — on root, 

 stem or leaf — may so disarrange the economy of its host that certain 

 functions cease production of fruits or of latex for instance. 



Some fungi grow on living plants only ; the bigger bulk of 

 undesirable parasites fall within this category. Others, called 

 saprophytes, can live on dead matter only ; the greater number of 

 these are useful, disintegrating dead plant and animal remains into 

 a form assimilable by young plants, but many, such as those which 

 attack timbers, are harmful. There is a third group which can 

 exist on living or dead plants indifferently. Thus Fomes semitostus 

 Berk is said to travel through the soil from old jungle stumps on 

 which it grows to the young Para root, ultimately killing it. 



A particular parasitic fungus can infect only one species of plant, 

 or at least only closely allied species, but this feature is of little 

 moment where areas square miles in extent, as in most tropical 

 estates, are covered by the same kind of plant. 



Treatment is usually most effective along lines of prevention. 

 The object is rather to hinder further infection. As a rule 

 satisfactory remedies can be advised only when the life-history of the 

 plant is pretty well known. In some part of its career it may offer 

 a vulnerable point which can be victoriously attacked. A method 

 certain to eradicate the parasite is often impracticable on grounds 

 of cost or applicability, but it may be cheapened and simplified as 

 new facts are brought to light. The life of many parasites is in 

 two stages, each on a different host. One of the hosts may be of 

 no economic value, and its destruction may be easier as well as less 

 costly than that of the valuable plant. Spraying with chemical 

 mixtures is usually an efficient preventive of the further progress of 

 leaf diseases. For root diseases, and often for advanced cases of 

 stem disease, the total destruction of the plant is the only hope, 

 then the soil is treated with quicklime which hastens the destruction 

 of organic matter ; if nothing is planted in the spot for a time, the 

 fungus is starved out. As an example of another method one 

 might cite the smut of corn, where it was found that infection came 

 from spores which remained attached to the seed-grain and being 



