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Diseases originated by insects form a very ’arge 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 mic roscopically 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 
>r Imperfect development of parts, such as blasted frruits or distorted 
leaves, may result. The parasite may destroy thd leaves and as a 
consequence the host, as a leafless plant can no longer elaborate 
food ft ' itself. Without causing actual death a parasite — on root, 
stem otdeaf — may so disarrange the economy of its host that certain 
functions cease production of "fruits or of latex for instance. 
Some fijujgi gro/w on living plants only ; the bigger bulk of 
undh parasites fall within this category. Others, called 
saprophytes, "'an live on dead matter only ; the greater number of 
thqse are use m disintegrating dead plant and animal remains into 
a form assim ilable by young plants, but many, such .as those which 
attack timbers, .are harmful. There is a third group which can 
exist on living of dead plants indifferently. Thus Foines 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 , ely allied species, but this feature is of little 
moment where areas square miles in extent, as in most tropical 
estates, are covered bv the came kind of plant. 
Treatment; is^jg^ally most effective along lines of prevention. 
The o6]ect is rathk to hinder further infection. As a rule 
satisfactory remedies caP be advised only when the lifo-history of the 
plant is pretty well know 11 - l n some part of its career it may offer 
a vulnerable point vvbich can be victoriously attacked. A method 
certain to eradicate the parasite is often impracticable on grounds 
of cost or applicability, bufk may be cheapened and simplified as 
new facts are brought to ight. 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 viable; plant. Spraying with chemical 
mixtures is usually an efficient preventive of the further progress of 
leaf diseases. For root diseases, an A 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 attache^ to the seed-grain and being 
