16 Introduction to Fungi [CH. 
from one district are often sent to other districts is 
very favourable to the spread of fungus diseases. 
The fungus known as EHrysiphe graminis, which 
causes mildew on cereals and grasses, really consists of 
several different fungi resembling each other in structure 
but differing in their behaviour towards plants. 
Put some spores from a mildewed wheat plant on 
to a healthy wheat plant under warm and moist 
conditions—infection takes place. Now repeat the 
experiment using a healthy barley plant, and no 
infection of the barley results from the wheat mildew. 
Similarly the mildew from barley will infect barley 
plants but not wheat plants. 
Take some spores from a mildewed wheat plant 
and put them on a barley leaf which has been bruised, 
the leaf now becomes infected with mildew. 
The fungus causing potato disease attacks only 
plants belonging to the same order as the potato. 
The spores are blown about in the air and fall on a 
number of other plants but no infection takes place. 
It is not known why a fungus is capable of infecting 
some plants and not others. 
Some fungi are confined to definite portions of the 
plant such as the root, the stem, or the leaf, whereas 
others attack the whole plant. Some cannot enter 
the healthy surface of a plant but can enter by means 
of awound. The fungus Nectria ditissima, which causes 
“apple canker,” is a wound parasite. Bad attacks of 
canker often follow an attack of “woolly aphis,” which 
causes wounds through which the fungus can enter. 
In order to deal successfully with a disease it is 
first necessary to find out its cause. If it proves to 
be a fungus we must then learn all we can about its 
