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ELEMENTARY AGRICULTURE 
entire wheat fields have been rendered worthless by 
severe attacks. 
Galls and Knots are caused by fungi or bacteria in 
the woody parts of trees or shrubs. The infection is 
made possible through injuries. Black knot is a common 
disease of plum trees. These knots often grow so thickly 
that branches of the plum are killed. Galls are found 
upon the rose. Many young trees have a root gall, and 
a crown gall is a common disease of apple trees. 
Fig. 196. —The effects of plant lice upon corn. Ants carry the lice and 
place them upon the young corn roots and care for them while they rob the 
corn plant. The larger corn in the background is not infected. 
A large number of fungi grow upon wood after it has 
been made into lumber or posts. The white rot attacking 
railroad ties and fence posts causes rapid decay. There 
are many rots and cankers of trees which are caused by 
fungi. 
A list of plants and diseases often attacking them, fol¬ 
lows: 
Alfalfa, leaf spot, root gall, root rot. 
Apple, black rot, blight, canker, brown and pink rots, 
scab and mildew. 
