PLANT DISEASES 187 
fungus that bears the red spores earlier in the season. The 
spores produced late in the season live through the winter and 
start life the next spring by infecting some host plant again. 
The mildews cover the leaves of plants with a white sub- 
stance either of a powdery or of a woolly form. The leaves 
of our lilacs nearly always show mildew 
except in early summer, and the grape, rose, 
woodbine, and willow are often affected. 
Of course, the larger part of the fungus is 
out of sight within the tissues of its host, 
and there it does its real harm to the health 
of the plant. To prevent mildew we may 
dust the foliage with dry sulphur. 
The blights cause the leaves of plants to 
shrivel and blacken, looking as if overheated 
by the sun or by fire. Fire-blight attacks 
pear and apple trees, and when the disease 
appears the affected twigs should be cut 
off and burned. The potato blights belong 
to this type of disease and may be prevented 
by spraying with Bordeaux mixture. 
The wilts are diseases that make their 
host plants wilt and droop as if suffering 
from lack of water. In a short time they 
die. This wilting is caused by fungi or Reser 
bacteria that grow in the sap tubes of the Leaf and stem of 
plant and thus choke off the flow of mois- wheat affected with 
ture to the upper parts. Flax wilt is the 
most destructive disease of this type in the Northwest. 
When a piece of ground has produced a crop or two of 
flax, it generally becomes infected with the germs of the flax 
wilt fungus, and we say it is flax sick. To control flax wilt 
