(\:>L0HAi)0 Plant Diseases 
29 
the underside. The twigs may 
be entirely covered with the 
same powdery growth. This 
powdery substance is made 
up of millions of small white 
spores (conidia) produced 
by the fungus which is 
growing over the surface of 
the leaf. These spores when 
blown from leaf to leaf ger- 
minate and produce new in- 
fections. In the late summer 
another form of spore is 
formed in small spherical 
spore cases (perithecia). 
When the perithecia are 
formed they change the 
white grayish spots to dark 
smoky patches. The spores 
borne in perithecia on ac- 
count of the protection af- 
forded by the spore case are 
able to live over the winter. 
They are set free in the 
spring by the breaking open 
of the walls of the perithecia, 
and are scattered to healthy 
plants where they start the 
disease anew. 
I ■ Powdery mildew is a difficult disease to control. All infected 
leaves and twigs should be cut out and destroyed. Ordinary lime- 
j' sulfur spray, to which has been added three pounds of iron-sulfate, 
; has proven effective in some sections of the country. The spray 
' should be applied first just after the petals have fallen and this 
; should be followed by two additional applications at intervals of 
I two or three weeks. 
li 
i Crown Gall (Pseudomonas tumefaciens) (13) — Crown gall is most 
j • injurious in the nursery. The disease is characterized by large rough 
1 .j? galls, occurring most commonly at the ground line. Galls are formed 
I ,'t^ most frequently where root grafts are carelessly made. The galls 
