PEAR DISEASES 339 
Cause. 
The causal organism is the fungus Mycospherella sentina; 
it is better known as Septoria Pyricola, although the former 
name is now accepted. 
The fungus passes the winter in fallen leaves which were 
diseased the previous summer. From these leaves ascospores 
are discharged into the air, and they are then carried to the 
new leaves. Within about fifteen days spots will become visible 
to the naked eye. During this period the fungus spores have 
germinated, the germtubes have penetrated the leaf, and the 
developing mycelium has established a food relation with the 
pear leaf-cells. The attacked cells are killed, and in a few days 
after the lesion becomes evident summer fruiting bodies begin 
to appear. They are formed beneath the upper surface of the 
leaf. They are composed of interwoven hyphal threads and 
break through the leaf-surface at maturity. When thus ex- 
posed they appear as black, flask-shaped bodies and are easily 
seen with a lens or the naked eye. Within two weeks after 
the effects of the fungus on the leaf begin to show, countless 
mature pycnospores are developed within the pycnidia. When 
the fruiting structure is moistened, these spores coil out through 
the mouth of the pyecnidium and are readily disseminated by 
the rain. They fall on other leaves and new infections result. 
This repetition continues throughout the summer. In the 
autumn the leaves fall to the ground, and those affected by 
the leaf-spot fungus carry the mycelium with them in their 
tissues. In the fallen leaves the fungus becomes saprophytic, 
living throughout the winter on the dead tissues. With the 
advent of spring perithecia are formed, within which asco- 
spores are matured for the first infections. It is held by some 
authorities that the pycnospores may winter over. The evi- 
dence on this point is as yet incomplete. 
Control. 
Neglected trees are worst affected. It has been found that 
