PEAR DISEASES 339 



Cause. 



The causal organism is the fungus Mycosphcerella 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 pycnidium 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 



