36 BULLETIN 1380, U. S. DEPARTMENT OF AGRICULTURE 
and showing no sign of infection would be green or olive-green 
spotted on the fourth day following. 
The appearance of the green or olive-green velvety coating on the 
infected spots is due to the development of an immense number of 
conidiophores, minute erect stocks with swollen bases bearing the 
1 -celled or 2-celled conidia. These may appear on both sides of the 
leaf but usually in greater number on the under side. They may 
develop directly through the epidermis or appear one or more to- 
gether through the stomatal openings and in greater numbers at the 
margins of the infected areas. The mycelium may extend superfi- 
cially beyond th% immediate point of infection from which conidio- 
phores develop. A section through the infected tissues shows that 
the mycelium is intercellular, with palmate or branched haustoria 
invading the cells. On emerging the mycelia become light or dark 
brown, conspicuously septate, and may entirely obscure the surface 
of the leaf. The conidia are of varying shapes, but when fully 
mature are generally oblong-elliptic. They are produced in great 
numbers and are believed to be chiefly instrumental in propagating 
the fungus. Since new and old leaves are generally present, either 
on the same or different individuals, continuous propagation by the 
wind-borne spores is made possible. The spores germinate immedi- 
ately, forming an expanded disk, from which a germ tube is devel- 
oped, which rapidly penetrates the young leaves and again produces 
conidia. In damp, rainy weather they may be the cause of a rapid 
spread of the disease, and young trees may become entirely defoli- 
ated. The second crop of leaves is infected, and the process goes on 
until the twigs and branches are depleted of food material and either 
die or fall a prey to Diplodia, which finally causes the destruction of 
the tree. Stahel reports that three successive defoliations in a period 
of six months caused the dying back of the crown of trees 5 to 6 
years old. In the case of older trees defoliation of branches resulted 
in loss of vigor and reduction of latex. 
If the young leaves, owing to their rapid growth, are not entirely 
parasitized or spot infections on more mature leaves have not in- 
creased in size from the same cause, small spherical black bodies 
appear around the edges of the original areas of infection. These 
are the pyenidia, re presenting the second stage of development 
(PI. XV. B). The pyenidia which develop from brown septate 
mycelia may appear singly or may become united in masses on a pseu- 
doparenchymatous stroma, completely incrusting the edges of the 
spots. They are very rarely developed on the under side of the leaf. 
The pyenospores are developed on short stocks arising from a hya- 
line inner layer contrasting with the dark stromalike walls of the 
pyenidium. The spores are very small and are germinated with 
difficulty. They probably take little or no part in the propagation of 
the fungus. 
From one to two months after the appearance of the second or 
pyenidial stage larger black spherical bodies appear around the mar- 
gin of the old spots, or they may be grouped in the center of small 
brownish hypertrophied areas scattered over the surface of the leaf. 
This is the third or perithecial stage and represents the last and high- 
est development of the fungus, the production of ascospores (PI. 
XV, E). The perithecia are either grouped in stromatic masses or 
