PEACH DISEASES 309 
thirds of the way around. Older lesions are brownish or pur- 
plish brown, then dark-brown, and ultimately are purplish 
black or jet black and sunken to some extent. Occasionally 
open cankers form on peach twigs, but, like those on the 
apricot and nectarine, are much less common than on the 
plum. 
Cause of black-spot. 
This is one of the many bacterial diseases which affect plants ; 
the pathogene is known as Bacterium Pruni. The. causal 
relationship between Bact. Prunz and black-spot or shot-hole 
has been known nearly as long as has the disease itself. 
The black spots or cankers on the twigs are the chief sources 
of the inoculum in the spring. In other words, the bacteria 
pass the winter largely on the twigs in the lesions of the previous 
year’s formation. They may also hibernate in the buds in 
some fashion, and in fallen leaves, if the same are well protected 
from the sun and air, but these are not important sources of 
the inoculum in the spring. Rain, dew and insects carry the 
bacteria from their hibernating quarters to the lower surface 
of young leaves, in the month of May or later, where they gain 
entrance through the stomata. The twigs are penetrated 
by way of the young lenticels. Moisture is essential for this 
process. Within a week, more or less depending upon the 
temperature and moisture conditions, and upon the organ 
attacked, evidences of the disease are visible to the naked eye. 
In this interim, that is, from the time the peach is entered until 
the disease is evident, the bacteria multiply rapidly and secrete 
a solvent which soon breaks down the cell walls, giving the 
pathogene a good food-supply. In a short time a small cavity 
is formed beneath the surface of the spot in which many 
bacteria obtain. These finally escape to the surface, through 
stomata or crevices, where they lie in sticky masses. They 
are disseminated to other points, and new infections arise on 
the current season’s twig-growth. With the advent of autumn 
