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. Pruni 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 



