414 



BULLETIN 371 



As the season advances the disease becomes more prevalent with each 

 period of wet weather, and, on nursery trees especially, a large part of the 

 foliage may be killed by midsummer. The fungus lives over winter 

 in the old diseased leaves that fall to the ground. During the winter 

 there are formed on these fallen leaves special fruiting bodies, known 

 as perithecia (Fig. 87), which become mature about the time when the 

 trees develop the first new leaves in the spring. Within the perithecium 

 are numerous sac-like bodies, known as asci (Fig. 86, B), containing 

 spores which escape through the opening in the top of the perithecium. 

 These ascospores (Fig. 86, c), being very minute, are carried by the 

 wind or by spattering drops of rain to the new foliage, where they produce 

 the first infections of the year. From ten to fifteen days later the 



characteristic leaf blotch le- 

 sions begin to appear, and 

 soon pycnidia are formed 

 which are a source of further 

 distribution of the fungus. 



vSmall trees in the nursery, 

 on which the foliage is near 

 the ground, often show numer- 

 ous spots early in the spring. 

 This is due to the fact that 

 Proportion of the 



of the winter Stage 



(perithecia) are able to reach 



the newlv developed leaVCS 



and produce infections during 

 periods of damp, cloudy weather. In mature trees the chances that the 

 spores may reach the newly developed foliage from the perithecia in old 

 leaves on the ground are usually limited, but at least a few leaves generally 

 become infected in this manner, and the pycnidia with spores, which 

 develop in the lesions, are the source of new infections whenever conditions 

 are favorable. When there is but little rainfall throughout the summer, 

 very few spores find suitable conditions for germination and thus but 

 little of the foliage is attacked. For this reason the disease may be more 

 prevalent in certain years than in others. 



CONTROL 



In determining control methods for a fungous disease it is necessary 

 to consider the relation existing between the host and the parasite that 

 causes the disease. In general, in the case of leaf blotch of horse-chestnut, 

 weather conditions that influence the trees affect also, to some extent, 



FIG. 87. CROSS SECTION OF A HORSE-CHESTNUT 

 LEAF THROUGH A PERITHECIUM (MUCH MAG- SpOrCS 



shows the sac-uke bod.es (asci) with a s cospo rc s. 



In early spring the spores escape through the opening in 

 the top of the perithecium 



