174 MANUAL OF FRUIT DISEASES 



diameter, and they may be scattered over the whole blade or 

 confined to one portion. Within a week or so the affected 

 tissue becomes dark-red or reddish brown in color. Later 

 developments may be one of two types; either the affected 

 portions drop out, leaving circular areas in the leaf, or the whole 

 leaf turns yellow (Fig. 48, center) . A single leaf may show both 

 types of symptoms, but with cherries, the yellowing of the 

 leaves is the more common, whence the common name yellow- 

 leaf. An allied trouble affects plums, and while the leaves 

 show both a yellowing and a shot-hole effect, the latter symptom 

 is the more common. During periods of wet weather whitish 

 masses appear on the lower surface of the leaf-lesions (Fig. 48, 

 center). Sometimes these pustules are found in the center of 

 the spot on the upper surface. Any time after the last of June 

 premature defoliation is likely to occur on affected trees. 



Affected pedicels show spots one-fourth of an inch or less 

 in length, which extend one-third or more of the way around, 

 often girdling the pedicels. The presence of such lesions causes 

 the fruit to ripen unevenly. Lesions on the fruit are unusual 

 and probably are never important. They manifest themselves 

 in the form of dead, brown spots with whitish fruiting bodies 

 in their centers. 



Cause. 



The leaf-blight of sweet cherry is caused by Coccomyces 

 hiemalis, a fungus with a not unusual life-history. The leaf- 

 blight on mahaleb seedlings is caused by a very similar organism, 

 Coccomyces lutescens. These fungi hibernate in the old fallen 

 leaves as sexual structures called apothecia. In the spring 

 ascospores are ejected from the apothecia, and are carried by 

 the wind to the new foliage on which spore-germination occurs. 

 About a week or ten days later signs of leaf-blight are visible 

 to the naked eye. The reader will recall that this has occurred 

 by June 1 at least. The spore on germination sends its tube 

 into the young leaf and mycelium rapidly develops. The 



