61 



noticeable as soon as the leaves appear in the spring. If severe, the 

 disease causes the young leaves to become enlarged, discoloured, thickened, and 

 distorted in short, curled. The variety of colour displayed by the curled 

 leaves is great. They generally assume a livid or white look ; but sometimes- 

 they are red, sometimes purple, and sometimes mottled with various com- 

 binations of green and red and purple. The surface of the diseased leaves,. 

 instead of having the dull lustre characteristic of healthy peach-leaves, be- 

 comes shiny and brilliant. This feature combined with the brilliance of colour 

 makes the diseased leaves very striking objects. (See coloured plate.) As the- 

 disease advances, the amount of overgrowth and distortion increases, until 

 finally the leaves become bulged out 

 on the upper side to such an extent 

 that pockets half an inch to one inch 

 in depth are formed, into which one 

 can easily insert the end of the 

 thumb. About this time the purple 

 colour, which is more common in the 

 earlier stages of the disease, begins 

 to disappear, and the lustre also 

 becomes less brilliant. This is owing 

 to the fact that the fungus which is 

 the cause of the disease is now pre- 

 paring to fructify. When this opera- 

 tion is finished the diseased leaves 

 appear as if covered with frost, or 

 with an exceedingly fine white 



j ^ - j.- -..r ii Fiff. 76. Microscopic details of the fungus (Exoascus- 



powder. On examination with the l jo . nimus) causing leaf . curl of the pea ch. The- 



microscope this apparent powder is figure shows a small portion of a section through 



found to be composed of very minute a diseased peach leaf ; a, the mycelium of the 



Sacs packed together as close as ^ngus growing on the surface of the leaf, d; b, a 



.r . -IT sac or ascus arising from the mycelium a, ana 



possible in a Single layer On the SUr- containing eight spores ; c, an empty sac; e, e,. 



face of the leaf. Each sac when ripe mycelium inside the leaf growing among the cells ;: 



Contains eight egg-shaped Spores. f> $> h > ' > ascospores budding in water ; fc, the 



These sacs and spores are of such a 



size that, according to a calculation I have made, a single square inch of' 

 diseased leaf may bear no less than twenty million spores, a number so large 

 that it is not easy to comprehend. One comprehends it to a certain extent 

 on reflecting that if one spore was placed on each square inch of the surface- 

 of the soil, twenty million spores would suffice to cover an orchard more 

 than a mile square. But this number of spores frequently grows on a single- 

 diseased leaf, while on a single peach-tree thousands of such leaves may often 

 be seen. The imagination is stunned by the numbers that represent the- 

 prolificness of this fungus. 



When the disease has culminated that is, when the fungus has fruited 

 the spore-sacs burst and the spores escape ; then the leaves, which have 

 thus far remained succulent, begin to dry up, turn brown, and fall. These- 

 leaves having fallen, they are succeeded by others which, as a rule, are 

 less severely attacked, and thus the disease goes on decreasing in severity 

 until in mid-summer the tree may have a crop of fairly healthy-looking 

 leaves, and may bear a partial crop of fruit. In the severest cases, however, 

 the fruit falls about three weeks after setting, and not a peach is left to ripen. 

 This occurs on trees in which the disease is chronic and severe. Such 

 trees are worthless, nay, worse than worthless ; they are a constant menace 

 to all the peach-trees in the neighbourhood. The sooner they are cut down. 



