MAINE ArPLE DISEASES. 353 



expected and not as something which can and should be pre- 

 vented. 



The apple scab fungus may attack the flowers, twigs, leaf- 

 stalks, leaves and fruit but it is upon the last two that its appear- 

 ance is most prominent. While severe attacks on the leaves do 

 much to weaken the trees, often causing considerable defolia- 

 tion, the direct monetary loss to the orchardist is greatest from 

 the effects of fruit injury for the reasons mentioned above. 



Scab appears on the leaves in the form of a superficial, some- 

 what velvety, olive-colored growth, darker than the leaf green.* 

 This growth is more likely to be observed on the under side of 

 the leaf but both sides may be attacked. It may occur in spots 

 but is frequently more abundant along the line of the mid-rib 

 and large veins. The later stages may be compact, thin-scurfy, 

 or more frequently especially if viewed with a magnifying glass 

 of low power it will be seen to be ramifying and much branched 

 giving a beautiful, delicate, "moss agate" efifect. Badly attacked 

 leaves may be more or less curled and crinkled and where defoli- 

 ation occurs a pronounced yellowing may appear. Fig. 247 

 shows the characteristic appearance of a well developed leaf 

 attack where the spots have run together along the veins. 



Scab on apple leaves was very abundant in Maine during the 

 summer of 1910 and samples showing the disease were received 

 by the Experiment Station repeatedly from all of the apple grow- 

 ing sections of the State. Cool, moist weather either in the 

 spring or summer favors the development and distribution of 

 scab, while hot, dry winds and sunny days tend to keep it in 

 check. 



Two forms of spores are produced on apple leaves. If some 

 of the olive-colored growth from the living leaves where the 

 fungus is growing parasitically is scraped off and examined 

 under the microscope a large number of the summer stage spores 

 are found. These, and like spores produced on the fruit spots, 

 are responsible for the summer spread of the disease. In the 

 winter the fungus develops saprophytically in the fallen leaves 

 under the trees and there produces an entirely different type of 

 spore. While the summer spores can live for some time and 

 doubtless it is not impossible for them to remain alive over win- 



* Before tlie spores begin to form tlie aflfected areas are of a ligliter 

 green than the healthy parts of the leaf. 



