46 MISC. PUBLICATION 168, U. S. DEPT. OF AGRICULTURE 



that affected twigs have a scurfy appearance very much like that 

 of scabby pear twigs. 



On the fruit the disease (pi. 1, A, B) appears as irregularly 

 circular spots, % to V2 inch in diameter, which have a dark-green 

 to nearly black velvety surface in the early stage. Spots typically 

 dark green and velvety generally show a ragged, papery fringe of 

 skin which has been loosened from the tissues beneath by the 

 growth of the fungus. Later they have a brown, russeted, rough 

 surface with merely a fringe of light to dark green or even black 

 around the margin. The spots are usually most numerous around 

 the calyx end, and when infection is severe they may coalesce and 

 form large, irregular lesions 1 inch or more across. Badly diseased 

 fruits are often misshapen, because of dwarfing on the side where 

 the infection occurs. During several months of storage some 

 lesions enlarge in diameter slightly — rarely by more than % inch. 

 It may be hard to distinguish lesions that enlarge in storage from 

 those that develop entirely in the orchard. 



Scab that develops during cold storage, known as storage scab, 

 looks different from that which develops in the orchard. Usually 

 only a small amount of such scab develops. The greatest develop- 

 ment results from late infections in the orchard. Some varieties 

 of apple are susceptible until picked and may become infected 

 late in the season if unusually long rainy periods occur then. 

 The resulting spots may appear soon after the fruit is stored. 

 If inoculation in the orchard is very late, they may appear after 

 several months of storage. Such spots vary in diameter from 

 Vie to 1/4 inch. They are circular and darker in color than the 

 orchard lesions and have definite borders (pi. 1, C). Often the 

 cuticle above the scab spot is not broken and the surface is shiny, 

 although it is usually roughened by the pressure of the mycelium 

 beneath. Dark-brown to black spots, on account of their color, are 

 sometimes confused with blotch. They can be distinguished from 

 it by the facts that no fruiting bodies are developed and the 

 edges of the spots never have the marked feathery, fringed ap- 

 pearance characteristic of the early stages of blotch. On overripe 

 fruit the scab lesions may enlarge rapidly. 



CAUSAL FACTORS 



Apple scab is caused by the fungus Venturia inaequalis. This 

 fungus overwinters mainly on fallen leaves and produces during 

 rainy weather in the spring immense numbers of spores; by 

 means of these the disease is spread to all susceptible parts of 

 apple trees. Although most of the infection, as well as the most 

 severe infection, takes place in the spring about blossoming time, 

 some infection is caused throughout the growing season by spores 

 produced on leaves and fruit that were infected in the spring. 

 The condition most favorable for infection is gentle, steady rain 

 followed by a period of cool, cloudy weather. Heavy rain followed 

 by clearing is much less favorable, the important factor being 

 the length of time the tree remains wet enough to permit spore 

 germination. 



The disease does not develop to any marked extent in transit, 

 but it may develop in storage, especially if late infection has 

 occurred in the orchard. Scab lesions afford ready entrance points 



