42 MISC. PUBLICATION 16 8, U.S. DEPT. OF AGEICULTURE 



On the fruit the disease appears as irregularly circular spots 

 one eighth to one half inch in diameter which have either a dark- 

 green to nearly black surface, or in later stages a brown, russeted, 

 rough surface with merely a fringe of dark green or black around the 

 margin (pi. 3, D). The difference between the two is due partly to 

 difference in age (the velvety areas being the younger) or to a rub- 

 bing off of the dark-colored material (fungus threads and spores) 

 during the processes of picking and packing. Spots typically dark 

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

 outermost part of the slrin, which has been loosened from the tissues 

 beneath by the growth of the fungus. The spots are usually most 

 numerous around the blossom end, and when infection is severe they 

 may coalesce to form large irregular lesions an inch or more across. 

 Badly diseased fruits are often misshapen because of dwarfing on 

 the side where the infection occurs. 



CAUSAL FACTORS 



Apple scab is caused by the fungus Venturia inaequalis. This 

 fungus winters over on twigs and fallen leaves, producing during 

 rainy weather in the spring immense numbers of spores, by which 

 the disease is spread to all susceptible parts of the tree. Although 

 most of the infection takes place in the spring, there is some through- 

 out the growing season, because of the spores produced on leaves 

 and fruit that were infected in the spring. The spores produced on 

 the dead leaves in early spring are borne in minute sacs or asci 

 (whence their name ascospores), in special flask-shaped bodies called 

 perithecia ; the spores produced during the growing season are known 

 as conidia and are borne free on the ends of short branches of the 

 mycelium. Each kind can give rise to the other, and each produces 

 mycelium which is the cause of the one disease, apple scab. 



The severest infection by the scab fungus occurs in early spring, 

 about blossoming time. It is caused mainly by ascospores discharged 

 from dead leaves of the preceding year. The condition most favor- 

 able for ascospore (primary) or conidial (secondary) infection is a 

 gentle continued rain followed by a period of cool cloudy weather. 

 A heavy rain followed by clearing is much less favorable, the impor- 

 tant factor being the length of time the tree remains wet so as to 

 favor spore germination. 



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

 it may develop in storage (storage scab), especially if late infection 

 has occurred in the orchard. On apples from the Northern and 

 Northeastern States it is often followed by pink-mold rot, which 

 develops both in storage and in transit. 



The development of scab on fruit in cold storage is usually small. 

 Some lesions enlarge slightly during several months of storage, but 

 rarely increase their diameter by more than one eighth of an inch. 

 It is usually impossible to distinguish the lesions that have enlarged 

 in storage, since their characteristics are only slightly different from 

 those of lesions produced in the orchard. The greatest development 

 of scab on stored fruit results from late infections. Some varieties 

 of apples are susceptible until picked and may become infected if 

 there are unusually long rainy periods late in the season. The result- 

 ing spots may appear soon after storage, or, if inoculation in the 



