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MASS. EXPERIMENT STATION BULLETIN 214. 



Apple Scab. 



Apple scab presents one of the most serious problems of the commer- 

 cial apple grower of Massachusetts. The disease is caused by a fungus 

 which attacks the leaves, flowers, fruit, pedicels and twigs. It may attack 

 any variety of apples, but is exceptionally severe on the Mcintosh. 



Every orchardist should endeavor to familiarize himself with the first 

 symptoms of apple scab as they appear on the leaves, so that the disease 

 ma.y not reach the epidemic stage before he realizes the danger. Scab 

 usually appears first on the lower side of the leaves as grajdsh or olive 

 webby spots or blotches, darker than the normal surface of the leaf. 

 The color deepens with age to dark brown or black. The spots on the 

 upper surface of the leaves are first noticed as yellowish green discolora- 

 tions, gradually deepening with age through ulive brown to black. They 

 are velvety, somewhat definite in outline, smaller than spots on the 

 lower side, and have a tendency to become raised or convex. 



The Causal Fungus. 

 The scab fungus passes the winter on the dead leaves, under the trees. 

 In the aufriunn after the leaves fall the fungus continues growing, pene- 

 trating the interior of the leaf. Sometimes, in November, it begins to 

 form the flask-shaped bodies (perithecia) in which mature winter spores 

 (ascospores) are developed by the following spring. During the rainy 

 periods of spring these spores are discharged, and, being extremely light, 

 are carried upward by the air to the under surface of the leaves. The scab 

 spots produced by this infection appear from eight to fifteen days later. 

 These spots, almost as soon as they are noticeable, produce the summer 

 spores in great quantities. These spores cause rapid spread of the 

 disease. 



Table I. — Dates of Discharge of Winter Spores and of the First Appear- 

 ance of Scab in 1921 and 1922. 



1921. 



1922. 



First discharge of winter spores 



First appearance of scab 



Last discharge of winter spores 



April 26 

 May 12 

 June 10 



May 2 

 May 18 

 June 15 



During both years the first spots were discovered on the lower side of 

 the leaves at the time of the calyx spray. In other words, the first spots 

 appeared as the petals were dropping. 



