592 ANNUAL REPORT OF THE Off. Doc. 



times result in a considerable spread of Scab even after the apple 

 is past its most susceptible stage of develoitment. Such late infec- 

 tion comes from the new crop of summer spores formed on leaves 

 or fruit infected earlier in the season. Fortunately in Pennsylvania 

 we suffer less from Scab than do sections northward and westward, 

 "^riiis is due, I surmise, to diflerences in climatic conditions, and to 

 the fact that many of our commercial varieties, such as Grimes 

 Golden, York Imperial, Ben Davis, Jonathan, etc., are naturally less 

 susceptible to scab than certain vaiieties extensively grown else- 

 where. 



We thus see that for Scab to develop three general conditions 

 must be met (1) There must be present the jiving spores or repro- 

 ductive parts of the fungus; (2) these must reach susceptible parts 

 of its proper host plant, the apple, and (3) the general environ- 

 mental conditions must be favorable to the development of the 

 fungus plant. These same general conditions must be met when- 

 ever any of our crop plants falls a victim to any fungus disease. 

 Effective and economical control of any fungus disease must take 

 into account the peculiarities of parasite and host with reference to 

 these three conditions. The devising of particular methods, and 

 their most successful application will depend on intimate knowl- 

 edge of all these factors, which usually requires close study by 

 the specialist. 



In the case of Scab, it has proven possible, by burning the old 

 leaves, or by turning them under before blossoming time, to elimi- 

 nate the source of early infection thoroughly enough for the control 

 of the disease; but this means is hardly practicable for general use. 

 And so we resort to protecting the susceptible parts duiing the 

 period when conditions are likely to be favorable to infection, with 

 a spray mixture that is unfavorable to the development of such 

 spores as may reach the parts. The life history of the fungus and 

 experience shows that, on varieties highly susceptible to scab, in 

 districts where scab prevails greatly, the first application should be 

 made just before the blossoms open. In Pennsylvania, for most 

 varieties and sections, and in an average season, it is sufficient to 

 begin with the second application of the full treatment, made just 

 after the petals fall, with the addition of arsenical poison for the 

 codling moth. It is advisable to follow this with another application 

 two weeks later. Lime-sulphur at a strength of 1.008 specific grav- 

 ity, or Bordeaux mixture, 3-3.50, is a satisfactory material to use for 

 this purpose. In considering fungicides, let us remember that the 

 prime requisite is effectiveness for the intended purpose, which can 

 be determined only after repeated trials under a variety of condi- 

 tions and will vary for different fungi; closely second is non-injuri- 

 ousness to the crop; and at a greater distance are such considera- 

 tions as cost of materials and convenience, which, because they are 

 apt to impi'ess us more immediately, sometimes influence us too 

 much. Bordeaux mixture is generally more effective than the lime- 

 sulphtrr preparations, but it sometimes injures certain varieties of 

 apples, while on others it is safe. 



Apple Sooty Mold and Fly Speck, probably two stages of the 

 jiftme fungus, are characterized by irregular, sooty, black blotches 

 that may run together, and by chrsters of dots resembling fly specks. 

 They develop superficially on the skin of the apple and may be 



