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The Ginadiaii Hortlcu lturi^ 



ol. XXXVII 



JANUARY, 1914 



No. 



The Apple Scab— How the Fungus Spreads 



* 



L. Caesar, Provincial Entomologist, Ontario 



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I 



APPLE scab, or Fungus as it is 

 sometimes called, is by far the 

 most destructive apple disease 

 found in Ontario. It occurs in every 

 part of the province where the apple 

 i,rro\vs. It is not the same disease as 

 the Pear Scab, so common on Flemish 

 Beauty and some other varieties of pears, 

 but is very closely related. Its presence 

 is of course most familiar to us in the 

 form of the black spots on the fruit, the 

 skin of the apple always being destroyed 

 beneath these spots. 



It attacks the leaves just about as 

 readily as the fruit. This fact is per- 

 haps not so well known to fruit growers. 

 On the leaves it causes at first small 

 nearly circular areas about one-fourth of 

 an inch in diameter, and of an olive col- 

 or. .After a while the affected parts of- 

 ten become somewhat elevated making 

 the surface of the leaf irregular or more 

 or less crinkled. Before long these spots 

 die. Sometimes there are nume'rous 

 spots on the leaves. I have seen leaves 

 of crab apple trees so badly attacked on 

 blade and petiole or stem that most of 

 them fall off by about the first of July. 



♦Kxtract from an address delivered at the re- 

 cent annual convention of the Ontario Fruit 

 Growers' Association. 



A fresh set soon took their place. Oc- 

 casionally but not oYdinarily the tender 

 twigs themselves are attacked. 



LOSS CAUSED BY THE DiSEASE 



Loss oomes in the following ways : 



First: Scabby fruit must be rejected, 

 as culls at any rate can never go as 

 number one. 



Second : In moist warm autumns the 

 scabby areas on apples in a barrel will 

 soon become attacked by a whitish or 

 pinkish mould, known as pink rot. This 

 makes the apple not only unsightly but 

 unmarketable. Greenings arc especially 

 subject to the rot. Even apart from this 

 disease scabby apples will not keep so 

 well as clean apples. 



Third : The scab fungus commonly at- 

 tacks the stems of the fruit while it is 

 still small and causes large numbers to 

 fall. Sometimes it is evidently in a large 

 degree responsible for the failure of a 

 crop. 



Fourth : By attacking the leaves and 

 killing areas on these it not only inter- 

 feres with the power of a tree to manu- 

 facture food (the food of a tree is manu- 

 factured chiefly in the green leaves) but 

 also permits spray injury around the 

 areas where the protecting skin has 



Ijeen destroyed. Consequently the \'ig()r 

 of a tree may be greatly lessenf'd by 

 these combined injuries to the leaves. 

 The following year the chances of a 

 good crop are, therefore, greatly less- 

 ened through the failure of a tree to 

 form fruit buds. This is one of the rea- 

 sons why well sprayed orchards regu- 

 larly yield larger crops than unsprayed 

 and are healthier unless injured by over 

 cultivation or over fertilizing and conse- 

 quent winter injury. 



LIFE HISrum OF THE FUNGUS 



The fungus which causes apple scab 

 is a very small microscopic plant which 

 unlike green plants cannot manufacture 

 its own food but feeds entirely upon 

 other plants, or in other words is a para- 

 site. It passes the winter almost en- 

 tirely upon the old diseased dead leaves 

 on the ground beneath the tree or wher- 

 ever they may be blovim by the wind. 

 Occasionally it may also winter on the 

 twigs. In the spring, about the time the 

 leaves are expanding, the diseased spots 

 on the dead leaves by a peculiar device 

 liegin to shoot out into the air in moist 

 weather tiny little spores which are car- 

 ried by the Wind especially to the lower 

 leaves. 



These spores correspond to seeds, and 



\ Portion of an Eighty-Acre Orchard in the Tranton Di»trict of Ontario 



Dj vv. A. rr.wrei. apple ilistrittB of the oontincui. 



