The Canadian Horticultun^ 



ft 



Floral Edition 



1. XXXIX 



TORONTO, DECEMBER, 1916 



No. 12 



The Apple Scab Problem 



L. Caesar, Provincial Eatomologist, O.A.C., Guelph, Ont. 



ONE of the big problems in the 

 minds of most Ontario fruii- 

 gi-owers this fall is "How can 

 we control apple scab next year in our 

 orchards?" Quite a few men, utterly 

 discouraged by the poor results they 

 have obtained from their spraying the 

 last two years will jump to the conclu- 

 sion that spraying is useless, and so 

 will cease to spray for a few years un- 

 til the results obtained from some more 

 thoughtful neighbor or neighbors will 

 again convince them that spraying is 

 an absolute necessity if the orchard is 

 to be a commercial success. 



Successful control of apple scab 

 under all kinds of weaither condi- 

 tions requires thait each grower be 

 constantly stud3ang all he can - 

 about the disease, so that he may 

 be able from the knowledge 

 gained to so time and apply 

 his spraying that he will be 

 able to keep his fruit clean. 

 I wish, therefore, to re- 

 peat in this article some 

 things that have already 

 appeared in the columns 

 of The Canadian Horti- 

 culturist, and also to 

 give some further infor- 

 mation gained as a result 

 of the last two years' ex- 

 7)erience with this disease. 

 Brief History of the Disease. 



Apple seafc winters in little 

 l>lack spots on both the upper and 

 lower surfaces of the leaves on the 

 jround. In the sipring, sometimes about 



le time the leaf buds have burst, some- 

 les not until the blossoms are open- 

 ?, these vspores after or during a rain 



^gin to be shot xvp a little distance from 



le leaf and are then carried by the 



5rid here and there through the air. 

 le Off tbem light on the young leaves 



^■d. if the blossoms are open or nearly 

 ady to open, on anv pairt of these. 



fere they do no harm unless there 

 les a period of dark, rainy or foggy 



leather lasting: for about two days, 

 _ that there will be moisture sufficient 

 to enable the spores not only to ger- 

 niinate but also to work their way 

 thrnntrh the skin of the leaf or of the 

 fi-nit blossom. Tf thev have started to 



germinate and bright weather comes 

 they peris'h unless they have first been 

 able to penetrate the skin. Spores 

 from the leaves on the ground are not 

 all scattered by the wind in one day, 

 but continue to be scattered foir some 

 weeks if there are rains to provide the 

 necessary conditions of moisture. 



Once the little gei'm tube has pene- 

 trated the skin it continues to grow- 

 just beneath the surface. There is no 

 sign on the surface of fruit or leaf that 

 it is there until at least a week and 



Fig:. 1.— Apple showing; tlu- black spots 

 caused by apple scab. 



often not until two weeks have elapsed. 

 Fruitgrowers should therefore remem- 

 ber if they are watching the little 

 apples for the appearance of scab spots, 

 that the first spots they see will have 

 come from an infection that on an aver- 

 age had taken place fully ten days be- 

 fore these spots were visible. 



The scab spots that develop on fruit 

 and leaves get their dark color from 

 the mass of little new spores that cover 

 their surface and that now form a new 

 source of danger as they are daily car- 



ried from place to place by the wind. 

 Once more these will do no harm unless 

 they too can get a couple of days of 

 constant moisture to enable them to 

 germinate and have their germ tubes 

 work through the skin. Once beneath 

 the skin no spraying can prevent tlieir 

 forming a scab spot later. Hence spray- 

 ing must prevent, not destroy, scab 

 spots. 



From the foregoing bi-ief account 

 we may draw the f oUoiwing inferences : 

 First : No matter how abundant spores 

 are in an orchard they cannot germin- 

 ate without plenty of moisture, usually 

 tAvo wet days at a time, or two wet 

 nights and a wet day between. 



Second : The greatest period of 

 danger of scab infection will be 

 from the time the blossoms are 

 ready to burst until about two 

 or three weeks after the 

 bloom has fallen. This is 

 the time when the foliage 

 and young fruit must be 

 kept covered to ward oflE 

 the disease. The reasons 

 that there is more danger 

 in this early period are that 

 the months of May and 

 June are nearly always 

 wetter than the rest of the 

 growing season for apples. 

 The days are then shorter and 

 the nights longer and the ground 

 more moist with heavier denvs, so 

 that there is less chance, especially 

 in dark weather, for the moisture 

 to dry off than later in the season. 

 The hairy nature of the young fruit en- 

 ables it to hold more moisture than the 

 bare, smooth skin of the apple later on 

 could hold. 



Third : The apple itself is formed 

 from the receptacle of the flower, that 

 is, from the little green pa.rt just be- 

 low the calyx. As this becomes ex- 

 posed as soon as the cluster of blossom 

 buds begin to separate it must be pro- 

 tected right from this time. If it be- 

 comes infected there will later be a 

 scab on the fruit. 



Fourth : The object of spraying is 

 to put on the leaves and young fruit a 

 substance such as lime-sulphur which 

 will remain there a considerable time, 



