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MKtOCOrr RKOIUTION TiST CHART 
 
 (ANSI and (SO TEST CHART No 2) 
 
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 /1PPLIED IM/GE Inc 
 
 K-. 5 tait Mam St'Mt 
 ;ochMle<. r^a York 1 •fi 
 M61 4^J - 03(X) - Phpr* 
 ■16) 288 - M09 - fa. 
 
EXHIBITION CIRCULAR NO. 44. 
 
 (June, 1915) 
 
 DOMINION OF CANADA 
 
 DEPARTMENT OF AGRICULTURE 
 
 EXPERIMENTAL FARMS 
 
 J. H. GRISDALE, B.Agr.. 
 
 Director. 
 
 Division of Botany. 
 
 H. T. GCSSOW, 
 
 Dominion Botanist. 
 
 I 
 
 POTATO SCAB 
 
 BY 
 
 JOHN ADAMS, M.A., 
 Assistant Dominion Botanist 
 
 I. COMMON SCAB. 
 
 trihiJii' ^if ^\°^ P°t«t°^« '« «"« «f the commonest and most widely dis- 
 tributed. It makes the tubers unsi. htly. and therefore decreases the, r 
 value^for sale purposes-in some cases as much as fifty or evt sevSity five 
 
 f„h»J* *«'^^f.the form of more or less circular areas on the surface of the 
 
 ^fow The sThbeT*^' f °" ''' ''T^' '^^^' °^ ^^^ ^'^■"' sometimes unk 
 oeiow. ihe scabbed spots vary much in size and freauentiv fhpv ^r-T 
 
 rzTLr ™" '"'° '■* ""■-- ^^' -™'™' cxroV^hTsrc 
 
 Common Scab is caused by a minute organism to which the name 
 
 skin'TtrnoTatT H^'""?"^ '^^" '^'^•^"- ^^'« organism tes on tl 
 m, Hini ^ i "^^'■' ^"'^ "'*"'"" '^'^ ^^"« of the skin, when young to 
 multiply or divide up in an irregular manner, thus forming ?heTab or 
 pustule instead of a smooth skin. 
 
 attac? bi?rnlpl"f ''''' t^^r^"'^"' '=«»«'n^ «cab on potatoes can also 
 c^ps asteir It J: T'";'"^-^'^!' ""' '^''''' ^^'^ P^"b^b'y ^^'"^ other 
 
 15 
 
 1! 
 if: 
 
 ■fl 
 
 31 
 
If the potatoes, when planted, are free from scab and have not been 
 contaminated by coming in contact with tubers bearing the disease, and if 
 the soil and manure are free from the scab organism, the resulting crop 
 of potatoes should be smooth-skinned, 
 
 If, however, scab is present on the potatoes planted, or in the soil, the 
 nature of the soil and fertilizers used may either increase or diminish it. 
 Wood-ashes, lime, potash, stable manure of all kinds, and substances of an 
 alkaline nature, will, as a rule, intensify the disease. On the other hand, 
 seaweed and commercial fertilizers either have a tendency to diminish the 
 disease, or, at any rate, do not make it worse. 
 
 Heavy moist soils appear to favour the disease, and it is more likely 
 to be prevalent in a wet season than in a dry one. 
 
 Green manuring increases the acidity of the soil, and consequently has 
 a tendency to check the disease. 
 
 The manure of animals fed on uncooked scabby potatoes will probably 
 carry the disease. Sacks, barrels, etc., which have been used for scabby 
 potatoes will, if used subsequently for clean potatoes, convey the contami- 
 nation to the latter. Ploughs and other implements may carry infected soil 
 into a clean field. Cellars that have been used for a scabby crop may convey 
 the disease to a crop apparently free from scab, unless they are disinfected. 
 
 Disinfection of the above may be carried out as follows : — 
 
 Sacks, etc., may be disinfected with boiling water or they may be dis- 
 infected in the same way as described below for potatoes. Ploughs, potato 
 diggers, etc., should first be washed with a stream of water and then 
 sprayed with formalin, 1 pint to a gallon of water. 
 
 Cellars should be thoroughly cleaned and the walls and floors washed 
 or sprayed with a fresh lime wash containing one pound of copper sulphate 
 (bluestone) to every 15 gallons of the wash. 
 
 Small quantities of potatoes may be disinfected by immersing them in 
 sacks or wooden crates in a solution of commercial formalin — 1 pint to 30 
 gallons of water for two hours, or in a solution of corrosive sublimate 
 (bichloride of mercury), 2 oz. to 15 gallons for li hours. The formalin 
 solution made as directed can be used ten times in succession without loss 
 of strength. The corrosive sublimate is intensely poisonous, and must not 
 be prepared in an iron vessel. The same solution should not be used more 
 than three times in the latter case. 
 
 For disinfecting large quantities of potatoes, the formalin gas method 
 is the most convenient. This should be carried out in an air-tight shed, 
 which can be easily constructed out of rough lumber lined with building 
 paper. An estimate must be made of the number of cubic feet inside the 
 building in order to determine the amount of chemicals necessary to use. 
 
The potatoes should not be placed over or within three feet of the generator 
 
 Tt^ ll K " ""/" '''*"'• ^^"^ '^""•'^ ^^ ■" ^^^J^^' «r crates, or shallow 
 slat-work bins not more than one foot deep. The amount of potatoes to be 
 dismfected must not be too small in proportion to the size of the disinfect- 
 ing chamber, as they are liable to be injured in that case, brown sunken 
 nfp„L?f '"*" their appearance around the eyes. If the tubers have com- 
 menced to sprout at the time of fumigation, they are much more likely to 
 be injured by the gas. The potatoes should be kept in the disinfecting 
 chamber for twenty-four to forty-eight hours before the doors are opened 
 and the temperature during this time inside the chamber should not be 
 allowed to fall below 50=F. The tubers should be dry when they a?e di. 
 mfectod The dishes used should have wide bottoms and shoud be suffi-" 
 ciently deep to prevent the liquid from bubbling over when the gas is givTn 
 off. Tm or galvanized iron dishes can be used. Three pints of commercial 
 formalin and 23 oz. of potassium permanganate should be used fTr 167 
 bushels of potatoes per 1000 cubic feet of space. The permanganate should 
 
 ISZ '".' ''-"m ^'^ "" ^'^ '•'""'" «^ ^^^ P^"' the formalin poured over 
 It and stirred quickly, and then the door should be tightly closed. 
 
 sistanc'eTsc^b" ' '"' ''''' "^"'"'^^ ^" '''' *« ^^^'^ ^^ ^^^ °^ - 
 Remedies : — 
 
 1. Use only clean-skinned potatoes for seed and disinfect them to make 
 
 sure that any germs adhering to the skin are killed. 
 
 2. Disinfect annually all cellars used for storing potatoes. Disinfect all 
 
 sacks, barrels, etc., before use, unless these are new. Disinfect 
 all implements used in an infected field. 
 
 3. Plant potatoes only in land that has not had a crop of potatoes or other 
 
 crop liable to the disease durir<? the previous five years. 
 
 4. Attend carefully to the manures used, and boil all scabby potatoes 
 
 before feeding to live stock. 
 
 II. POWDERY SCAB. 
 This disease occurs in the Maritime Provinces of Canada, and. like the 
 
 ZZT'h!""^'"' 'V'''' '^""''^ ^^ ^ '"'""t" '^^^^^'^^ known as Sponge 
 spora subterrmea Lag. parasitic on the skin of the tuber, and in rare 
 cases also on the roots. The pustules produced on the tober are more uni! 
 form m size and shape than in the case of Common Scab, and are usually 
 darker in colour. The skin of the tuber is slightly raised around the 
 
 which f,nd' ?.' ''"''■"' "''* °' *'' ""* '' P""^^"'^ '« '"-de up of a powder 
 which, under the microscope is seen to consist of spore-balls These snore 
 balls readily distinguish it from all other potato diseases 
 
 'If! 
 
 -II- 
 
 ■m 
 
 
 If; 
 
This organism may attack the tubers when they are only as large as a 
 hazel nut. It only attacks the parts of the plant below ground — usually the 
 tubers — and does not infect any other crop but potatoes. 
 
 It has been found that the presence in the soil of an excess of lime tends 
 to favour the disease, but it has not yet been determined whether other 
 fertilizers have a similar or opposite effect. An attack of the disease is 
 usually more severe in a moist soil or a rainy district. 
 
 The spore-balls can retain their vitality in the soil for at least three 
 years, but how much longer they can live in the soil has not been 
 determined. 
 
 Experiments have shown that the spore-balls can pass through the 
 digestive system of pigs, and afterwards infect a potato crop in the manure. 
 Boiling diseased potatoes before feeding to live stock would prevent this. 
 
 Sound potatoes, as in the case of Common Scab, may be contaminated 
 by thf^ use of infected cellars, sacks, implements, etc. The same methods 
 of disinfection should be employed as in the case of Common Scab, and the 
 remedies suggested are the same except that a crop of turnips, mangels, 
 etc., can take its place in the rotation as these are not affected by Powdery 
 Scab. 
 
 So far as is known at present, all varieties of potatoes are equally 
 susceptible to this disease.