Common Scab of Potatoes. 



53 



results were obtained by digging in a crop of Eye and planting 

 the potatoes soon after the operation. There appears to be no 

 reason, however, why other sources of organic matter of vege- 

 table origin should not be used wiQi equally good etiect. Leaf 

 mould is used by some gardeners with good results, and since 

 carrying out this work the writer has been informed that speju 

 hops are employed in some localities for- the same purpose. 



How -Grass Inhibits Scab. — Various theories were formulated 

 during the course of the investigation to account lor the preven- 

 tion of Scab by grass and plant residues, and this part of the 

 work proved far the most diiiicuit. J^xperiments on it were carried 

 out concurrently with the repetition trials of the treatment, but, 

 for a full account, the reader must be referred to the Depari- 

 mental Eeport. The conclusion there drawn is that the fungoid 

 organisms responsible for the Common Scab are primarily sapro- 

 phytic, that is to say, they feed on dead organic matter in the 

 soil, where they thrive mainly on plant residues and aid in the 

 early stages of its decomposition. Only when these natural 

 suppKes of food are exhausted do they become porasitic on the 

 potato tubers. 



It is not difficult to see that this deduction will account for 

 many of the observations made in regard to Common Scab. Two 

 of those made earlier in this article may be recalled. Scab is 

 most prevalent on light sandy or gravelly soils and is practically 

 unknown on peat soils. In soils of the first type, especially where 

 these have been liberally supplied with lime, organic remains 

 tend to disappear rapidly and the scab organisms being left with 

 a deficiency of food, attack the potato. The introduction of fresh 

 supplies of plant residues remedies this defect, and may be con- 

 sidered to act as a decoy for the fungus. The potato crop in this 

 way escapes attack. In a peat soil, however, large reserves of 

 organic matter are present naturally, and there is consequently 

 no fear of the crop being atti^ckod. 



Other phenomena relating to Scab, in particular the influence 

 of lime and ashes on the disease, may be accounted for in :i 

 similar way. More work is being caiTied out on the subject, 

 and it is hoped to publish a further report of the experiments in 

 due course. 



(1) " Common Pcab of Pofofocs,'' UniiJersitij of Leeds (ind Yorl-sJu're 

 Council for Agricultural Efhication, Report 118. 



(2) Connecticut Aqric. E.rpt. Station, Report for 1800. 



(3) Invesfir/ations on Potato Diseases, 0th Report, Dept . of . biric, 

 and Tech. Instruction for Ireland, Vol. XV., pp. 41)l-52<'). 



(4) University of Leeds and Yorkshire Council for Agricultural Education^ 

 Reports ,55, 03, 70. 



