»74 



IRISH GARDENING. 



Potato Scab. 



IN the Aug-List number of Irish Gardening 

 reference was made to what may, perhaps, 

 be called the latest disease in potatoes 

 (though it has been known in England for 

 about twenty years) — namely, " black scab " or 

 " warty " disease. This disease has not yet, so 



This, although 



Fig. I. — PeVFATO Tubers (from England) showing the characteristic 



WART-LUvE Ol'TGROWTHS OF BlACK SCAB. 



far as is known, been discovered to be in Ireland, 

 but there is every probability that it is present 

 with us and that it will be observed before very 

 long. Although during the past few years the 

 disease has been found attacking the field 

 potato crop, it is especially in gardens that it 

 has hitherto been most prevalent in Great 

 Britain. Hence, Irish gardeners in particular 

 should continue to keep a sharp look out for 

 the appearance of the disease, and especially 

 so seeing that it has now been scheduled by 

 the Department of Agriculture and Technical 

 Instruction for Ireland under the Destructive 

 Insects and Pests Act, 1907. 



To be forewarned is to be forearmed, and the 

 accompanying illustration (Fig. i) will give a 

 good idea of the general appearance of diseased 

 tubers. The warty outgrowths usually have a 

 characteristic " flabby " feeling, and to the touch 

 are somewhat like soft indiarubber. In colour 



they are usually darker than the rest of the tuber, 

 but parts of them may be light in colour, and 

 then they somewhat resemble a piece of a cauli- 

 flower head ; indeed in some districts the name 

 " cauliflower disease " is in vogue. They may 

 also somewhat resemble the " convolutions " of 

 a brain. 



mdoubtedly the most to be 

 feared, is not the only scab 

 in potatoes. In Ireland there 

 are at least two other forms 

 of scab, one of which is 

 also illustrated, for purposes 

 of comparison. This scab — 

 the ordinary potato scab — 

 has been extremely pre- 

 valent this season, probably 

 as a result of the compara- 

 tiv'ely dry summer, for this 

 form of scab is favoured by 

 dry seasons, and is more 

 characteristic of light soils 

 than heavy ones. It can 

 scarcely be said to be more 

 than " skin deep," and be- 

 yond somewhat unsightly 

 disfiguration of the tuber it 

 does not appear to do much 

 harm. Many people, in- 

 deed, believe that potatoes 

 attacked in this way are of 

 particularly good cooking 

 quality. The cause of this 

 disease is at present some- 

 what obscure. Formerly it 

 was supposed to be due 

 merely to mechanical irrita- 

 tion of the skin, by rough 

 substances in the soil com- 

 ing into contact with the 

 growing tuber. It is, how- 

 the disease is caused by a 

 living organism, for scab-producing soil can be 

 made to grow perfectly clean tubers by being 

 first sterilised. The organism is probably one of 

 the " thread-bacteria," and it is favoured in its 

 growth by comparative dryness of the soil and 

 by the presence of lime, which renders the latter 

 alkaline. Experiments have shown that it may 

 be prevented bv laying wet sawdust in the drills 

 at the rate of about two and a half tons to the 

 statute acre, and laying the sets on this. Pro- 

 bably the acidity due to the slowly decomposing 

 sawdust keeps the scab organism at bay. Peat 

 may also be used in the same way. 



Another form of scab, to which the name of 

 "corky scab" has been given, somewhat re- 

 sembles ordinary scab, at least superficially, 

 although the organism causing it is a very 

 diff"erent one. The microscope is almost a 

 necessity for diagnosing this form of scab with 



ever, certain that 



