May, 1922 



SCIENTIFIC AGRICULTURE. 



311 



tissues. Sometimes only one or two 

 haulms in a hill are affected but general- 

 ly the whole plant shows the symptoms. 



An inspection of the root system proves 

 that root tips and feeding roots are rotted 

 off and larger roots when sectioned are 

 seen to have browned vascular tissues. 

 Lesions occur on the stolons owing to at- 

 tacks from the outside. The organism 

 can grow from the affected stem or stolon 

 into the vascular tissues of the tuber and 

 here it gives rise to "net-necrosis'". The 

 necrosis of the vascular tissues of the tub- 

 ers varies from slight, when it only pene- 

 trates a quarter of an inch from the stem, 

 to severe when it may extend from the 

 stem-end over half way through the tuber. 

 In the latter case tuber rot may occur if 

 moisture and temperature conditions dur- 

 ing storage are suitable. In the former 

 such tubers used for seed develop *' spind- 

 ling" sprouts. 



Sources of Infection. 



There are two sources of infection : — 

 (1) the soil and (2) diseased seed-tubers. 

 In the first case wilting does not usually 

 occur until late in the season if it occurs 

 at all, and this is the condition generally 

 here. In the second case either the plants 

 are so spindling that they cannot grow 

 or wilting may occur earlier in the season 

 when tubers are about lialf grown. 

 Resistant varieties. 



Little is yet known of varieties actually 

 resistant but it is important to notice 

 that early potatoes escape the disease. 

 Thus Irish Cobbler tubers are fairly mat- 

 ure bj' the time Fusariose becomes preval- 

 ent while Green Mountain tul)ers are just 

 forming. 



Control. 



1. If the soil is knoMni to be infested 

 crop rotation must be practised and if the 

 infestation is severe a long rotation of 

 five or more years is necessary. 



2. Tubers should be selected for 

 soundness. All germinated tubers show- 

 ing a tendency to ''spindling sprout'' 

 should be discarded. If the tubers have 

 not been sprouted the stem-end of suspi- 

 cious tubers can be clipped and those 

 with net-necrosis discarded, or the stem- 

 end half only discarded if the bud end 

 is healthy. 



3. Plant disease-escaping varieties, i.e. 

 early types. 



(e) Common Scab. 



In Europe, Africa, Australasia, and 

 North America common scab occurs pro- 

 bably every wliere the potato is grown. 

 The disease is caused by a fungus closely 

 related to the higher bacteria and known 

 as Actinomyces scabies (Thax.) Gussow. 

 It is often present to such an extent as to 

 prohibit potato culture but usually the 

 chief loss comes from depreciation in the 

 sale value of the tubers. Estimates show 

 that there is a variation from 5 p.c. to 

 75 p.c. of the crop Avhidi is unfit for sale. 

 Symptoms. 



This is a tuber disease and the symp- 

 toms vary from shallow,, rough pit'tings 

 if the attack occurred when the tubers 

 were nearly mature, to deep rough pittings 

 with furrows and cracks if the potatoes 

 Avere infected when young. At first the 

 scab appears as a minute red-bro^Ti spot. 

 It gradually extends outward, becomes ir- 

 regularly corky and deeper brown in color. 

 Scab mites, white grub and wire worms 

 enlarge and deepen the injury, making 

 the tubers still less valuable. 



Life History of the Organism. 



The fungus lives overAvinter in the scab 

 spots on the tubers or in the soil. In the 

 soil it may persist for many years and in 

 that case long rotation is not sufficient 

 but soil treatment must be considered. 

 Alkaline soils favour the fungus and acid 

 soils, check it. Thus the addition of lime, 

 stable manure, potash or a.shes to soil is 

 dangerous when it is known that the scab 

 organism is present. 



Other Hosts. 



Turnips, beets and mangels are suscep- 

 tible to attack by this organism and there- 

 fore these crops cannot be used in a 

 rotation. 



Varietal susceptibility. 



Irish Cobbler, Sir Walter -Raleigh and 

 Carmen among others are fairly resistant 

 to scab. 



Control. 



Two essentials are clean seed tubers 

 and clean soil. If the soil is clean great 

 care should be taken regarding the seed 

 tuber.s and when there is any doubt dis- 

 infection should be thoroughly performed. 

 Even in the case of infected soil it is 

 essential that clean seed tubers be used 

 since otherwise the infestation is being in- 

 creased. Long continued investigations. 



