January, 1922. 



SCIENTIFIC AGRICULTURE. 



165 



and the protuberance on the tuber. In 

 root and stolon galls tiie parasite event- 

 ually reaches the phloem in which most of 

 the food manufactured is conducted and in 

 this tissue most of the hypertrophy^ occurs. 

 The tuber is somewhat modified because 

 of its function as a large storage organ 

 and the plasniodia do not, as a rule, reach 

 the i)hl()em but attack cells of the hypo- 

 ilermis and cortex. 



Where a cell is covered by a Plasmodium, 

 or part of one, the wall swells and softens. 

 That it is changed chemically is shown by 

 the modified staining. Through the soft- 

 ened wall the Plasmodium penetrates by 



Figure 8. — Black-stem of potato. Note the 

 blackened tissues at A and also the rolling of 

 the leaves. (After Coons, Mich. Bull. 85; Fig. 

 9 also.) 



protoplasmic protrusions which Kunkel 

 calls ''infecting pseudopodia". Once in- 

 side the ceil the, naked protoplasm of the 

 myxomyce^e penetrates the limiting mem- 

 brane of the host protoplast, and the two 

 protoplasms are to an extent miscible. It 

 is when the cell is thus infected that hyper- 

 trophy begins. As the vegeta'^ive growth 

 of the potato gradually ceases and matura- 

 tion commences the spores of Spongospora 

 subterranea are formed and these spores 

 are aggregated into round masses known 



as spore balls. Such groups of spores are 

 highly resistant to adverse conditions and 

 are known to be able to live over in the 

 soil at least three years. 



Before leaving this discussion mention 

 should be made of dry rots following 8. 

 subtenanea. If, in the development of the 

 pustule, it happens that the cork layer 

 which cuts off the parasite is not laid 

 down, drying-out or dessieation follows in 

 storage. The severity of this depends up- 

 on the number' of pustules on the surface 

 of the tubers which are not limited by a 

 cork layer. It often happens that the spore 

 balls are still present in the pustule in 

 storage and if the temperature conditions 

 are satisfactory for germination the spores 

 in the balls wall germinate and the result- 

 ing Plasmodia will destroy cells surround- 

 ing the old pustule. 



Naturally such pustules are open places 

 for the entrance of Avound parasites and 

 one of the most important of these, Phoma 

 tuherosa Melhus et al, causes serious dry- 

 rot injury. 



It must be pointed out that, secondary 

 Spongospora injury, dry-rot and dessiea- 

 tion may occur together in some cases.. 

 Varietal Susceptibility. 



Nothing can yet be said regarding re- 

 sistance of this disease since it is probable 

 that all the well-known potato varieties 

 may be affected. Seasonal differences 

 may account for some varieties escaping 

 infection during one .season but being af- 

 fected another. 



Other Hosts. 



It is interesting to note that the roots 

 of tomato are suceptible to infection by 

 «S[. suhtenajiea and that galls are formed 

 similar to those on potato roots. 



Control. 



The following points are important in 

 control : 



1. Seed tuber selection. 



2. Seed tuber treatment (formaldehyde, 



hot or cold). * • 



3. Destruction of spore balls in diseased 



tubers by boiling before using as 

 feed. 



4. Sulphur 900 lbs. per acre applied 



broadcast reduces the amount of in- 

 fection. 



5. Practise long rotation if the attack is 



severe. 



