371 



heavy soils are aerated with difficulty and if substances are present in the 

 soil, which require large amounts of oxygen, they take it from the living 

 plant when a sufficient amount is not found in the soil. Refuse, sewage, 

 animal manure, ferrous oxid compounds, etc., must be considered as sub- 

 stances which require a great deal of oxygen. We find examples where a 

 piece of land fertilized \\ ith stable manure yielded scabby potatoes, while 

 unfertilized land surrounding it yielded a crop free from scurvy ^ 



However, in the decomi)osition of sewage and other animal refuse, 

 injurious sulfur compounds are produced in the soil, which will naturally 

 act poisonously on the root system and yet favor certain groups of bacteria. 

 As soon as such processes set in, the scurvy bacteria, wdiich prefer neutral 

 or alkaline soil, will thrive. 



Such conditions may also be produced in clay soils in times of intensive 

 heat and drought ; or they can be brought about by the addition of marl 

 containing iron. In this way might be explained the appearance and often 

 the annual repetition of the scurvy, which may appear after marling but 

 does not always set in. All the above named factors favoring scurvy 

 can actually develop it in certain cases and not in others. The good 

 effect of lime, already observed in many cultural experiments-, may be 

 explained by its characteristic flocculating action in heavy soil, with a conse- 

 quent improvement in physical texture. The soil becomes warmer, more 

 porous, more easily aerated, while the animal manure is more protected 

 from unfavorable decomposition. The easily aerated sandy soils, which do 

 not long contain highly concentrated soil solutions, are usually free from 

 scurvy. Therefore, the various substances, said to favor scurvy, arc not 

 injurious in themselves but only in certain combinations, which direct soil 

 decomposition into unhealthy channels. 



A\'e have been led to the point of view here expressed by our own 

 experiments", which were intended to answ'er the question, as to whether 

 scurvy can be retained constantly in the soil and can spread there. The 

 result was negative. In the two successive experimental years not only the 

 tubers obtained from healthy seed, but, with a very few exceptions, even 

 those originating from scabby potatoes were healthy. Thus it is clear that 

 the condition of the seed does not necessarily determine that the scab dis- 

 ease will be present in open land, and so the much recommended steriliza- 

 tion is unnecessary. The recommendations for combatting the disease must 

 be based on a change in the constitution of the soil and especially on the 

 avoidance of substances which favor scurvy. In regard to the oft-asserted 

 injuriousness of lime, my experiments have proved that tubers, some of 

 which were brought directly in contact with the lime, remained perfectly 

 smooth skinned and healthy. Recently, substances have been introduced 



1 Arb. d. D. Landw.-Ges. Jahresbericht d. Sonderausschusses f. Pflanzen- 

 schutz 1904. 



- Kriiger, Fr., Untersuchungen iiber den Gurtelschorf der Zuckerruben. Zeit- 

 schrift d. Ver. d. Deutsch. Zuckerindustrie. Nov. 1904. 



3 Zeitschr. f. Pflanzenkrankh. 1899, p. 182. 



