44 BACTERIA IN RELATION TO PLANT DISEASES. 



tested were boiled and unboiled. They were exposed to the oxydizing action of the air. The 

 slices of potato after exposure for 1 2 hours in these juices were inoculated with pulp from a rotting 

 tuber. Blanchard and Early Rose rotted readily. Simson exposed to the juice of Zeland rotted 

 some, Simson exposed to the juice of Preciosa (cooked and uncooked) did not rot. Zeland exposed 

 to the juice of Preciosa (cooked and uncooked) did not rot. Zeland exposed to its own juice (raw) 

 lost its natural immunity and rotted, but resisted after exposure to its own juice cooked. The most 

 interesting result is the supposed immunity acquired by Simson on soaking in the juice of Preciosa. 

 (This conclusion seems to have been based upon a single experiment.) 



His general conclusion is that the resistance of potato tubers is due to the existence of some 

 soluble substances in the cell-sap, the r61e of which can be destroyed by alkaline solutions. The 

 total acidity of the juice of the tubers does not correspond to the action of these protecting substances.* 



The bacillus in question, i. e., that identified as B. coli, is very widely distributed, rarely capable 

 of living parasitically on tubers of potatoes, and then only when the tubers have been deprived of 

 resistance by exceptional cultural conditions. Its virulence varies greatly under different condi- 

 tions. In no case were infections secured on normal tubers or roots when the inoculation material 

 was taken from artificial cultures, even a passage through a slice of cooked potato suffices to suppress 

 the parasitic tendency. The virulence does not continue to increase after 5 or 6 passages through 

 raw potato. Laurent gives a list of 30 compounds from which this organism was able to take its 

 carbon food, and a list of 14 from which it could not obtain carbon. 



"Tous les melanges organiques que je viensd'&iume'rer, et dans lesquels le bacille s'est de'veloppe', 

 ont 6t deposes en quantity notable a la surface de tubercules de Marjolin coupe's en deux, et 16gere- 

 ment excave's afin d'empecher le liquide ensemence' de tomber. Pour beaucoup de solutions, les 

 essais ont et^ rdpetes plusieurs fois en variant les concentrations. Jamais le bacille ainsi cultive' ne 

 s'est developpe sur tubercules vivants qui n' avaient subi aucune preparation spdciale. 



"Memes rdsultats lorsqu' aux tubercules de Marjolin, on a substitue' ceux de variete's Early Rose 

 et Blanchard, cependant si peu rdsistantes." 



As already stated, the result was quite otherwise when portions of these cultures were placed 

 on the cut surface of tubers previously treated with i per cent caustic soda or potassa. 



Light lessens the virulence : Thus tubers of Marjolin inoculated with bacilli from the thirteenth 

 passage, placed under a bell-jar in the sunlight, remained intact even after the cultures were 

 replaced in the thermostat. 



Heat beyond a certain point diminishes and even suppresses the virulence. Heating for 10 

 minutes at 45 and 50 does not retard development, but when 55 and 60 of heat were used, the 

 small colonies which started growth soon ceased to grow and the tuber healed. The bacillus, how- 

 ever, can not attack the potato at a temperature of 40 C., although it grows up to 45. 



Passage through different kinds of roots decreases the virulence. Thus after passing through 

 the acid media afforded by turnips, radishes, or onions, the bacillus seems unable to secrete the 

 alkaline substance necessary for the destruction of the middle lamellae of potato. 



Inoculations on various plants, roots, stems, and fleshy leaves gave slight development only 

 around the point of inoculation. In the inoculated Opuntia, however, large brown, decayed spots 

 appeared and the whole plant was finally destroyed. 



A section through a diseased tuber showed between the pulp and the healthy tissues a zone 

 free from bacteria yet beginning to disorganize. This was due probably to secretions from the 

 bacteria. 



*Averna-Sacca, who studied in Italy (St. Sp. Ag., 1910) the resistance of grape leaves to Oidium, Peronospora. 

 and Erinose found the more resistant sorts had the greatest acidity of cell-sap. The acidity of the leaves expressed 

 in terms of tartaric acid was as follows: 



Sorts. Per cent of dry weight. 



Average of 19 resistant varieties (Rupestris, Riparia, Berlanderi) 6.565 



Average of 31 non-resistant varieties (mostly European sorts) 1 .372 



An examination of the acidity of the must gave equally striking results : 



Grapes tested. Per cent of acid in the must. 



Average of 7 resistant American varieties 20.811 



Average of 10 non-resistant European varieties 7 . 895 



On a field rich in lime these diseases were more prevalent than on a sandy loamy field and the quantity of acid 

 in vine leaves from the sandy field was double that in leaves from the other field. 



The author's conclusions are: (i) The resistance of the grape vine to the attack of parasites must be ascribed to 

 the acidity of the juice of their organs: (2) This resistance is not stable but may undergo changes with cultivation or 

 even be annulled completely, wild varieties being most resistant. 



