330 BACTERIA IN RELATION TO PLANT DISEASES. 



from a study of cabbage and other cruciferous seeds in a natural state, as they come from 

 the seedsman, and most easily before they have left his hands, i. e., in the field on maturing 

 plants observed to be diseased. 



Since the above paragraph was written Harding and his associates have thrown a flood of 

 light on this subject by actually obtaining Bacterium campestre from the surface of cabbage- 

 seed harvested separately from four diseased plants obtained from a field of seed-cabbage 

 on Long Island. This shows that the organism may reach and contaminate the seeds in a 

 certain number of plants reserved for seed, and renders it likely that the entire crop of a 

 given seedsman would be more or less contaminated if some diseased plants came to maturity 

 in his fields and were harvested along with the sound plants, since the dust of the threshing 

 would disseminate the organisms widely. Can the organisms thus disseminated remain 

 alive for a long time on such seed i. e., from autumn until spring? We do not yet know, but 

 the probabilities are in favor of such a belief, since these experimenters have also shown 

 that when pure cultures of this bacterium are placed on cabbage-seed and dried, some of the 

 bacteria (a small proportion it would seem) remain alive for at least 13 months. These 

 discoveries, while not wholly conclusive, render it extremely probable that the disease is 

 very often disseminated on seed, since the persistent vitality which has been demonstrated 

 experimentally in the laboratory using pure cultures is quite likely to occur naturally, at 

 least sometimes, on contaminated seeds offered for sale in the open market. If the organ- 

 isms remain alive on such seeds over winter in any great number we have a very satis- 

 factory explanation of the wide dissemination of this disease in the United States during 

 the last 15 years. 



Harding has now made out so good a case against the seedsmen that these gentlemen 

 should take very special precautions to avoid harvesting seed from infected plants, and 

 from badly infested fields. Such fields should not be used for production of seed-cabbage. 

 The growers, on the other hand, as a matter of ordinary precaution, should disinfect all 

 cruciferous seeds before planting them. This may be done, it is said, by soaking the seeds 

 for 15 minutes in 1:1000 mercuric chloride water, or in full strength formalin diluted with 

 water in the proportion of 1:240. 



It seems likely that this short treatment will prove effective only in case the organisms 

 are confined to the surface of the seeds. If they are sometimes in the interior a longer treat- 

 ment will probably be necessary, and to avoid this (since it might prove disastrous to 

 germination) it would be advisable to screen out and reject all shriveled and inferior seeds 

 before planting, since it is probable that the plump seeds will be less likely to be infected 

 within than the thin or shriveled ones. 



In Farmer's Bulletin No. 68 (which may be had on application to the U. S. Department 

 of Agriculture), the curious reader will find various pieces of evidence tending to show 

 spread of the disease from the seed-bed, persistence of the organism in the soil, and infection 

 of land by way of the dung-pile, and by refuse from cabbage storehouses and pits. 



To summarize: Avoid infected seed, soil, and manures; destroy insect carriers of infec- 

 tion; if the plants are attacked, harvest early, and use at once, or store in a very cool house. 



PECUNIARY LOSSES. 



It is difficult of course to arrive at any very definite conclusions respecting the losses 

 due to this disease. In September 1904, the writer received the following statement con- 

 cerning black rot from an extensive cabbage-grower near Chicago : 



"I send you by mail some small samples which I hope will be sufficient in order for you to 

 determine the trouble. In some cases as high as 40 or 50 per cent of the cabbage is diseased and in 

 one case which I have noted I should think that over 90 per cent of the cabbage had died from this 

 disease. The ground from which these plants came is comparatively new ground and has only been 

 under cultivation for 2 or 3 years, though large quantities of stable manure have been placed upon it." 



