8 A SPOT DISEASE OF CAULIFLOWER. 



spots do not become more than 2 to 3 millimeters in largest diameter, 

 though where crowded the spots usually coalesce, forming diseased 

 areas of considerable size. 



^Mien only the upper surface of leaves was subjected to the inocu- 

 lating spray very few uifections resulted, while inoculations on the 

 lower surface gave numerous infections (PL III). So far as observed 

 in sections the infection takes place only through stomata, and 

 mostly from the lower surface. The older and the very young leaves 

 appear to be partially or even completely immime, wliile those of 

 intermediate age (on the same plant) may be seriously affected. 



The diseased leaves become yellow and fall off in from three to five 

 weeks. The younger leaves and new growi^h are healthy. Under 

 our rather dry hothouse conditions in no case Avas there evidence of 

 infection on any but the inoculated leaves. 



From spots developing on leaves inoculated with the original 

 culture the organism was repeatedly isolated by means of petri- 

 dish poured plates, and subcultures from colonies were again used 

 for inoculations, always with the production of the characteristic 



infection. 



The checks in all cases remained healthy; also numerous other 

 cauUflower plants in the same greenhouse. 



Cabbages inoculated A\-ith .this organism became infected in the 

 same manner as the cauhflower ^nth one exception, \dz, the spots 

 were darker. From spots appearing on the inoculated cabbages the 

 organism was isolated and tested on cauUflower, with the result of 

 the production of the disease. 



Durino-lSlav and June of 1909 all inoculations resulted in successful 

 and typical infections. 



On July 10, 1909, seven plants were inoculated in the usual manner 

 and, contrary to expectations, no infections resulted. The conditions 

 were the same as in previous inoculations, except that the tempera- 

 ture was liigher at tliis time, 26° to 34° C. (78° to 93° F.). Subse- 

 quent experiments show that the bacterium causing the disease 

 refuses to grow in artificial media at 29° C. (84° F.) or above. It is 

 probable, therefore, that in this case the high temperature prevented 



infection. 



After July no more plants were available for inoculation until Janu- 

 ary 24, 1910, when 10 plants were inoculated. No infections resulted. 

 On February 2, 1910, 10 more plants were inoculated and no infec- 

 tions resulted. All conditions se«med favorable, and loss of virulence 

 in the culture was suspected as the cause of failure to infect. The 

 cultures used for inoculation were descended through numerous 

 transfers made during the winter. An agar-stock culture which had 

 not been transferred since September, 1909, was then tried. Fresh 

 agar-slant cultures were made from the September stock and used 



225 



