34 BACTERIAL DISEASES OF PLANTS 



bacteria in diseased plants by supposing that they must enter 

 the plant through the lumen of fungous hyphae. In this he 

 was wrong, certainly, if it be stated as a general proposition, 

 since many bacteria are able to attack and do attack, unassisted, 

 but it appears to be clear that in some cases the two types of 

 parasites work together, the fungus invading first, and the bac- 

 terium following hard after and often doing the major part of 

 the damage, as in potatoes attacked by Phytophthora infestans. 

 The reverse of this also occurs, the bacterium entering first and 

 the fungus following, as in crown gall followed by Fusarium. 

 Parasitic bacteria are soon followed by saprophytic bacteria, 

 which complete the destruction of the tissues, and, if the dis- 

 ease is somewhat advanced, cultures from the tissues may 

 yield only the latter (potato rots, lettuce rots, etc.). Also, 

 as in animals, one bacterial disease may follow another and the 

 second be more destructive than the first, e.g., fire-blight on 

 the apple following crown gall. 



EXTRA-VEGETAL HABITAT OF THE PARASITES 



Here is perhaps the place to say a few words about the non- 

 parasitic life of the attacking bacteria. 



All are able to grow saprophytically, i.e., on culture media 

 of one sort or another, and probably all live or may live for 

 a time in the soil. Very few, however, have been cultivated 

 from it. The vast mixture of organisms present in a good 

 earth rather discourages search. In some of the unsuccessful 

 attempts failure may have been due to having undertaken 

 isolations at not exactly the right time, or in not just the right 

 place, or on not just the proper medium, but more often prob- 

 ably to the swamping tendency of rapidly growing saprophytes. 

 How long a parasite is able to maintain its virulent life in a soil 

 must depend largely on the kind of competitors it finds. I 

 have used the term virulent, because it is conceivable that an 

 organism might remain alive in a soil long after losing all 

 power to infect plants, just as we know it can in culture media. 

 Bacterium solanacearum causing brown rot of Solanaceae and of 

 many other plants, Bacillus phytophthorus causing basal stem- 



