Family Cucurbitaceae 245 



or root of the diseased plant, its interior fibrovascular 

 bundles will be browned. The browning indicates 

 the presence of the parasite. 



Wilt is not always confined to the older plants. 

 In badly infected fields young seedlings begin to die 

 at an early age (fig. 42 d), resulting in a very poor 

 stand. From the investigations of the writer, it has 

 been found that Fusarium citrulU is more active on 

 seedlings than are the other two species of Fusarium. 

 This, however, is not intended to convey the idea that 

 F. niveum and F. Poolensis are not capable of pro- 

 ducing wilt on the younger seedlings. 



The Organisms. The three species of Fusarium 

 which produce wilt of watermelon may be readily 

 distinguished when grown in flasks on commeal. 

 Fusarium citrulli is entirely different from the two 

 others in that it forms a glistening, flat, compact, 

 flesh-colored dry growth confined to the surface of 

 the commeal. Growth is slow, and no color is pro- 

 duced in the substratum for a considerable time, 

 about two months or more. Fusarium Poolensis at 

 first greatly resembles F. niveum in growth and in 

 color. Later, however, F. Poolensis takes on a deep 

 blue to almost indigo which is retained indefinitely. 

 The three species of Fusarium have been definitely 

 proved by the writer to be the cause of the water- 

 melon wilt. Infection can take place only on water- 

 melons and not on any other cucurbit hosts, nor on 

 cotton; okra, or cowpea, the wilts on all of which are 

 caused by different species of Fusarium. It is possible, 

 however, that a sick watermelon field may also be 



