THE ANGULAR LEAF-SPOT OF COTTON: TECHNIC 331 



looking stage of the disease. Do any of the accompanying 

 saprophytes favor the growth of the parasite? 



Only yellow colonies should be considered. Moreover all 

 yellow ones that have a wrinkled surface, all that reduce nitrates, 

 redden litmus milk or fail to throw down the casein must be re- 

 jected on the start. Potato cylinders also should be used as a 

 means of separation, all orange-colored and slow-growing, non- 

 starch-consuming organisms being rejected. On agar-poured 

 plates besides the white colonies and wrinkled yellow ones, cir- 

 cular pale yellow colonies of two or more types may appear. 

 Those having a smooth surface and a mottled interior, i.e., light 

 and dark aggregations, are likely to be the right organism, 

 especially if they give a copious pale yellow growth on potato 

 and precipitate casein from litmus milk without development of 

 an acid. Those pale yellow surface colonies having from the 

 beginning (2d day, 3d day) a uniform inner structure and yield- 

 ing a scanty creamy growth on the potato cylinders should be 

 rejected. Fortunately, young rapidly growing cotton plants 

 are quite susceptible and sub-cultures from the various colonies 

 may be tested out quickly, by means of stem and leaf inocula- 

 tions, care being taken, if the sprayed plants have been shut up 

 over-long in cages, that suffocation spots are not mistaken for 

 bacterial spots, since the former may occur. Sections (Fig. 261) 

 will quickly show whether or not the spots contain bacteria in 

 numbers. 



Cotton for the inoculations should be planted in a warm 

 hothouse six weeks to tw^o months before it is needed. Many 

 varieties are subject to the disease. I have had good success 

 with inoculations on Rivers, Sunflower, Durango and Columbia. 

 The disease is readily induced on young rapidly growing stems 

 and bolls by delicate needle punctures introducing the parasite, 

 and several times I have produced it on leaves simply by gently 

 rubbing my infected fingers over their undersurface (Fig. 241aa). 

 Generally, however, for the leaf infections, the spraying of water 

 suspensions of young agar-streak cultures upon plants shut up 

 48 hours in spraying cages will prove most satisfactory. Cages 

 are not necessary, however. I have not seen the leaf spots due to 

 stomatal infections appear earlier than toward the end of the 



