1 70 Journal of Agricultural Research voi. iv. no. 2 



first appear are generally brown, rarely red, in color. The latter color 

 suggests that the lesion was first produced by some injury which probably 

 caused the formation of carotin, the fungus later gaining an entrance. 

 Typical spots grow rapidly and after 10 days or 2 weeks the black, 

 somewhat erumpent pycnidia develop. 



During the growing season of 1912 at Rocky Ford, Colo., observations 

 were made of the different types of Phoma spots that were found to 

 occur. Small brown spots were first noted on old mature leaves in the 

 early part of July. Cultures made from such spots dcA'eloped colonies 

 of Cercospora heticola and Phoma hetae. Several small red spots collected 

 somewhat later gave either pure cultures of P. hetae or a mixture of 

 Phoma and an Alternaria. It would seem that the earliest sjDots con- 

 tained more than one organism, owing probably to the fact that insect 

 wounds made it easy for various fungi to enter. These spots frequently 

 did not enlarge, showing that the organism had gained no sure foothold. 

 By the last of July or the first part of August large, light-brown, typical 

 spots yielded pure cultures of P. hetae. Such spots always occurred on 

 those leaves which were old and showed symptoms of yellowing. Con- 

 sequently on a normal beet plant only a few leaves were infected, but 

 on a plant that was physiologically weakened as a result of rot caused 

 by Rhizoctonia solani or of some other factor inimical to plant growth 

 many leaves were found to have typical spots. This obserA^ation was 

 confirmed during the season of 19 14 at Madison, Wis., where many of 

 the leaves on the "mother beet" plants were found to be infected with 

 Phoma. The roots from which these plants had grown had been more 

 or less affected by various storage rots during the preceding winter, and 

 consequently the vitality of the plants was greatly lowered. 



The leaves attacked on the normal and abnormal plants showed the 

 same symptoms of age. Thus, it would seem from field observ^ations 

 that the age of the leaf becomes the important factor in its susceptibility 

 to the disease, and this is upheld in controlled experiments. 



AGE AS A FACTOR IN LEAF SUSCEPTIBILITY 



Practically all inoculation experiments carried on in 191 2 to determine 

 the connection between Phoma hetae and Phyllosticta hetae gave negative 

 results. In all the preliminary studies the pycnospores of Phoma from 

 the root and of Phyllosticta from the leaf were suspended in sterile 

 water and either sprayed or smeared on leaves of all ages. Out of 150 

 inoculations thus made there were only four infections, and these occurred 

 on old, yellow leaves, indicating that the organism is only rarely able to 

 penetrate the unbroken epidermis. Later work has shown that even 

 at the most favorable age the great majority of infections take place 

 through some lesion on the leaf surface. 



