274 MASS. EXPERIMENT STATION BULLETIN 245 



tion on young leaves. The leaves most suscei)tihle to the l)light fungus were 

 those which had reached such an age as practically to cease growing. 



Blight causes little loss on hunch carrots for they are harvested before tlie 

 leaves have reached the most susceptible stage. Carrot leaves up to eight 

 weeks of age are relatively resistant to the disease and after that suscepti- 

 bility increases with age. 



Young leaves of carrot are, however, not resistant to leaf-sj)()t, and i)lants 

 as young as sixteen days old may be infected. But leaf-spot alone is of less 

 importance on the carrot here than is l)light, and the symptoms of the former- 

 early in tiie season are rarely consj)icuous. 



Overwintering of the Fungi Which Cause Blight and Leaf-spot 



Tlie sjjores of tlic blight fungus, Murrosporhun rarotite, \\ere foimd to li\e 

 through tlie winter in tiie soil. Spores collected in tlie field during the winter 

 and spring always germinated. Freezing the spores in ice did not kill tlieni, 

 but rather was followed by an increased percentage of germination. 



The mycelium of tiiis fungus was found to be living in carrot refuse in tlie 

 field as far into the winter as January 15. But attempts to isolate the fungus 

 the following April from mycelium in carrot refuse in the field were not 

 successful. 



The leaf-spot fimgus, Cercospora upii carotae, was found to live through 

 the winter in fragments of carrot leaves in tlie form of mycelium. Leaves 

 with leaf-spot were buried in the soil in the field in October. The following 

 April the fungus was isolated from the mycelium in these leaves. 



The spores of the leaf-spot fungus obtained in the winter and spring from 

 fragments of carrot leaves in the field did not germinate. 



Since blight and leaf-spot sometimes occur on the inflorescence of tiie car- 

 rot, the possible relation of carrot seeds to overwintering of the fungi was 

 considered. However, neither of the fungi was found in or on the seeds. 

 Carrot seeds from diseased plants were plated on agar hut neither of the 

 fungi developed. 



In this connection it may be noted that Klotz (5) did not find on the seeds 

 of celery the closely related fungus, Cercospora apii, which causes early blight 

 of this crop. 



Since the fungi were not found on carrot seeds, there is probably no reason 

 for market gardeners to disinfect the seeds of carrot for the prevention of 

 blight and leaf-spot. In any case, it is unlikely that seed treatment would 

 materially reduce infection in the field because both fungi are generally 

 present and can winter-over in fragments of diseased carrot leaves in the soil. 



Dissemination of the Blight Fungus 



In the fall when carrots are harvested, crumbling fragments of decaying 

 leaves which have been lying on the soil form part of the dust in the air. 

 Great numbers of spores of the blight fungus. Macros porium carotae, are 

 present. Slides coated with adhesive were exposed 100 yards from carrots 

 when they were being harvested, and many spores of this fungus were caught 

 upon them. The fungus is doubtless disseminated in other ways, but the 

 spores which are borne by the wind are probably an important means of 

 disseminating the fungus to fields not previously used for carrots. 



