272 MASS. EXPERIMENT STATION BULLETIN 245 



grayish weft of tlie causal fungus is soinetinies visible to tiie naked eye. Leaf- 

 Sj)ots on the petioles are more elongated and are slightly darker in color than 

 tlie spots on the blade of the leaf. 



The blight disease appears first as indefinite brown areas with pale yellow 

 centers near the margins of the leaves. Individual teeth of the leaflet turn 

 brown followed by the browning of the leaflet and entire leaf. Sometimes 

 entire leaves turn yellow and then brown and black witliout the conspicuous 

 presence of the scattered brown areas. Lesions of blight are not as nearly 

 circular nor as sharply defined as are tiiose of* leaf-spot. The blight is the 

 more severe of the two diseases and may cause tlie death of entire leaves. 

 The mass symptoms of blight are more striking than are those of leaf-spot. 

 The foliage of a carrot field in which blight is severe may apjDcar as if 

 burned by heat. Both diseases are nuicli more couunon on the older leaves 

 of the plants. 



Of the two diseases on the main crop of carrots, leaf-spot appears earlier 

 in the season and is generallj^ found during the middle of the summer, and 

 blight does not often appear until late summer. This is probably due prin- 

 cipally to differences in susceptibility coincident with growth of the foliage, 

 a point which is further discussed. Although either disease may be found in 

 the absence of any appreciable amount of the other, it is more common for 

 blight to be prevalent following the earlier appearance of leaf-spot in a field. 



Causes of Leaf-spot and Blight 



Leaf-spot of carrot is caused by the fungus Cercospora apii carotae Pass. 

 (3). Blight of carrot is caused by the fungus Macrosjyorium carotae Ell. 

 and Langlois (4). 



These fungi have been found constantly associated with the respective dis- 

 eases in Massachusetts and their pathogenicity here has been established. 



Plants Attacked 



There is no evidence that the fungi which cause bligiit and leaf-spot of 

 carrot occur on any other cultivated plants. 



Leaves of carrot, parsnip, parsley, and celery were inoculated with 

 JSlacrosporium carotae isolated from carrot foliage, but of the plants inocu- 

 lated only carrots became infected. 



Klotz (5) has shown that Cercospora apii, from celery, is not transferable 

 to carrot, and he did not secure infection on celery when he used as inocuhun 

 , the Cercospora isolated from carrot. 



The fungi which cause blight and leaf-spot occur on and have been isolated 

 from both wild and cultivated carrot. Cultivated carrots have been inoculat- 

 ed and infected with these fungi isolated from wild carrots. The wild carrot 

 is a conmion weed in Massachusetts, and if it grows near a field of cultivate:! 

 carrots it may serve as a source of infection for the latter. 



Relation of Temperature and Moisture to the Germination 

 of Spores of the Fungi 



Since the germination of fungous spores is prerequisite to infection, a brief 

 consideration of some of the factors which affect the germination of the 

 spores of these fungi is of interest. 



