BLIGHT AND LEAF-SPOT OF CARROT 273 



Tlie sixires ot tlie hligiit fungus, Jlacrn.sporium ccrotae. were found to ger- 

 minate well at all temperatures between 61° and 82° F. They germinated 

 l)est at temperatures between 72° and 75° F. The temperatures most favor- 

 able for the germination of the spores of the blight fungus are relatively 

 high. 



The spores of the leaf-spot fungtis, Cercosporn apii carotae. germinated 

 best at a temjierature of 75° to 78° F. 



The spores of both fungi were found to germinate well through a relati\el\' 

 long range of temperature, and it is not likely that summer temperatures are 

 often an important factor affecting infection by these fungi in Massachusetts. 

 In the case of botii diseases, tliere is more likelihood of the weather late in 

 the siunmer being too cool than too warm for their development. 



"Water on the leaf is necessary for germination of the spores of the blight 

 fungus, Marrosporiu)!! carofoe. An average of only 6 per cent of these spores 

 germinated in moisture-saturated air as coniiiared with 100 per cent which 

 germinated in water. The leaf must be wet, but these spores germinate 

 quickly and two liours in water is long enough to permit germination. 



The spores of the leaf-spot fimgus, Cercospora apii carotae, germinatetl 

 well in moist air, but only after they had been previously soaked in water 

 for one or two hours. 



Tliese findings are in accord with the field observations to the effect thai 

 the severity of these diseases on carrot is usually proportional to the rainfaii 

 of the season. 



Infection 



The germ tubes which gro\\ out of the spores during their germination 

 penetrate the carrot leaves and infection results. 



The average length of time between inoculation and the first appearance 

 of the symptoms of disease on the leaves, or the period of incubation, was 

 found to be eight or nine days in tiie case of the leaf-spot fungus, Cercon- 

 pora apii carotae, and about fourteen days in the case of the blight fungus, 

 Macrosporiuni carotae. The air temperatures during the period of incubation 

 were between 67°F. (nights) and 86°F. (days). 



As was anticipated from experiments on germination, it was found that the 

 leaves must be wet and must remain wet for more than two hours for blight 

 infection to occur. If, during the germination of the spores of the blight 

 fungus, MacroKporium carotae, on a wet leaf, the leaf became dry and re- 

 mained so, the germinating spore was killed. A short rain immediately fol- 

 lowed by drying sun and W'ind may result in the death of these spores rather 

 than in their germination. However, carrots are planted so thickly that any 

 rain causes the foliage nearest to the soil to remain wet long enough to permit 

 some infection. , 



There is an interesting and rather important relation between the age of 

 carrot leaves and the degree of their susceptil)ility to blight. The oldest and 

 the youngest leaves were renio\ed from plants, placed in moist chambers and 

 inoculated with the spores of the blight fungus, Macrosporiuni carotae. None 

 of these leaves showed any evidence of natural infection, and uninoculated 

 check leaves in moist chambers never became infected. The experiment was 

 repeated five times and in each case three weeks after inoculation the number 

 of blight spots on young and on old leaves was counted. There was an aver- 

 age of twenty-four points of infection on old leaves for each point of infec- 



