Bean Anthracnose 161 



the spore gets sufficient water in such cases to permit germination, but 

 he suggests that when the evaporation is not too great it may absorb 

 water from the host plant by imbibition and later by osmosis. 



If the soil is wet and cold, the bean seed germinates slowly, but the 

 spores of the fungus when present on the seed under such conditions will 

 germinate and penetrate the host before it appears above ground. If dry 

 weather follows the appearance of the plants grown from spotted seed 

 and infection has not yet taken place, the chances are good that the 

 spotted cotyledons will drop off before any infection occurs. When this 

 happens the plants are as free from infection as though they had come 

 from healthy seed, but the affected cotyledons on the ground may serve 

 as a source of inoculum for a time. Often after wet weather the juvenile 

 leaves of the seedlings become very badly spotted. These in turn drop 

 off, and if the plant has not been infected otherwise it may continue to de- 

 velop entirely free of the disease (fig. 17). If, on the other hand, rainy 

 weather should occur following the germination of spotted seed and 

 before the cotyledons fall, some of the seedlings will surely become 

 infected and the disease will be likely to spread to neighboring plants. In 

 fact, an instance is known in which, of two fields planted from the same 

 lot of seed but at different times, one field became badly diseased and 

 the other remained healthy because rainy weather succeeded the planting 

 of the one and dry weather the planting of the other. 



If infection is to take place in plants inoculated with the spores of the 

 fungus, a moist condition must prevail for at least eighteen consecutive 

 hours at some time before the spores succumb to desiccation so that they 

 may germinate and form appressoria. This statement is based on spore- 

 germination tests recorded earlier (page 117). In the inoculation experi- 

 ments conducted by the writer, the plants were usually kept in an inocu- 

 lation chamber for about forty-eight hours, although successful results 

 were obtained when they were left for but twenty-four hours. That the 

 spores may endure a period of dryness before favorable conditions arrive 

 was shown by an experiment in which seedlings growing outdoors were 

 inoculated during the morning of a bright, hot day. The next morning, 

 before the dew had evaporated, each plant was covered with a shaded 

 lamp chimney plugged with cotton at the upper end. All these plants 

 showed lesions after six days. 



