326 Journal of Agricultural Research voi. xrn.No.e 



LIFE HISTORY OF THE CAUSAL ORGANISM IN RELATION TO PATHO- 

 GENESIS AND CONTROL MEASURES 



PRODUCTION OF CONIDIA 



As has been described previously, conidia are formed in the pycnidia 

 sometimes even before the symptoms of the disease are conspicuous. 

 They are usually abundant by the time the yellowing is distinctly visible. 

 It appears that they are produced in greatest amount while the leaf tissue 

 surrounding the pycnidia is still alive. When the diseased area dies and 

 dries, conidia production appears to cease. When the entire leaf is dead 

 the conidiophores are usually found disorganized, though the cavity 

 may still be filled with spores. But it is doubtful if the production of 

 conidia always ceases with the death of the leaf. In a few instances, 

 while examining structures that appeared like undeveloped apothecia 

 on dead leaves in the summer; and in one instance in the spring, these 

 structures were found to be filled with typical conidia borne on conidi- 

 ophores. Since conidia production has been found under such circum- 

 stances but two or three times, it is assumed that it is not a frequent 

 occurrence. There has been no opportunity to determine whether these 

 conidia are capable of germination. Ordinarily by the time that 

 apothecia are mature the conidia have completely disappeared from the 

 diseased leaf. 



PRODUCTION OF ASCOSPORES AND OVERWINTERING 



Since the experimental evidence indicates that the ascospores are the 

 only source of infection, a careful study of the conditions under which 

 they are produced becomes important. Apothecia containing spores that 

 will be discharged after a few hours of soaking have been found most 

 abundantly in the autumn, and even in the early winter, especially in 

 the late growth of alfalfa which has become well infected before killed 

 by frost. If these leaves are not subjected to frequent wetting, spores 

 will be retained in a viable condition, at least over the winter. This has 

 been shown by the fact that collections made in October, 191 5, and 

 wintered in a cage out of doors were used successfully to produce infec- 

 tion the following February, and spores were discharged from this mate- 

 rial as late as the following August. But in the field the spores formed 

 in the fall appear to be discharged long before spring, and no new spores 

 are formed until after a period of warm weather. On May i, 1916, 

 apothecia were collected at Madison, Wis., on overwintered leaves in the 

 field, but no mature spores were found in them until they had been kept 

 four or five days in a damp chamber at room temperature. On May 10 

 several collections of apothecia containing spores were made. By May 

 28 the disease was abundant in the localities where the apothecia had 

 been found on May 10. On the latter date a few apothecia with mature 



