252 FUNGOUS DISEASES OF PLANTS 



demonstrated through sections. The perithecium contains at ma- 

 turity the ascospores, or perfect stage, of this fungus. Owing to 

 the dense structure of the knot, it is almost impossible to follow 

 closely the stages of development in the hymenial tissue and in 

 the formation of the asci. The asci, at any rate, develop during 

 the winter, and the spores are ripe during midwinter or later, de- 

 pending upon the region. Each mature ascus is about 120/A in 

 length, and contains eight spores, the spores being two-celled by 

 a cross wall which separates unequal portions. They may be vari- 

 ously arranged in the ascus but are often obliquely uniseriate, each 

 being 16-20 x 8-io/i. Paraphyses are always present. These are 

 filiform, nonseptate structures with a slightly enlarged tip. Other 

 spore stages, a stylosporic and a pycnidial stage, have been found 

 associated with the two already described, but they are not of com- 

 mon occurrence, and may not represent fixed and common stages 

 in the life history of this species. 



The conidia and the ascospores germinate in plum juice or 

 upon various nutrient media, and pure cultures may be readily 

 made upon solid media. The spores also germinate in water. 

 Humphrey succeeded in developing a pycnidial form upon nutri- 

 ent gelatin which differed from any stage of the fungus found 

 on its natural hosts. 



In spite of the good work which has been done upon the de- 

 velopment of this fungus, there is opportunity for much more 

 careful morphological study. It would be necessary to study the 

 plant upon different hosts and upon various culture media in pure 

 cultures in order to determine the ultimate relationships of the 

 different spore forms which have thus far been described. 



Control. It is evident that since the conidial stage is produced 

 abundantly during late spring and early summer, pruning out of 

 the developing knots just prior to the season mentioned would 

 largely control the spread of this fungus by the conidial stage. 

 A similar careful pruning should be given prior to the develop- 

 ment of the ascospores, if any knots have been overlooked. It 

 would be well, however, to make several prunings during the year 

 if this method of eradication alone is practiced. The suppression 

 of black knot has been a subject of legislation in many states, and 

 in those in which it is fairly well under control, pruning is usually 



