i5o 



FUNGOUS DISEASES OF PLANTS 



Symptoms. The effects of the fungus are somewhat various 

 upon the different hosts. Upon the shepherd's purse the stems 

 are enlarged and distorted, while no unusual malformations of 

 floral organs and leaves generally occur. On the radish the floral 

 organs may be strikingly hypertrophied (Fig. 49), ovary sacs 

 greatly enlarged, stamens, petals, and sepals distended and some- 

 times becoming leaf-like. Upon nearly all hosts the porcelaneous 



Fig. 50. Conidial Stage, Fertilization, and Germinating 

 Oospore of Cystopus. [b and c, after De Bary) 



conidial cushions, characteristic of the family to 'which this species 

 belongs, are prominent. 



The fungus. The conidial cushions occur upon leaves, stems, 

 and floral parts, or fruits. On the majority of hosts, such as 

 shepherd's purse, horse radish, etc., oospores generally occur only 

 in the stems, yet upon some other hosts, particularly upon certain 

 mustards in the western United States, oospores alone are com- 

 mon. The mycelium is considerable, and constantly intercellular, 

 with abundant knob-like haustoria. The mycelium develops abun- 

 dantly at some points just beneath the epidermis, and there are 

 produced numerous short, erect, basally branched conidiophores. 

 The latter give rise to simple chains of spores in basipetal suc- 

 cession. These are usually separated one from another by slight 



