EUTHALLEPHYTA EUMYCETES OOMYCETES 205 



Many members of this group are destructive parasites to cultivated plants 

 like the potato rot fungus (Phytopthora infestans), the onion mildew (Peron- 

 ospora Schleideniana), the lettuce mildew (Bremia Lactucae), the mildew of 

 the sunflower (Plasmopara Halstedii), the Clover mildew (Peronospora tri- 

 foliorum) which may be injurious to animals, the millet mildew (Sclerospora 

 graminicola) which may also be injurious. As a type of this family the 

 downy mildew of the grape (Plasmopara viticola) may be taken. It appears 

 during the early summer and continues till frost. Leaves, berries and stem 

 are affected. The upper surface of the leaf shows yellow patches, underneath 

 a white frosty mould. A section through the leaf will show the mycelium 

 vegetating between the cells. The mycelium gives rise to the fruiting branches 

 of the fungus, the conidiophores, which pass out through the stomata. The 

 conidiophores are dichotomously branched, and at their ends bear the conidia. 

 When these conidia are placed in water they begin to change, at the end of an 

 hour, they swell and the contents divide. According to Dr. Farlow "at the 

 expiration of an hour and a quarter the segments had resolved themselves into 

 a number of oval bodies" which before long succeeded in rupturing the cell- 

 wall and making their escape from the mother cell. Each of these zoospores 

 is provided with two cilia. In some, zoospores are not produced, but the whole 

 mass passes out, which soon produces a tube. The zoospores produce germ 

 tubes which probably pierce the leaf of the grape. The temperature most favor- 

 able for germination is between 25 and 35 C. Inoculation experiments with 

 the grape vine mildew show that on the second day the disease appears. Sexual 

 method of reproduction takes place later in the season and occurs in the leaf. 

 A slight swelling appears at the ends of the branches of the mycelium, which 

 is spherical in shape, the cell-wall being thick and pale yellow in color. The 

 whole rounded mass is called the oogonium. The central part is the oosphere. 

 A small body is developed from another (or the same thread) which lies along- 



Fig. 49. 1. Downy Mildew. Peronospora calotheca. Mycelium between the cells send- 

 ing haustoria into the cells, x 390. 2. Potato Rot Fungus (Phytophthora infestans), 

 omdiophores, conidia borne on the branches. 3. Single conidium forming, zoosporangium 



the zoospores. 4. Discharge of zoospores. 5. Single ciliated zoospore. 6. Oogonium 

 (o) and antheridium (a). 7. Oospore and antheridium (a) of Peronospora alsinearum x 390. 

 . Conidiophore and conidia of Basidiophora entospora found on leaves of Erigeron x 200. 

 9. Germinating conidia of Bremia Lactucae (the Lettuce Downy Mildew). 10. Conidium 

 of Peronospora leptosperma germinating x 300. 1-7 9-10 after DeBary, 8 after Cornu. 



I 



