480 CLOVER PERONOSPORA. SEEDLING ROT. 



13. Clover poronospora, {Peronospora irifoUorum, DeBary). 

 A dirty white or purple-brown mold, often completely covering 

 the lower surface of the leaves of clover, alfalfa, none-such, etc. 

 (See Fig. 175.) 



The life history of this species is quite 

 similar to that of the last, though they differ 

 greatly in appeai'ance. The leaves that it 

 occurs on are paler than the others, and the 

 threads that escape through their stomata and 

 bear conidia are so numerous and bushy as to 

 form a dense coating on their under side. 

 Fig. 175. Oospores are produced in smaller numbers 



than in the last species, and, as they are thin-walled 

 and nearly colorless, they are only to be found after careful 

 microscopical examination. Another species of the same genus 

 {P. vicial, Berk.) is found on the leaves of vetches and of the 

 pea. 



14. Seedling-rot, {Pytldum deharyanum, Hesse). Causing 

 young plants of clover, millet, corn, and many other species to rot 

 close to the ground or " damp-off," as it is called in greenhouses. 



Several species of Pytldum attack living plants. The present 

 species is said to be widely distributed in garden soil in Europe 

 and causes serious trouble by attacking seedling plants. It can be 

 recognized by its effects on the plants, which quickly decay near 

 the ground. They contain a delicate, colorless mycelium that 

 fruits on the surface of the deling parts, when these aro kept 

 damp, producing conidia, swarm-spores, and oospores. 



15. Fairy-Ring Fungi. — Bright green circles, several feet in 

 diameter, closely surrounded by a narrow strip of dead or dying 

 grass, are frequently seen in lawns or pastures, and are commonly 

 called "fairy-rings." They are caused by several species of 

 toadstools (the commonest is Marasmius oreades) that spread a 

 short distance outward every year, their mycelium destroying 



