i io DISEASES OF CULTIVATED PLANTS 



If a given plant is only slightly injured during the first 

 season, the mycelium hibernates in the crown of the root and 

 infects the leaves the second season. For this reason roots 

 that have grown in an infected plot should not be planted 

 for the purpose of furnishing seed. 



Conidiophores repeatedly branched, terminal branchlets 

 straight, blunt, conidia ovate, tinged brown, 20-24 X 15-18 //. 

 Oospores globose, the thick wall smooth. 



Closely related to Peronospora effusa (Rab.), differing more 

 especially in the oospore having a perfectly smooth wall. 



As oospores are produced abundantly in the tissue of the 

 dead leaves, all diseased plants should be collected and 

 burned. Rotation of crops should be followed in the case of 

 land that has produced a diseased crop. 



Peronospora affinis (Rossm.) forms delicate, dingy lilac 

 patches of mildew on the leaves of Fumaria officinalis. 



Conidiophores much branched, ultimate branchlets pointed, 

 conidia elliptical, 22-26x15-18 p. Oospores globose, wall 

 brown, smooth. 



Peronospora grisea (De Bary) occurs on plants of the order 

 Scrophulariaceae, more especially on the speedwells ( Veronica], 

 where it forms dense, dingy, lilac tufts on the under surface of 

 the leaves. 



Conidiophores thick, many times branched, ultimate 

 branches generally unequal in length and slightly curved, 

 oospores elliptical, 25-30X15-22 p. Oospores globose, wall 

 thick, smooth, 30-40 /x diam. 



Peronospora urticae (De Bary) forms small, dingy lilac 

 patches of mildew on the under surface of nettle leaves. 



Conidiophores repeatedly branched, branches wavy, ulti- 

 mate branchlets pointed, curved, conidia broadly elliptical, 

 20-30 X 17-24 p. Oospores globose, brown, smooth, 21-25 I 1 

 diam. 



The conidia have generally a suggestion of a papilla at the 

 apex. 



Peronospora ficariae (Tul.). On many wild buttercups. 

 Often covering the entire plant with a delicate greyish 

 mildew. 



Conidiophores repeatedly branched, ultimate branchlets 

 usually curved, conidia broadly elliptical, 20-25x15-18 /*. 

 Oospores globose, brown, wall almost smooth, 30-35 ^ 

 diam. 



