COLOCASIA DISEASE 79 



tion of the disease is to cut the crop before oospores are 

 formed. 



Peronospora parasitica, De Bary. This species attacks 

 numerous wild and cultivated plants belonging to the 

 Cruciferae. Among the latter may be enumerated turnips, 

 radishes, rape, cabbage, wallflower, cress, etc. Diseased 

 parts are covered with a very delicate white mould, and 

 are often distorted. 



Peronospora schachtii, Fuckel. The central young 

 leaves of beet and mangold are frequently destroyed by 

 this parasite ; and if its attack is continued, favoured by 

 damp weather, the plant is killed. Serious injury has 

 been done on several occasions to the beet crop in 

 France. The conidiophores form dense, greyish-lilac 

 patches of mildew on the under surface of the leaves, 

 which soon become dry and perish. Numerous resting- 

 spores are formed in the dead leaves, hence it is important 

 that all diseased plants should be collected and burned, 

 and not allowed to lie and rot on the ground. Alternation 

 of crops is the best course to pursue after the disease has 

 been present. 



Peronospora effusa, Rab., forms delicate greyish-lilac 

 mouldy patches on the under surface of spinach leaves, 

 and when present in quantity proves very destructive, as it 

 spreads rapidly. It also occurs on wild plants, as goose- 

 foot {Chenopodium). Spraying is practically impossible in 

 this case, but diseased plants should be collected and 

 destroyed, otherwise the resting-spores formed in the 

 decayed soil find their way into the soil, and are liable to 

 infect future crops. 



