n6 DISEASES OF CULTIVATED PLANTS 



culate, 2-3 times trichotomously branched ; conidia small, 

 obovate, or subglobose, 12 x 10 /*; oospore globose, epispore 

 brown, with raised ridges anastomosing to form an irregular 

 network, 35-40 /x. 



The tubers should be thoroughly dried before storing. 

 Only sound tubers should be used for propagation. Land 

 that has produced a diseased crop should not be planted with 

 colocasias again for two or three years, 



Morris and "Massee, Journ. Linn. Soc. (1887), p. 45. 



Maize mildew (Peronospora maydis, Racib.) is responsible 

 for a very serious disease of maize or Indian corn, which, so 

 far as is known, is at present confined to Java. Young plants 

 only appear to be attacked. As a rule the first two or three 

 leaves are healthy, later ones becoming pale green, or more 

 or less bleached, soon after which they droop and die. The 

 conidtophores emerge from the tissue of the leaf through the 

 stomata. The disease appears in twelve to eighteen days 

 after a leaf has been intentionally infected. It is assumed 

 that the disease has passed on to maize from some native 

 grass, but this assumption has not yet been proved to be 

 correct. 



Conidiophores emerging through the stomata, 0*3 mm. 

 high and up to 25 /x thick, with 1-3 main forking branches, 

 which are divided at the tip into 3-6 conical pointed 

 branchlets, each bearing a smooth, hyaline, globose conidium, 

 15-18 p diam.; oospores globose, membrane smooth, 

 14-24 //, diam. 



Oospores present in the soil are considered to be the prin- 

 cipal cause of infection. This is probably due to diseased 

 plants being left to rot and decay on the ground. 



Raciborski, Ber. d. deutsch. Bot. Gesell., 15, p. 475 (1897). 



Cabbage and turnip leaf rot. One of the downy mildews 

 called Peronospora parasitica (De Bary) during certain seasons 

 causes serious damage to turnips, cabbages, cauliflowers, 

 radishes, wallflowers, and other cultivated plants belonging to 

 the Cruciferae. Nearly all our wild plants belonging to this 

 family of plants are also attacked, the shepherd's purse 

 (Capsella borsa-pastoris) more especially suffering severely. 

 The fungus appears as white downy patches on the under 

 surface of the leaf, these patches increase in size until within 



