132 DISEASES OF CULTIVATED PLANTS 



cabbage, cress, and among weeds the shepherd's purse 

 suffers most severely. The fungus attacks the plants when 

 seedlings, entering into the tissues through the stomata of 

 the cotyledons or seed-leaves. The mycelium grows up with 

 the plant, and at a later stage produces fruit on leaves, 

 stem, and flowers alike. When the stem and inflorescence 

 are attacked they usually become much distorted and curled ; 

 the same also applies to the fruit. The fruit of the fungus 

 first appears at the surface of the host as snow-white, polished 

 patches which soon become powdery as the conidia mature 

 and become free. 



Conidia forming the chains all similar in size and shape, 

 globose, 1 0-16 fj. diam. Oospores subglobose, brownish, 

 wall with large warts which sometimes run out into ridges 



35-45./* diam - 



If it is remembered that the infecting bodies are zoo- 

 spores, can only make progress when a film of moisture is 

 constantly present on the host-plant, and further that in- 

 fection can only be effected during the seedling stage of 

 the host, it will be seen that seed-beds should occupy a fairly 

 dry, open situation. 



As oospores are usually produced in abundance, all 

 diseased plants, as far as practicable, should be removed and 

 burned. Finally cruciferous weeds, more especially shep- 

 herd's purse, should be kept down. 



Cystopus tragopogonis (Schrdt.) forms white patches on 

 the leaves of goat's-beard ( Tragopogon pratensis\ also on 

 Convolvulus and Ipomaea. 



The white sori or pustules are often grouped in broken 

 concentric rings. Terminal conidium of chain larger than 

 remainder, thick walled, sterile, remainder of conidia shortly 

 cylindrical, each with a transverse thickened ring, 19-23 /x. 

 Oospore globose, brown, with large wrinkled warts, 45-60 /t 

 diam. 



Var. spinulosus forms white patches on thistle leaves, and 

 is distinguished by the elongated conidia, and the very 

 prominent warts on the wall of the oospore often bearing 

 outgrowths or spines. 



Cystopus lepigoni (De Bary) forms minute yellowish 

 patches on leaves of plants belonging to Caryophyllaceae, 

 as Spergula, Armaria, etc 



Terminal conidium of the chain larger than the rest, 

 sterile, remainder nearly globose, 18-21 /i. Oospore 



