128 DISEASES OF CULTIVATED PLANTS 



impossible, whereas if the leaves only are attacked, the plant 

 may recover. Gaps are often made in seed-beds by this 

 fungus, which spreads rapidly when once introduced. The 

 extension of the disease is favoured by damp weather, and is 

 retarded by drought. 



In this species the mycelium is furnished with minute, 

 roundish haustoria, which pierce the cells and absorb nourish- 

 ment. Lemon-shaped, papillate conidia are produced on the 

 surface of diseased portions. These are conveyed by wind, 

 rain, animals, etc., to neighbouring plants, where they 

 germinate, enter the tissues, and extend the disease. Sexually 

 produced oospores or resting-spores are also formed in the 

 tissues of the host-plant. 



Hyphae variously branched ; conidiophores slender, simple, 

 or sparingly branched, often nodulose at intervals below the 

 apex; conidia lemon-shaped, 50-60X35-40 /, liberating on 

 germination up to 50 zoospores; oospore globose, smooth, 

 yellowish-brown, 24-30 /<. diam., often in clusters. 



Seed-beds should be freed from shade if the disease 

 appears, as if the young plants dry quickly the conidia are 

 prevented from germinating. All diseased plants should be 

 carefully and promptly removed and burned. Hartig states 

 that oospores retain their vitality for four years, hence land 

 that has borne diseased plants should not be used again. 



De Bary, Beitr. Morphol u. Phys. der /'/.'-.. (1881). 

 Hartig, Unters. aus don Forstbot. Inst. (1880), p. 33. 

 1 Luti- and Somerville, Diseases of Trees, p. 38. 



Cacao-pod disease. This disease, caused by Phytophthora 

 omnivora (De Bary) has been present for a considerable 



time in the West' Indies, but of lab years has become 

 much more general and destructive in Trinidad. The same 

 disease has been proved to be present on cacao-pods in 

 Ceylon, and is probably present to some extent wherever 

 cacao is cultivated. The fruit is the part attacked, and the 

 symptom of disease is a blackening of the ' shell ' of the 

 pod, which almost invariably commences at one end, and 

 gradually spreads over the entire surface. After a while the 

 fruit of the fungus appears on the surface of the shell as 

 a very delicate white mould, located mostly in the furrows of 

 the shell. The white mould represents the conidial fruit of 

 the fungus, and continues to produce fruit for some time. 



