DISEASES CAUSED BY FUNGI 



biologic specialisation in regard to host plants is sometimes 

 exhibited in this family, and resistant varieties are therefore 

 possible. 



The Downy Mildews {Phytophthora). 



Members of the genus Phytophthora are the most rapidly 

 destructive of plant parasites. One of them, P. infestans, caused 

 the potato famine of 1845 ^^ Ireland. P. faheri is the cause of 

 cacao canker and pod-rot. P. palmivora is the cause of a destruc- 

 tive bud-rot of palms in India, and has been recognised on coconut 

 in Jamaica. Another, or possibly more than one species causes in 

 the West Indies a boll 

 rot of cotton which is 

 very destructive in 

 wet weather. 



The mycelium of 

 Phytophthora is hy- 

 aline, non- septate 

 when young, much 

 branched, and has a 

 characteristically 

 coarse and vigorous 

 appearance under the 

 microscope. Living 

 tissue is infested and 

 rapidly killed. The 

 common form of re- 

 production is by large 

 lemon-shaped conidia, 

 developed one by one 

 at the apices of coni- 

 diophores produced on 



the surface of the medium. The massed conidia on the surface 

 of a cacao pod or cotton boll have a granular glistening appear- 

 ance ; the mycelium, in very moist conditions, may grow out 

 and form a white downy covering. The conidia typically ger- 

 minate by liberating a number of zoospores, which swim 

 actively in a surface film of water before settling down and 

 producing a germ tube. While in the mobile condition they are 

 splashed about by rain, blown about in drops of water by the 

 wind, and may conceivably be distributed by birds or insects. 

 In the absence of free water the conidia sometimes produce 

 germ-tubes direct. 



In addition to the conidia, resistant chlamydospores and 

 oospores may be produced on the mycelium buried in the infested 

 tissues. 



The occurrence and prevalence of Phytophthora diseases is 

 dependent on a high degree of humidity in the air about the 

 plants. Close planting, shade and shelter, the inducing of the 



From a drawinglpy] 

 Fig. 2 



[J. B. Rarer 

 Phytophthora Faberi 



