24 DISEASES OF CROPS. 



ground in the autumn, and hibernate until the summer, 

 when they germinate on clover leaves, giving rise to 

 internal hyphse and mycelia, which in due time bear 

 conidia. 



The growth of this mildew injures the clover plant in 

 several ways : (1) By simple contact it causes putre- 

 factive changes to occur within the tissues of its host. 

 (2) It clogs up the organs of assimilation and transpiration 

 (the stomata), and thereby greatly interferes with these 

 physiological functions, which are necessary for a healthy 

 and vigorous growth. (3) This mildew produces brownish 

 spots on clover leaves. 



Prevention. — (1) Avoid thick planting, for it favours the 

 attacks of P. trifoliorum. (2) Burn all clover refuse, as 

 it harbours the resting-spores of this fungus. (3) The 

 land should be well drained, as P. trifoliorum is " fond " 

 of moisture. 



Peronospora exigua was discovered by Mr. W. G. 

 Smith, F.L.S., and is very similar in its life-history to 

 the clover mildew. It is much smaller than Peronospora 

 trifoliorum, and causes putrefactive changes within the 

 cells of the leaves of clover. This fungus, whose conidia 

 are almost round, produces oospores like the clover 

 mildew. 



Besides the two species of Peronospora described, 

 several other fungoid growths are, or have been, put 

 down by various observers, as the cause of " clover sick- 

 ness." From what has already been said, there is little 

 doubt that " clover sickness " is due to a variety of causes, 

 and not to any single parasite (animal or vegetable). 



(3) The Parasites of Lucerne {Medicago sativa). 

 The animal and vegetable foes of lucerne are the same 



