42 MASSACHUSETTS HORTICULTURAL SOCIETY. 



In selecting a subject for this address, it has seemed to me 

 better, and probably more profitable, to choose a quite restricted 

 field which I can hope to cover in some detail, rather than to 

 attempt to string together a few facts gathered in a necessarily 

 very superficial survey of a larger one. This moderately detailed 

 examination of a single small corner of the whole great field of 

 plant diseases will also, I believe, give a truer idea of its extent 

 and of the need of further exploration, than any general, outline 

 sketch of the whole could do. The word " mildew" is used very 

 loosely, to designate either of several appearances or effects due 

 to widely different causes. 



But as our knowledge of the various parasitic diseases of plants, 

 and popular interest in them, have increased, botanists, yielding 

 to the demand for vernacular equivalents for the technical names 

 of the various groups under which the parasites are classed, have 

 pretty generally adopted this word as the English name of one 

 or two groups. If we accept the authority of the United States 

 Department of Agriculture, we shall apply the term to two groups, 

 botanically quite distinct, and shall distinguish the two by prefix- 

 ing descriptive adjectives. The Reports of the Department speak 

 of the doivny mildews and the poivdery mildews. The structure 

 and effects of the organisms composing these two groups are ver}' 

 different, and it would be impossible to discuss them together. 

 I shall, therefore, confine myself to that group to which the term 

 mildews is by some restricted — the downy mildews. This group, 

 known to botanists as the Peronosporece, is a rather small and 

 very natural one, and the life history of its members is much more 

 completely known and understood than is the case with most of the 

 vegetable parasites. For this result we are indebted chiefly to the 

 model researches of the great German master and pioneer in this 

 work, De Bary, whose untimely death a year ago has left a con- 

 spicuous vacanc}' in the ranks of the workers. 



The members of the Peronosporece may be regarded, for our 

 purposes, as separable into two smaller groups, or genera, known 

 by the names Peronospora and Cystopus. Those included in the 

 latter genus difl^er in several important particulars from the mem- 

 bers of the former, and are often denominated the White Rusts. 

 They do not, then, concern us at present, and may be ignored. 

 We thus restrict the term mildew to those parasites included in the 

 genus Peronospora^ which we maj- now proceed to discuss. 



