106 REPORT OF THE COMMISSIONER OF AGRICULTURE. 



yard — at least the injury is slight as compared with that wrought by 

 the Downy Mildew. 



This season I have observed it in all its phases of development, both 

 ujDOn vines in the open air and upon those cultivated under glass here 

 at the Department. Upon the foreign varieties in the grapery it was 

 most abundant and its injurious effects most apparent. 



The term "Powdery Grape-Vine Mildew" was first applied to this 

 fungus by Prof. C. V. Riley,* and as it is descriptive, and at the same 

 time clearly distinguishes this mildew from Peronospora viticola, to 

 which the same author has applied the name of "Downy Mildew," it 

 has been employed here, with the hope that it may become generally 

 adopted by those who prefer English to Latin names. 



It ha^ frequently been discussed in our agricultural and horticul- 

 tural journals for many years past under the name of " O'idium 

 Tuckeri," or simply " O'idium/' it being supposed that our fungus 

 was the same as the European vine mildew of that name, but whether 

 the European O'idium is the same as our Uncinula or not is yet a 

 matter of question, owing to the fact that the mature or fruiting form 

 of the first named has never been discovered, the conidial stage alone 

 being known. De Bary has suggested that the European O'idium is 

 an importation from America. He says : f 



Concerning its first appearance and spread in Europe, it can be accepted as certain 

 that it was transported suddenly from some otlier flowering species introduced into 

 our vineyards from abi'oad. Most prohahly its iximigration is from Amernca. [Tlie 

 italics are mine. — F. L. S.] In spite of its destructive spreading over the wliole 

 vine-growing portion of Euroi^e, the most careful investigations in this countiy 

 have nowhere led to the discovery of any indication of perithecia; the entire inva- 

 sion takes place by means of the conidia, produced in great abundance, the form 

 of which has procured the fungus the name of O'idkwi (O. Tuckeri). The perithecia 

 are probably found in North America on the native sorts of Vitis, and have been de- 

 scribed as^. {Uncinula) spiralis, Berk, and Curtis; yet this is not certain. 



The Powdery Mildew consists of a mycelial growth, that rests wholly 

 upon the surface of the parts of the vine supporting it, and the repro- 

 ductive bodies or spores, of which there are two kinds. The threads 

 or hyphse of the mycelium have a uniform diameter of about c-oVo of 

 an inch, are much branched and interlaced, and are provided with 

 frequent septa or cross-walls. Where this mycelium is applied di- 

 rectly tO'the epidermis of the supporting plant there are developed 

 at short intervals irregular protuberances or suckers, by which the 

 fungus fastens itself to the host and through which it imbibes its 

 nourishment. 



If the fungus be examined early in the season, say in June or early 

 in July, short branches will be seen arising from the threads at right 

 angles, or nearly so, to the plane of their growth. The branches are 

 divided into several oblong cells by cross-walls. The uppermost cell 

 is slightly larger than that immediately below it and is rounded at 

 its upper extremity. If this terminal cell be v,^atched we will soon 

 see its lower end becoming rounded like the upper, forming thus a 

 stricture between it and the next cell below, from which it is soon 

 completely separated and falls off. The next cell of the branch 

 quickly passes through the same changes noted in the first, and in 

 this way a number of conidia are formed in rapid succession. Like 

 the conidia of the Peronospora, those of the Uncinula serve for the 

 immediate propagation and dissemination of the fungus; but damp- 



* Proc. Amer. Pom. Soe. , session of 1885, p. 49. 

 f Verg. Morph. u. Biol, der Pilze, &c., pp. 244-245. 



