262 



STATE HORTICULTURAL SOCIETY. 



CEYPTOGAMIC BOTANY. 



Agricultural Experiment Station of Cornell University. — Bulletin No. 15. 



I. THE ONION MOLD. 

 Peronospora Schleideniana, De Bary. 



So far as we can certainly ascertain tliis destructive dis- 

 ease was observed in New York last year for the first time. 

 It was forwarded to ns from Madison, N. Y., by Mr. E, A. 

 Goodrich, on July 29, who writes, * * * " It is very 

 destructive. I have a bed of onions in my garden the 

 tops of which are now all dead and dry from the effects of 

 this parasite. It first appeared last year, but has not 

 become general in this section. The plant is killed by the 

 time the bulbs are from one half to one inch in diameter. 

 (natQrarsize!) showing In Setting out this Spring the small onions saved from last 

 ^^lifideniaiaTonlhi J^^^''^ crop, I fouud Several of the bulbs completely covered 

 upper part. by this mold and threw them away. It shows that the mold 



can be kept over winter." On August 6th he writes: " Very little of it 

 can be found at present. The season of its gi-owth seems to be the months 

 of June and Jiily." 



About this time it was observed by ourselves on the onions in the gar- 

 dens on the university farm, where it was not particularly destructive, 

 rarely causing half a leaf to wither, or attacking all the leaves. Several 

 small fields of the crop were noticed in Ithaca, all of which were 

 unaffected by the mold. 



It is common and greatly dreaded in Europe; but was not noticed in 



America till 1883, when observed 

 by Prof. Trelease* in Wisconsin. 

 Since then it has not often been 

 mentioned, and seems to have been 

 largely confined to the west. Even 

 in the Connecticut river valley, 

 the only disease of the onion 

 attracting attention appears to be 

 the "onion smut" {Ui^ocysUs 

 Cepulae, Frost. ) 



The mold first appears on the 

 ^ upper part of a leaf as a velvety 

 ^grayish outgrowth, being visible 

 to the unaided eye, (Fig. 1). This 

 pilose appearance is due to the 

 fruiting branches, or conidi- 

 ophores, growing out of the 

 stomates, ( Fig. 2 ) from the myce- 

 lium in the interior of the leaf. 



Fig. 2.— A few epidermal cells (ep.),and stomates (st.) The stouiates are UUmerOUS, and 

 of the onion. A conidiophore of the mold growing- Tno-nAr T^lmi+c i^anh <^n<a ic nnr'^^ 

 through a stomate from the mycelium (m. 1 bears a conid- ^ many piHlllS LdCll OUC lb OCCU- 



ium (CO.) The cell-walls (c. w.) pied by a fruiting branch, hence 



* In the " First Ann. Rep. of the Agr. Exper. Sta. of the Univ. of Wise." (1883) this author summarizes 

 what was known concerning the Onion Mold or " Onion Rust," and gives the best account of it avail- 

 able to American gardeners. 



