EXPERIMENT STATION BULLETINS. 263 



the velvet-like aspect of the diseased part. The gray or smoky tint is 

 due to the lavender color in the cell-wall of the conidial spore, ( Fig. 2, co. ) 

 The direct effect of the fnngus is to cause the upper part of the leaf to 

 become pallid, slowly shrivel, and finally, in the antnmn, to become nearly 

 white. 



This pcroiioi^pora is nearly related botanically to several destructive 

 forms of fungi, such- as the downy mildew of the grape, the lettuce mold, 

 and the more destructive potato-rot fungais. Its habit is similar. Its conidial 

 spores germinate within a few hours after maturity if they fall on the sur- 

 face of the onion leaf and find it moist. The germ tube enters a "breathing- 

 pore," or stomate, when it forms an extensively branching and ramifying 

 mycelium, running among and praying upon the leaf -cells. A few days 

 suffice for its maturity, when its conidiophores, already mentioned, grow 

 through the stomates, and immediately develop the conidia on the ends of 

 the ultimate branches, as shown in the figure. The rapid maturing of 

 these conidia furnish another crop of spores for fresh infections. 



The rapid and the fatal progress of the disease as indicated in the letter 

 from Mr. Goodrich, is thus accounted for. If it has not already, in many 

 c|uarters, been as severe as at Madison, where it destroyed the crop, it is 

 likely to develop at any time into a dangerous enemy, if we may judge 

 from the experience in England and France. 



An important link in its history, its mode of passing the winter, has 

 not been definitely ascertained. Besides the conidial spores, which are 

 ephemeral, it produces rarely "oospores," or spores which will live over 

 winter in the dead tissues of the onion leaf. But these are probably not 

 the active propagators of the disease. They rather insure the sj^ecies 

 against complete extinction at any time. 



Mr. Goodrich and others have found the onion bulbs in the spring 

 afl^ected, apparently by this fungus. We have examined the tissue of fresh 

 leaves growing on diseased plants in October and November, and found 

 vigorous mycelium among the cells, althougl there was no external sign 

 of the fungus, nor was there any disposition to send out fruiting branches 

 when the onion plant w^as brought into a warm room. If the mycelium 

 hibernates in the leaves or bulbs and begins to fruit in the spring from 

 this source, it is possible our future observations may ascertain the fact. 



No remedy has been tried. But the success in France of the copper 

 fungicides in the treatment of a related parasite, the Potato-rot, — (see repe- 

 tition of this important experiment, appended below), justify us in hoping 

 they will protect the onion also. No bulbs from a crop diseased the pre- 

 vious year should be set out; and the English are in the habit of sowing the 

 onion seed in the fall, thus enabling the young plant to get a good start 

 before the possible advent of the fungus in the spring. 



THE PREVENTION OF POTATO ROT. 



Col. A. AV. Pearson of Vineland, N. J., in "Garden and Forest" Dec. 4, 

 1889, reports almost completely siiccessful experiments during 1889, against 

 the common Potato Eot ( Phyfophthom infestans) by the use of either 

 of the following copper solutions: 



(1). The Bordecmx Mixture. The particular formula employed was as follows: (a) 

 Sulphate of copper (pulv.), 6 pounds, in 4 gallons of hot water, (b) fresh lime, 4 pounds 

 in 4 gallons of cold water. Mix. (a) and (b) slowly and thoroughly and dilute to 22 gal- 

 lons. 



