194 — 



The mold first appears on the upper part of a leaf as a velvety 

 grayish outgrowth, being visible to the unaided eye, (Fig. i). 

 This pilose appearance is due to the fruiting branches, or conidio- 

 phores, growing out of the stomates, (Fig. 2) from the mycelium 



in the interior of the 

 leaf. The stomates 

 are numerous and in 

 many plants each one 

 is occupied by a fruit- 

 ing branch, hence 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 di- 

 rect effect of the fun- 

 gus is to cause the up- 

 per part of the leaf to 

 become pallid, slowl}' 

 shrivel, and finally in 

 the autumn to become 

 nearly white. 



This Pero7iospora is nearly related botanically to several de- 

 structive forms of fungi such as the Downy Mildew of the Grape, 

 the IvCttuce Mold and the more destructive Potato Rot fungus. 

 Its habit is similar. Its conidial spores germinate within a few 

 hours after maturity if they fall on the surface 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 preying upon the leaf-cells. A 

 few da3'S 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 quarters, been as severe as at Madison, where it 

 destro3^ed the crop, it is likely to develop at any time into a dau- 



Fig. 2. — A few epidermal cells (ep.), and Stomates (st.) 

 of the Onion. A conidiophore of the Mold growing 

 through a stomate from the mycelium (m.) bears a conid- 

 ium (co.) The cell-walls (c. w.) 



