
293 
LetrucE MILpEw, 
Peronospora gangliformis DeB., 
This fungus at first appears 
in irregular patches half an 
inch or more across on both 
surfaces of lettuce leaves, 
either at first without altering 
the color, or changing it toa 
shade or two lighter green, and 
is visible as minute, colorless 
down or mold. The vegeta- 
tive threads of the fungus 
grow within the leaf and only 
come to the surface to form 
spores. It isrelated to the po- 
tato rot and grape mildew, hav- 
ing the same destructive hab- 
its on account of which it is 
sometimes called lettuce rot. 
When abundant it overspreads 
the leaves and soon makes 
them soft and rotten. At first 
i _ the outer leaves are affected 
Fig. 4.—A single branch of lettuce mildew aris- 
ing from the breathing pore of a leaf, and bear- and finally the whole plant 
ing spores; a end of a branch from which the may break down into a wet 
spores have fallen. Magnified 150 diameters. ; : 
riginal. putrid mass. The delicate 
white mildew on the surface, the only part visible to the unaided 
eye, is made up of tree-like branching threads (fig. 4) dilated at their 
extremities (fig. 4 a), each of which bears several colorless globular 
spores. The number borne by a single branch is very large; that il- 
Iustrated shows probably not more than half the original number 
of spores, for many dropped off in handling, and the others lay upon 
each other too thickly to be all drawn. 
The disease was observed on lettuce in the garden in September 
and October, and caused considerable rotting of thefoliage. It may 
occur throughout the season, but is most prevalent in spring and 
fall. It is not, however, likely to prove very troublesome to lettuce 
grown in the open air, although Smith* states that it sometimes en- 
tirely destroys the harvest of lettuce seed in England; but on early 
lettuce in the hot bed the reverse is true, and it may and often does 
sweep away the whole crop. It is especially troublesome through 
the Alantic statest but is also found westward], and may be expected 
wherever lettuce is forced. The best means known of controlling it 
is to keep the air about the plants as dry as possible. In the open 
ground the plants should be thinned and kept free of weeds, and in 
the hot bed the sash should be raised for the escape of the moisture. 
The application of lime to the foliage is also said to have some value. 
The fungus is common throughout this country and Kurope on 

- several species of wild lettuce and allied plants. It is on these that 
it forms its resting spores, which carry it through the winter and 
*Gardeners’ Chronicle, XX, p. 600. 
+Halsted, Rep. Conn. Bd. Agr., 1882; Farlow, Bot. Gaz., VIII, 1883, p. 318. 
¢Burrill, Bull. Il. Mus., 1, 1876, p. 55. 
