DISEASES OF PLANTS. 343 
The second kind is a whitish slime, which some 
species of aphis leave upon the leaves. 
As soon as there is the least appearance of mil- 
dew, all the leaves stained with it should be plucked 
off and burned. In scarce and delicate plants, the 
leaves ought to be washed. But where it is pro- 
duced by aphides a weak decoction of the dry leaves 
of tobacco will be found most serviceable. 
Jf, on the contrary, all parts of a plant are full of 
it, and the plant is hard and of long duration, then 
the parts must, according to what plant it is, be ta- 
ken off. If it is an annual, and of great delicacy, 
it will be best to wash it, with a brush dipped in 
the decoction of tobacco, and afterwards to expose 
it to the open air. 
§ 311. 
Melligo, or honey-dew, is a sweet and clear juice, 
which during hot weather is frequently found upon 
the leaves, rendering them sticky, and, especially 
when it docs not rain, causing them to fall off. 
This sweet matter is likewise secreted by aphides, 
from peculiar glands at the anus. 
In tender plants washing with water, or with the 
above decoction is of great benefit ; the fumes of to- 
bacco likewise kili the insects. 
§ 312. 
Rubigo, or a ved matter of the appearance of rust, 
is seen frequently on the leaves and stems of many 
plants.. It consists of yellow or brown stains, which 
when touched, give out a powder of the same cMfour, 
Y 4 which 
