In the March number, "J. McB.," of Boston, Mass., inquires about a disease affecting his Violets.

I have been quite familiar with this disease for four or five years past, and believe it exists to some extent in nearly every greenhouse in the country, generally in such a mild form as not to be noticeable.

Since discovering it, I have always carefully examined all plants received, and in most cases, have found it prevalent to some extent. Have received plants affected by it from Boston, New York, Philadelphia and Baltimore, besides other places. I have frequently thought it very singular it had not been noticed by gardeners before. A person not familiar with it, would not notice it, except in bad cases.

The violet is the only plant, I have observed, it killed outright; but it will attack many kinds of greenhouse and bedding plants, such as Verbenas, Petunias, Torrea Asiatica, Tea rose, etc, but these plants are not as seriously injured by it as the violet.

This disease is owing to the attacks of an exceedingly small insect, with the habits of the Phylloxera, but not more than one third the size. Although I was confident from the first it was caused by an insect, yet it was a long time before I was able to find one, for whenever I examined the roots, there were none to be seen. After repeated examinations, I found one running across the field of the microscope, and with considerable difficulty kept it in view long enough to see that it resembled the Phylloxera, much smaller, with longer legs and slimmer body, and much more rapid in its movements. I afterwards found them feeding on the roots, but they will run away as soon as disturbed.

It appears to be difficult to destroy them by applications to the soil, and I find the best way to get rid of them is in being very careful in selecting soil for potting, and to grow all stock from seed or slips, and not from division of the roots, and to keep the stock plants in a different house from the slips or young stock; also, to destroy all stock plants affected by them as soon as they can be spared.

I find it is not safe to use soil from the vicinity of the greenhouse, where I have found them attacking the tomato, several kinds of weeds, and even the tobacco plant; although I think any rich soil taken from near the greenhouse, is as good as soil taken from the field if properly treated beforehand. Heaping it in the spring and covering with a sufficiently heavy coat of mulch, to entirely prevent a growth of weeds through the summer, will effectually destroy them by fall.

Killing all plants by hoeing or storing the soil dry under a shed during the summer and autumn will also answer the purpose.

I think, from my experience, the good qualities of fresh soil from a pasture is very much owing to the absence of these insects. Where the soil is rich and the plants can get an abundance of nutriment, the effects of these insects are seldom noticed on the growth of the plants; consequently it is best to supply all affected plants with plenty of nourishment. By this means most plants, except violets, can be grown successfully, even where they are very numerous.

If they are very abundant on the violet, they will, in the autumn, eat the bark from the main roots or body of the plant, and cause it to rot off below the surface of the ground.

This trait is similar to one of the Phylloxera, the effects of which on the different varieties of grape vinis I have been studying for several years past - probably being one of the first to notice them in this country - having discovered them with a microscope, on some diseased grape-vine roots, in the autumn of 1869. My observations on this subject I propose to send to the Gardener's Monthly at some future time.

[In addition to the above, we have the following from Mr. R. Linsley, West Meriden, Conn.]

"I notice the enquiry of J. McB., of Boston, on page 77, March number, and send you a sample of a plant lifted last fall, the old roots 'clubbed.' as we call it - the young roots grown quite recently showing the same thing, - the ground occupied is a fine loam, well underdrain-ed. Some of the plants died in the garden, others after we lifted, and others in the same plot showed no disease, but have not flowered as well as usual. The disease affects them much as ' club ' does cabbage, but the process is not the tame at all, and is much longer in destroying the plant. I have forwarded a sample to Prof. C. V. Riley, St. Louis, Mo. There is at the same time a fungus on the leaf."