This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
In Professor Burrill's essay which you published in the December No. on this subject, he •says that the mould (mildew) and the black rust that appears on Verbenas have been often confounded. He must have been a very green hand indeed who would do so, for they are quite as distinct from each other as the mealy bug is from the green fly, and no observing boy of sixteen with a year's experience but would know the difference. The Professor says he is led to believe that I did not clearly identify the species of insect that causes the disease: perhaps my rough drawing of it in Practical Floriculture led him to think so, for I believe my genius as a draughtsman has yet to be developed. But I believe I first discovered and first published the fact of its being an insect, and my investigations which have covered a period of over a dozen years leads me to the belief, Professor Burrill to the contrary notwithstanding, that there is only one species of mite causing the disease on the Verbena; nor does that appearing on the Heliotrope, Petunia, Fuchsia, Pentstemon and a score of other families of plants show it to be different.
But how it looks, or what it is, or whether it belongs to the " order Acorenia," or any other order of microscopical insects is of very little consequence to the man making his bread and butter by the sale of plants; what he wants to know is a preventative, or if the trouble is present, a remedy. I am satisfied that the insect causing black rust rarely if ever attacks a plant in luxuriant health, and that it is probably a consequence rather than a primary cause of the disease, for we have found by actual experiment repeated so as to leave no doubt in the matter. If we take for example, 100 plants each of Verbenas, Petunias, or Heliotropes which have been' first potted in the usual 2-inch pot, and we take 50 of each and shift them into 3-inch pots, so as to move on their growth unchecked, that these will be entirely exempt from the insect, while those allowed to starve in the 2-inch pots will be less or more affected - this is for prevention; now the remedy. We have tried every nostrum supposed to be inimical to insect life for the black rust, and have never succeeded in checking it, except by stimulating the plant with liquid manure: this is undoubtedly a remedy if the plant is not too far injured by the disease.
We grow some hundred or more varieties of Verbenas, and this Fall we found the black rust affecting one variety only, which happened to be at the end of the house and was probably at some time allowed to be starved by •drying. We tried on it the Cole's " insect exterminator," which is death to every visible insect that attacks greenhouse plants, with no apparent effect. We then stimulated the plants with liquid manure, and in 20 days every trace of the living insect was gone, and the plants now show no indication of ever having been effected.