This section is from the book "The Gardener's Monthly And Horticulturist V21", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Head before the Illinois Horticultural Society.
This is in every way different from the mould or mildew found on the leaves of the same plant and which was the subject of a communication to this society some years ago. Yet different in origin and appearance as the two diseases are, they have been frequently confounded and sometimes by those who could and should have known better. The mildew gives the leaves the appearance of having been dusted with a white mealy powder, which in fact is very nearly the truth. The white material is composed of the threads and conoidial spores of a fungus named long ago by Schweinitz, Erysiphe verbe-nae, but which is most probably Erysiphe communis, (Schlecht), found on very many other plants. The black rust is found on house-grown plants, almost exclusively on the youngest portions of the vegetation - the terminal buds and rudimental leaves. These have a purplish tint and a stunted appearance; growth nearly ceases; there is very little deformity, but simply a dwarfing. It has been considered contagious and beyond remedy, other than the complete destruction of the plants in order to exterminate their enemy. Hundreds of dollars' worth have thus been sacrificed more than once in our country.
Dissap-pointment has intruded upon many a lively anticipation of the brilliant effect to be produced when the verbenas had grown larger and become full of flowers.
Peter Henderson long since asserted the disease to be due to a mite which creeps over the surfaces, and by puncturing and sucking, causes the difficulty. He rudely figures the mite in his book on Practical Floriculture. Since this was published much discussion, pro and con., has been indulged in on the subject. Charles Henderson, son of the former, repeated the mite theory in the Gardener's Monthly last year. Others were skeptical, and to convince them, Henderson, junior, sent specimens of the affected leaves to several persons, including Drs. Farlow and Riley and the editor of the Gardener's Monthly. However, it seems no one found the microscopic mischief-makers. Perhaps they instinctively eluded the intensely scientific eyes turned towards them. Again the two diseases of the plant were confounded and answers returned accordingly. But Henderson is correct, though his figure is not. The black rust of the verbena is assuredly caused by a lowly organized member of the animal world, belonging to the order Acarina, family Acaridae and according to some systematists' classification to the sub-family Tyroghyphidse or cheese-mite group. Its generic (perhaps) and its specific names are yet unheralded, but will be announced some time soon in an appropriate place and manner.
After careful examination I am led to believe that Henderson did not clearly identify the particular species causing the disease in question, but probably saw them as well as others, which may or may not have been injurious. At any rate this is the only explanation I can make of his statements, as the figure alluded to is widely different from a true representation of the real verbena pest. There is no other account of them, as far as my information goes. The following is from personal observations during the year now closing:
The verbena mite is too small to be seen without a magnifier, being one-hundredth of an inch long by one two-hundredth wide across the middle and widest part of the body, it is pale-yellowish white and sluggish in its motions; the legs and feet are unlike any described species and differ among themselves. The first pair are terminated by a single claw and double vesicles, while the second and third pairs have double claws and single vesicles, and the fourth pair have neither claws nor vesicles, but taper into a long bristle. They propagate their kind by eggs laid on the leaves, and most probably pass their whole life in this situation. They infest abundantly, plants growing in the open ground throughout the summer and in such situations work upon older full-sized leaves as well as the tender extremities of the plant; they rarely, however, are met with on the upper sides of such leaves. As long as the plants themselves survive in autumn the mites may be found living and working - this year (1878) as late as the first day of December, at Urbana, I11. Greenhouses and cold-frames are very liable to become stocked with mites upon plants taken from the open ground in autumn.
This is worthy of especial notice and should be thoroughly understood by those to whom the black rust is such a bugbear and the source of so much loss, estimated in dollars, as well as by the mental condition of him who doggedly fights against unknown but certainly existing foes. Besides the purplish color exhibited by the foliage, which is the principal evidence of the presence of the mite, older leaves have a scaly appearance from the death of patches of the epidermis and adjacent tissues; the surfaces become rough and the leaves look old, and are often ragged and upon handling stiff, liable to break. Badly affected plants give no satisfaction in the beds, though they struggle for life; the foliage is unprepossessing and the flowers few and small.
The ordinary "red spider," a true mite, though distantly related to our species, thrives only in warm; dry situations. Out of doors its worst deeds are done during the periods of very hot dry weather, but this does not seem to be the case with the verbena mite. We have seen that it continues its unwelcome services long after sharp frosts have occurred in autumn, even after the ground has been slightly frozen. Most probably it passes the winter under the shelter of any material on the surface of the ground, and is ready to begin again its work as soon as plants start in the spring. So far I have found it on nothing but the verbena species but sometimes as numerous on wild varieties as upon the cultivated plants. The common weedy wild species of verbenas, everywhere abundant enough, could be easily spared, and if the subject of our sketch will take notice of this, its reputation will hereafter be much better, now that it is really known at all, than if it persists in choosing the object which we animals of a higher grade, especially appropriate to ourselves.
For the greenhouses it will doubtless be wisest to practice prevention rather than cure, admitting no plant which from its appearance harbors the scourge; but cure is certainly not impossible. Dipping repeatedly at intervals of a day or two in hot water at 120° Fahrenheit must be useful in ridding them of the mite. A half pint of coal oil in two or three gallons of water, well mixed by rapidly drawing up and discharging with force from the syringe the contents of the vessel into itself and then quickly refilling the syringe with the agitated liquid and freely applying it to the plants, will not injure the vegetation, but will kill all kinds of insects which thus infest plants. As these mites suck the juices rather than bite the leaves, poisons, like Paris green, etc., cannot be relied upon. Thrifty-growing plants can much better withstand the injuries caused by such enemies, - hence are often supposed to be free from the disease when in fact they bear its cause as abundantly as their stunted neighbors.
Having freed ourselves from the cur-rent opinions that this black rust is in the constitution of the plant or comes from improper ventilation or any mismanagement of the houses, aside from that which prevents vigor and robustness of plant growth, we may place more faith in the success of proper treatment of the plants and more hope in the good that is to follow close and full investigation.