Mildew seems particularly to affect rose plants purchased in full growth in the spring, which not infrequently show the shock of change of condition and temperature in its development.
Control—It has long been known that any form of sulphur used as a fungicide will control the powdery mildew of the rose. The best form here recommended is that which is mentioned in the combined mixture described later for the treatment of black-spot, to which the inquiring amateur is referred.
Black-spot is the most definite obstacle to rose success in the United States. It occurs everywhere that roses are found, and frequently weakens and eventually destroys the plants because they are defoliated repeatedly through its adverse action. It shows in black spots on leaves and petioles, by the leaves yellowing and easily detaching from the plant.
The spots are more or less circular, and sometimes are a half-inch or more in diameter, with irregularly fringed margins. They show only on the upper surface of the leaves, and the disease tends to be more serious during late summer and autumn, in the presence of cool nights and heavy dews.
It should here be noted and remembered that black-spot lives through its mycelium within the structure of the leaf, and a leaf once attacked by it is of no further value to the plant, needing to be removed and burned, so that the fungus, which lives over the winter in dead leaves, on the ground, or clinging to the plant, may be destroyed.
Black-spot can be controlled, as will hereafter be noted, but it should also be noted that it cannot be cured as can mildew. The affected leaf can never again function as a leaf, though it may readily carry over the disease to do further damage another year.
Again it is repeated that fallen yellowed leaves and stems need to be constantly gathered and burned, and that the ground in which the rose plants grow be kept clean and clear.
Control—Much experimentation has resulted in providing a treatment, simply and easily handled, which does usually control black-spot by preventing it in most cases. The wise amateur will vigilantly watch his plants and immediately pull off and burn a yellowed leaf, or pick up a dropped leaf, from the very beginning of the season. He will also try the hard thing of properly dosing his plants with the protective application to be described, just when it seems most outrageous to interfere with the beauty of the rich young foliage.
Bordeaux mixture, commercially obtainable ready to use, is effective for controlling black-spot. Ammoniacal copper carbonate also is equally as effective. Both these substances, however, disagreeably mark the foliage, wherefore the application worked out by Dr. L. M. Massey, of Cornell University, at the instance of the American Rose Society, and first published in its Annual for 1918, is here recommended.
A simple powder-gun, costing a few dollars, can readily be obtained of the same source from which the proper chemicals may be had. It should be noted that certain roses are much more susceptible to black-spot than others. The Pernetiana class of Hybrid Teas shows this peculiarity. It is wise, therefore, to plant roses of the Pernetiana type together, so that they may be the more effectively controlled.
It should be noted further that with the admixture of one part of powdered tobacco the dust above described will take care of all the insects that bother the rose, save only the hated rose bug, or June bug.
It is proper here to state that the foregoing account of the insects and diseases that beset roses, with suggestions for their control, are adapted from the exhaustive study of the subject printed in the American Rose Annual for 1922, as made by Dr. L. M. Massey, the plant pathologist of Cornell University, at the request of the American Rose Society. The interested amateur is commended to a relationship with this Society, which brings constant access to advice concerning all rose troubles, with free consultation of authorities who keep posted on rose troubles as well as rose advances.