This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Of the many different species of parasites classified under the misapplied name of mildew, it is purposed to speak of only one - the Sphaerotheca pannosa, or rose-blight; that variety found mostly on plants of the rose family. The appearance of mildew to even the most careful observer is something which is extremely mystifying, and all conclusions arrived at pertaining to its solution have been of the most dissatisfactory and perplexing character. As the subject is one of importance, it is worthy the attention of lovers of botany generally.
It is a prevalent belief among many students of fungology and vegetable physiology that a plant can be attacked by mildew only when in a weakened condition. While it is true that a plant is most susceptible to the attack of mildew while in the condition named, it is also true that there are a number of other causes, physical or otherwise, which will affect plants growing under the most favorable circumstances. The question of draught so strongly urged by all gardeners, although to some degree exaggerated, is without doubt one of the most important points under consideration. On this subject the important fact must first be noted that the exercise of every function is restricted to certain definite limits of temperature, within which alone it can take place: all functions are brought into play only when the temperature of the plant, or the particular part of a plant, rises to a certain height above the freezing point of the sap, and cease when a certain maximum of temperature is attained, which can apparently never be permanently higher that 122° F. Hence the life of a plant - the course of its vital processes - appears to be confined in general within the limits of 32° and 122° F. In the growing of plants careful attention should be given that an evenness of temperature be maintained.
Plants that are grown at a temperature of 65° should not under any circumstances be allowed to go 20° or 30° higher, as it could not but have a dilibating or exhausting effect upon the plant; and if. while in this condition, draughts of air should be allowed upon them mildew would without doubt make its appearance. The injury resulting from too high or too low a temperature may under certain circumstances be indirect and slow in its manifestation. This will be the case when a particular function is too highly excited or too much depressed, and thus the harmonious cooperation of the various vital processes is disturbed. Thus growth may be so excited by too high a temperature that assimilation, especially when the light is deficient, is not sufficient to supply the necessary formative material, and the transpiration of the leaves may in addition be so much increased that the activity of the roots is insufficient to replace the loss. On the other hand, too low a ground temperature may so depress the activity of the roots that even small losses by transpiration from the leaves can no longer be replaced.
The syringing and watering of roses while the house is at its highest temperature, has a very deteriorating effect upon the plants, especially when the water is at its natural temperature; the atmosphere of the house when syringed, will, from observation, be found 10° to 12° lower. It, therefore, will be seen (apart from the injury to the roots) that it could not but have the effect of chilling the plants; and repeated waterings under the same conditions would put the plant in a condition favorable to the appearance of mildew. Roses should never be watered so late in the evening that they will remain wet till the morning, and they should never be watered in the heat of the day, for two reasons - the weakening effect upon the plants, and the injury resulting from the condensation of the heat rays of the sun by drops of water resting upon the leaf. The drops under such circumstances being usually globular, the focus of the concentrated rays will be found at the surface of the leaf, and the temperature of that particular spot will consequently be increased beyond the higher limit of growth.
This condition exists for a short time it is true, but probably long enough for the rapidly growing mycelium to effect an entrance to the cellular tissue, and secure a firm and fatal footing there. This cause is more hypothetical than the other cited, but like the other mentioned, might be the subject of some interesting experiments to corroborate it. Over-much dampness through imperfect drainage should also deserve attention. In the preparation of rose beds drainage is fully as important as the soil. The construction of houses should also be carefully attended to; the water from gutters of houses that leak will soon spoil the best system of drainage, and plants in the vicinity of such leaks are always in a mildewed condition. Many writers urge the injury resulting from excessive moisture, and refer fungus growth back to this as the main cause, but it does not occur to them that plants may grow and mature in water containing all the elements of plant food, and if the temperature of the water be maintained fungus growth will not appear. The cause, therefore, in this case is not the presence of water, but the loss of heat, due to the evaporation of water.
This will be observed especially after rain-storms or continued damp weather during that part of the year in which the houses are without artificial heat, for in such cases mildew always makes its appearance. This could, however, be remedied by attention to the causes before stated, namely, by thorough drainage, and by the main-tainance of an evenness of temperature. The condition of the soil also has a strong influence on the condition of the plant. A plant that from the exhausted condition of the soil cannot be kept in a growing condition must from necessity go backward. It will be observed that it is always the weakest plants, and the weakest parts of a plant that are first affected. The cure of the disease mentioned will be found in attention to the physical condition of the plant, which condition will vary in localities. When mildew does make its appearance they should be dusted with sulphur at once. While the beneficial effects of sulphur is greatly over-estimated, it is without doubt of some benefit, but its chief value lies in its preventative rather than its curative properties. S. Reynolds Hole, an English authority upon the rose, recommends the use of soot for mildew; he, however, neglects to mention the kind of soot.
Having tested soot from the anthracite coal to a slight degree, it proved satisfactory, but I have not tested it sufficiently to testify to its continued good results. Its use, however, is objectionable while the plants are in a flowering condition, on account of the disfigurement caused to the buds.
The question: Is mildew a permanent injury to the leaf? is one in which there is some difference of opinion. While a plant may overcome a slight attack of mildew, the injury is without doubt permanent in its character when the leaf curls. This happens from the portion of the leaf on which the mildew is situated being to all purposes dead, the remaining part of the leaf growing causes the leaf to curl. When the plant is in this condition its only help is a new growth, of leaves, and the removal of the causes favorable to the production of mildew. The secret of success in the matter of mildew is the same as that with other varieties of plants, that is, a study of the locality and attention to the physical conditions of the plant. Different localities possess different varieties of temperature, and consequently different modes of treatment The mysteries of plant growing cannot be compassed by books. There is no " royal road " in this respect any more than there is in any other. Success comes only by practical observation.