The Rose Manual (1933)
J H Nicolas
We are just recovering from a panic. In the spring of 1926 an unusual amount of "die back" was noticed — a phenomenon long observed but passed on as a peculiarity of certain varieties. The alarm was sounded, and by 1927 the panic was on. We now hear less of it. There are several cankers well defined by the professors, and the reader interested in the scientific definition of the fungi and details of the various rose cankers is referred to the masterly article of Miss Anne Jenkins of the United States Department of Agriculture, pages 161 to 182 of the 1927 Annual of the American Rose Society.
What concerns us most at present is the practical side of the canker question. Regardless of the number of parasites identified, they all amount to the same thing: a fungus gets under the bark in some way, eventually girdling the branch and causing the upper part to die. Brown canker, the most common, attacks the soft growth in the fall, which growth starts to "die back," the fungus sliding down until it reaches hardwood or the affected part is removed, and if the spores are carried over winter they will begin again their devastating career in the spring as soon as the weather gets milder. Very little damage is done during summer, probably because the fast-growing tissues heal the lesions faster than the spread of the disease, closing on the spores so to speak.
The first symptoms of brown canker are small spots or tumefactions, purplish at the center, running to pink at the edge. Soon a white dot appears — this is the "bloom" of the fungus, discharging its spores that scatter and carry the infection to other parts of plants. It is said that when the purple spot appears the canker is beyond reach and cure — the branch had better be cut and burned. If it appears on large canes, as it sometimes does, it may, if taken early, be carved out, and the wound rubbed or painted with straight Fungtrogen or any copper paste or Semesan paste. Brown canker is more prevalent in wet years; and, although not related to the presence of black spot during the summer months, it is in direct coincidence with it. Canker is the aftermath of an unchecked attack of black spot because the premature defoliation has stopped the hardening process of the wood and lowered the vitality and power of resistance of the plant — a condition very favorable to the spread and virulence of the disease. The most effective means of prevention or arrest is to keep the plants in full foliage. Fight black spot and you prevent canker. It is a proved fact that varieties having retained their foliage until frost seldom show canker lesions, although tips of late growth induced by a too late fertilization and checked by early frost or cold nights when full of sap may become a prey, but this is not serious as the winter would have dispatched these soft branches, anyway. Varieties like Pernetianas that naturally defoliate early will succumb quicker than the others.
Past experience seems to show that copper (Bordeaux or Fungtrogen) is more effective than sulphur in combating canker spores from getting a start when alighting on the bark.
To summarize, the fight to keep the foliage healthy is the best prevention of all diseases, and we repeat most emphatically: Save the Foliage and You Save All.
Summer defoliation is not always due to disease, although diseases will precipitate it. Summer defoliation did not bother our fathers before the introduction of Austrian Briar blood. Austrian Briars will lose their foliage early because it matures early. It is the nature of the beast that nothing can change, and all their hybrids inherit that character in a greater or lesser degree. When the foliage is mature, the "cortex" or film of corky material that heals the pores where the leaf is attached to the branch begins to form, the leaf receives less and less nutrition from the plant and drops at the least provocation, spray or dust notwithstanding. Some varieties when plants are healthy will push on new leaves at the top as fast as the bottom ones drop and the plant retains its "activity," but if the defoliation is premature and accelerated by disease faster than new foliage can be produced the plant goes dormant. Pernetianas are at best in regions of long vegetative season, like southern France, the birthplace of the race, divided in two distinct growing periods — an early spring and a long springlike autumn intervened by a hot summer, because there is enough growing weather when they awake from their summer rest enforced by natural defoliation to come again to a useful life, and the new growth has time to mature before winter, if winter there is. Pernetianas are out of place as autumn bloomers where the season is short between a late spring and an early winter.
Black Spot starts from the inside, but is induced and spread by outside atmospheric conditions. As proof of this Cochet offers the argument that black spot always attacks the older leaves as they mature. If it did start from outside it would attack the soft new foliage just as mildew does, but the moisture inside young foliage is in rapid motion to and from the roots and has no time to mold even when inoculated until that motion slows down in the process of maturity. Black spot is intimately related to the innate persistence of foliage, as above stated; long-lived foliages, such as pure Teas and species evergreen in their native habitats are immune. As Lambert said, "Have you ever seen black spot on bracteata, sempervirens, Cherokee?" When Pernetiana foliage has reached near maturity and the weather is humid, the moisture inside the leaves is not evaporated fast enough nor reabsorbed by the plant, and a fungus or mold forms just as clothes fresh from the aseptic laundry process will mold if put away moist.
Although black spot starts inside, the fungus fruits outside through punctures, and infection spreads rapidly to foliage already in process of maturing, there finding a fertile ground even if weather conditions were then normal for proper evaporation of excess moisture in the leaf tissues. This foliage has to be kept insulated by some medium. They do not like dust over there [in Europe] but prefer copper sprays. Another point brought to me to support that theory is that true Hybrid Teas do not take the spot until very late (if they do at all), when part of their foliage has naturally matured, which process is much later than Austrian Briar hybrids. Where summer weather is normally sultry, stick to the old line Hybrid Tea.
Cankers. Caused by fungi of various identities (see page 186). Like black spot, the disease starts from the inside. There is much analogy between the mechanical line of march of both diseases, too much moisture in the tissues. Canker is especially active in the autumn when late growth has been stimulated and, full of sap, is checked by frost or cold nights. In the spring it is brought about by late frost after the tissues have become full of sap. The sap is "killed," the inactive moisture becomes "stale," and fungi develop from the inside, but bloom outside and infest other plants or soft parts thereof. Where black spot is prevalent there is bound to be canker, and the control measure for one gets the other. In Lambert's place I noticed some wichuraiana climbers infested with a brown canker (not "brown" canker as defined by Miss Jenkins). I called his attention and he said that they have it in years when a late frost came after vegetation had started.