Diseases Affecting Rose Plants
Growers are fortunate in that there are ways of controlling most disease organisms that infect. Good cultivation not only helps to produce handsome roses but is also the gardener’s first line of defence against disease. As with insect pests, much can also be gained by a regular and close examination of plants for early symptoms of disease.
Gardeners generally find it is sensible also to take some positive preventive action against diseases. Coating both the upper and lower sides of the leaves with a fungicide will prevent diseasefrom germinating. When in active growth, roses produce new leaves fairly rapidly, so fortnightly applications may be necessary. Weather conditions also have to be allowed for, and if there has been heavy rain it may be necessary to renew the protective cover. This is where the systemic fungicides now available are particularly useful, as they are absorbed into the plant tissues and provide internal protection.
Dusting may be easier than spraying, but it is generally less effective. Spraying, however, should not be done in hot sunshine; if possible choose a calm, dry evening. You will get better coverage if you use rainwater for mixing or diluting chemicals.
Black spot has been aptly termed ‘clean-air disease’: it rarely attacks roses in industrial areas. Indeed, the Clean Air Act, led to a vast improvement in atmospheric conditions all over urban Britain, and was sometimes regarded with mixed feelings by rosarians contemplating the havoc caused by this disease.
Much has been written about black spot, which shows itself first as small spots with fringed edges. Appearing initially on the lower leaves, sometimes as early as May, it is generally more noticeable from early August through to October, especially in periods of hot weather. In a severe attack, the disease spreads higher up the plant; the leaves turn yellow and drop prematurely, so that the whole plant becomes defoliated. This weakens the bush, and the effect of this may be noticeable the following season. Attacks vary in intensity from year to year and in different areas. The southwestern region of England seems to be particularly favourable to the disease.
Protection before infection is all important, and some growers spray the bushes andsurface with a copper fungicide while the roses are dormant. After , spraying with maneb has proved effective, and many gardeners have used the partially systemic benomyl with success; zineb and captan have also proved effective, as has a new preparation containing bupirimate and triforine.
Powderyis easily recognized by the which resembles a sprinkling of flour on the leaves; when severe it becomes felt-like in appearance. Over-generous use of nitrogenous fertilizers, which produce young, soft growth, is a contributory factor, and dryness at the roots also encourages the spread of the disease. It is often particularly noticeable on climbers close to walls or fences where it is difficult for rain to reach. ‘Dorothy Perkins’ has become notoriously prone to , and in recent years has suffered a decline in popularity as a result.
This disease often overwinters in leaf buds near the top of the growth, and also on ramblers generally, especially if they are somewhat neglected. Rosarians usually shorten back their plants around November to prevent wind-rock, so it is a good idea to remove the mildew-infected tips at the same time. Be sure to burn these tips as soon as possible: it may save you much trouble in the following season. Probably the most effective fungicide is the partially systemic benomyl.
Rose rust seems to flourish in the same conditions as black spot, so it is most frequently seen in the same areas. Bad attacks, however, affect the plants much more seriously. Varieties bred from ‘Fashion’ and ‘Spartan’ (both of which were susceptible but are now seldom planted) seem inclined to infection, and indeed the attractive ‘City of Leeds’ has become suspect in some areas. The disease is most easily seen in summer, when the bright-orangeappear as pustules on the undersides of the leaves. These ripen and become black, the leaves crumple and fall, and the plant is seriously weakened. The spraying programme advocated for black spot should give some control. Some rosarians have succeeded in eradicating this disease by using a preparation called ‘Plantvax’.
This disease generally spreads owing to the careless handling of tools, and in severe cases it can cause the death of stems of old rose bushes. A corky callus surrounds the stem, preventing the upward flow of the sap. As there is no cure, these cankered stems must be cut off cleanly below the lowest of the cankers, and the diseased material burned. In order to prevent canker, all pruning cuts should be made with a well-sharpened tool capable of making a clean cut. Cultivators, hoes, and mowers should be handled with care to avoid damaging rose bushes.
Virus diseases have become so common in some plants that they have become a subject of detailed study by horticulturists. Fortunately, they have not yet become so serious a problem on roses in Britain as they have in the United States and Australia. Symptoms of virus are seen in mature leaves, and certain varieties seem more likely to be affected than others; ‘Masquerade’ and ‘Queen Elizabeth’ seem to be particularly susceptible.
In vein-banding rose mosaic, the pale yellow veins show up in strong contrast to the green leaf. Line-pattern mosaic shows up as pale, wavy lines, sometimes forming an oak-leaf pattern. Some floribundas also show a ‘ringspot’ infection. Also, the strawberry ring-spot virus is prone to attack R. rugosa. There is no means of curing this disease, and where the ringspots appear, the stems must be cut off and burned without delay.