Black spot

Black spot apears on a rose plant

Black spot is more common in certain geographic areas, and some rose varieties are more susceptible to it than others. More often, wet weather promotes the development of the disease because black spot spores need water to germinate. The combination of heat and high humidity creates even better conditions, so in humid and warm climates, black spot is very common. Also, morning dew significantly improves the conditions for black spot. In a dry and warm climate, this disease is much less common, but gardeners in such a climate have many other concerns. Some gardeners note that this disease occurs more often on chalky-lime soils and less often on acidic soils.

In recent years, more varieties resistant to black spot have been bred and introduced to the market. Therefore, if the use of chemicals in your garden is a big no-no for some reason, take a look at such varieties. Modern rose breeding has created a vast assortment of diverse roses resistant to this disease. However, not all varieties have such resistance. If your favorite variety is prone to black spot, but you cannot refrain from growing this rose, then you should probably reconsider your approach to roses. For example, only by giving up sprinkler watering can you significantly reduce the risk of this disease appearing in your garden.

To better arm ourselves against this widespread disease, let's learn more about it: black spot is a common rose disease caused by the fungus Diplocarpon rosae Wolf (conidial stage: Marssonina rosae (Lib.) Died.). It causes premature defoliation and leads to the weakening of plants. All varieties of roses are more or less prone to this disease.

Black spot apears on a rose 'Shoking Blue'

Historical records

The formation of spores (conidial stage of the fungus) was first reported by Elias Magnus Fries in 1815 in Sweden. This was followed by reports from Belgium and France, and in the second half of the nineteenth century the disease was also reported from Germany and Great Britain. At the end of the nineteenth century, reports came from Italy, North America and Australia. The disease was registered in China in 1910 and in 1914 in Japan. All these papers report the fungus in the conidial stage. The first report of the terminal stage came from the US mycologist, Frederick Adolph Wolf in 1912. He found apothecia of the fungus on an overwintered leaf in Ithaca (New York, USA). Now the fungus is spread all over the world and is even found on such remote islands as the Philippines and Hawaii.

'Red Intuition' rose is also susceptible to black spot

Symptoms and life cycle of fungus

From mid-summer, black spot appear on the leaves and, in some cases, on the stems. The leaves turn yellow and fall over time. Untreated plants may eventually die.

In the spring, the first infections are caused by spores that have overwintered in fallen leaves or in structures called acervuli on stems. Acervuli are fruiting body structures that the fungus develops in leaves to form conidia, initially subarticularly or subepidermally. Spores stick to the cuticle of the leaf, germinate, and penetrate the cells of the epidermis. The fungus forms strands of subcuticular hyphae and grows deeper into the leaf, with intercellular hyphae sending nutrient-absorbing structures called haustoria into plant cells. Small brown spots appear on the upper surface of the leaves, which turn into irregularly shaped spots with fringed edges.

While the brown spots develop on the leaves, small black acervulae with new spores (conidia) often form under the cuticle in concentric circles but are also irregularly distributed. The diameter of the neck can vary from 50 to 400 mm. Inside each acervulus are masses of two-celled conidia. The conidia are spread by rainwater, insects, or arachnids and cause infection on new rose leaves throughout the summer if humidity is high enough. The leaf tissue around the spots turns yellow, and the leaves drop at the slightest touch. Inside the yellow tissue, metabolic activity is very high. High enzymatic activity can be measured in the yellow tissue, and phenolic compounds and amino acids accumulate. Ethylene production is also associated with yellowing and leaf drop. Ethylene production in black-spotted rose leaves is very high early in the infection process but decreases as the leaves turn yellow.

In autumn, the fungus penetrates deeper into the tissues of the leaves, and new necks develop under the epidermis. Sometimes, single-celled spores (spermatia) are formed together with two-celled conidia. Subepidermal acervula can overwinter even at strong negative temperatures. If the conditions are favorable for the fungus, then the ideal stage of the life cycle will be formed in autumn. Ascopores (which are haploid and require mating (karyogamy) and meiosis) can be found the following spring; they are produced by a fungus on leaves in structures called apothecia. That is why the most effective method of combating black spot, if it has already appeared on the bushes, is to remove the affected leaves.

'Keira', a florists rose by David Austin has tendency for black spot as well

Prevention and control

Because black spot conidia must be in contact with water before they germinate, rose leaves should not be kept wet for several hours. If the roses are watered by sprinkling, then it should be done only in the bright morning. It is necessary to ensure good air circulation, avoiding dense planting, so that the leaves of rose plants dry faster. Of course, the ideal option for watering roses is watering under the root. Such watering does not create the best conditions for the germination and spread of the fungus.

Mulching the soil with humus, dry grass, peat, other recommended materials, laid in a thickness of 2.5 cm, is undoubtedly useful in that it prevents the spores from blowing up from the soil. In addition, this, of course, contributes to the preservation of soil moisture, better growth of the bush, as the mulch will decompose over time and enrich the soil around the plant.

If, after all, the rose got sick and you saw the first signs of the disease on the plant, you need to take more active actions. First, remove all infected leaves and stems and destroy them. Do not compost them, as the spores will remain and with the manure can spread to other plants or return to the roses. Instead, place infected leaves in a plastic bag and throw them away. Of course, the spores will not be destroyed in this way, they will simply move to a new place. The surest way to destroy sports is burning, but this method has its drawbacks, which we will not talk about in this article. Personally, I take soil from the allotment to shelter roses in winter from frosts, digging a fairly deep hole, into which I drop off affected leaves and branches, at a depth of more than 150 cm the spores will be reliably buried and will not spread.

If the plant had to be heavily pruned during the removal of affected leaves and pruning of affected branches, be sure to feed and water the plant well to promote rapid recovery and new growth.

Timely removal of infected leaves prevents the spread of the disease in the garden, especially to other rose bushes, even to those more resistant to this disease. It is especially important to remove the affected leaves in autumn, when the weather is more favorable for the development of the fungus, and in this way you will significantly prevent its spread next year. Black spot is a part of growing roses and cannot be completely prevented.

There are several fungicides that help prevent disease and contain it in the garden if it has already appeared. It should be noted that when using fungicides, you should strictly follow the instructions for their use, because exceeding the dosage norms can cause significant damage not only to the rose plants in the form of leaf burns, but also to you and to the environment.

'Black Beauty’, one of the darkest red roses and the one that might be severely affected

Fungicides

The use of fungicides, like any other chemicals, should always be approached seriously and carefully because, in any case, they carry a certain degree of contamination. However, it is not always possible to do without them. When working with them, you must always wear protective equipment, strictly following the instructions on the package.

After pruning in the spring, a protective fungicide should be applied to kill the newly spread conidia. Treatment should be repeated at intervals of 10-14 days when new leaves appear. If black spot appears on rose plants, it can be difficult to control. Currently, the following substances are used as fungicides against black spot: bitertanol, dichloranid, fenarimol, kresoxymethyl, tebuconazole, sulfur, cyprodinil, mandelamides, and triazoles.

From my experience, good results are achieved using triazole fungicides (Topaz, by Syngenta) and a mixture of penconazole and difencazole (Jack-Pot from Ukravit). At the same time, the first fungicide I use in spring is cyprodinil (the trade brand Horus, and it is perfect for this job). I spray my roses with it immediately after establishing stable positive temperatures. 10-14 days later, I repeat the treatment with Topaz, and after another 10-14 days, I apply Jack-Pot. Usually, this treatment scheme gives good protection against the germination of spores at the beginning of the season when the weather is more favorable for the germination of spores of black spot.

Further on, in addition to all basic agrotechnical measures for roses, I manage to prevent black spot disease on my roses by preventive spraying once every 30-40 days, alternating Topaz and Jack-Pot.

The fungicides that show great results in my garden

Selection of new varieties with high resistance to black spot

Since the mushroom is not a purely biotrophic parasite, it can be grown under artificial conditions. Isolates of the fungus can be differentiated by morphological features or by their interaction with different roses. Breeding for resistance to black spot requires the presence of resistance genes. After evaluating species and hybrid roses in some rose species such as Rosa rugosa Thunb. ex Murray and in R. rugosa Thunb., resistance genes have been found, and breeders are trying to transfer these genes into cultivars by crossing or incorporating resistance genes from other plants into transgenic roses.

However, breeding rose varieties resistant to black spot is a labor-intensive process due to the complex genetic structure of roses. Black spot resistance operates at two levels: mechanical and/or metabolic. A thick cuticle can prevent the penetration of the germ tube of the fungus. On the other hand, the plant may produce substances that inhibit the germination of conidia or stop the growth of the fungus after it has invaded the plant tissue. In breeding for resistance, the first step is to find sources of resistance to black spot. For this, it is necessary to evaluate rose species and hybrid roses. Resistant roses must then be crossed with other roses, and seedlings must be tested as early as possible using one of the grafting techniques, followed by selection for other traits such as flower shape, growth pattern, and scent.

That is why breeding varieties resistant to black spotting is a rather expensive and lengthy process, and at the moment, varieties with full resistance to this disease have not been bred. However, modern selection has managed to significantly improve resistance to black spot in the novelties of recent years.

Unknown rose variety which is very beautiful but heavily affected as well

Here, we have prepared for you a selection of rose varieties that have very good resistance to black spot.