The Management of Black Spot of Rose

The Management of Black Spot of Rose - Diplocarpon rosae Black Spots on Rose Leaves
Diplocarpon rosae Black Spots on Rose Leaves

With the article The Management of Black Spot of Rose, we are going to be able to satisfy an (ours) long-standing request regarding the implementation of our (in vain) promises to complete the tributes we undertake.

And for this reason, we offer the reader the present work, through which (after presenting the epidemiology, symptoms, methods, techniques and selected fungicides of conventional and organic agriculture that concern and eradicate Diplocarpon rosae, i.e. Marssonina rosae – its anamorph type), closes the circle of our comprehensive reference to the Black Spot of Rose.

A mycological disease, which is a problem mainly – but not only – for open field rose cultivation, where under favorable conditions for the fungus it can reach epidemic proportions.

So here – after the first – is the second and last tribute (for the indulgence and jubilation of its editors) to the malicious ascomycete Diplocarpon rosae or Marssonina rosae.

Although Black Spot itself rarely kills Rose plants, it nevertheless greatly weakens them.

Rose Plant with Severe Infection by Marssonina rosae (Diplocarpon rosae)
Rose Plant with Severe Infection by Marssonina rosae (Diplocarpon rosae)
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The Management of Black Spot of Rose - Epidemiology

Infection of the rose bush is carried out almost exclusively by the parasite’s spores of the asexual reproduction, through the conidia of Marssonina rosae.

Its teleomorph type, the Diplocarpon rosae, is not necessary, as it is rarely seen, and then only in a relatively small geographical area – mainly in the northeastern United States and southern parts of Canada.

The Black Spot of Rose first appears mainly in the spring.

Conditions for the Development of the Pathogen

Suitable weather conditions for the occurrence of the disease are the existence of frequent rainfalls together with high humidity, from March until the end of autumn, and temperatures ranging between 15 and 26 °C (59-78.8 °F). Once these conditions are met, the pathogen settles in the host and exerts its parasitic role.

Temperature

For the germination of the conidia of the fungus, temperatures ranging between 15 and 26 °C (59-78.8 °F) are favorable, while the optimum temperature is found at 18 °C (64.4 °F).

In the second case, conidial germination takes place within about 9 hours, and within a period of 36 hours it can reach the impressive rate of 96%.

High temperatures are not beneficial for the conidia, since above 30 °C (86 °F) they germinate but do not develop further, while at 33 °C (91.4 °F) they are killed.

For mycelium, the optimum temperature for its growth is 21 °C (69.8 °F). The mycelium continues to grow for about two months until the temperature reaches 29 °C (84.2°F) when it stops.

The manifestation of the disease on rose leaves occurs within 3 to 16 days. For this to occur, the prevailing temperatures must range between 22 and 30 °C (71.6-86 °F), while the temperature optimum is 24 °C (75.2 °F).

Moisture & Humidity

For the appearance of Black Spot of Rose, moisture, humidity and their duration are essential factors.

Thus, without their full wetting, the conidia of Marssonina rosae are impossible to germinate, even when the relative humidity of the air reaches 100%.

The wetting of the conidia must last at least 7 hours, and if this happens, 8 hours later and with a relative humidity below 90%, the process of infection of the rose begins.

According to the above, it is not at all surprising that the disease does not manifest itself if the surface of the leaves is completely dry for 7 hours, even after the germination of the conidia.

Pathogen Growth

Upon perforation of the cuticle, due to mechanical pressure from the germ tube of the conidia, the latter settle on the host, initiating primary leaf infections.

The growth of the mycelium takes place in the mesophyll – mainly in the upper part -, and is manifested by the appearance of spots on the surface of the leaf.

A dense intercellular network of mycelial hyphae is created, and through the haustoria that invade the epidermal cells as well as those of the palisade, the necessary nutrients are pumped.

At the same time, it has been shown that the mycelial cells produce both the gas ethylene, which causes defoliation, and abscisic acid, whose presence results in the premature defoliation of the rose.

From the beginning of the development of the mycelium and within a period of about 14 days, the acervuli bearing the conidiophores, which produce the two-celled hyaline conidia, appear on the formed spots.

The incessant production of conidia throughout the growing season (with favorable conditions), causes – through them – secondary infections.

In the rare cases where the infection is due to the products of sexual reproduction, there is a rupture of the apothecia shield and a forcibly discharge into the air of the two-celled ascospores. Ascospores settle mainly on the lower leaves of the rose.

The Management of Black Spot of Rose

The Management of Black Spot of Rose - Infestation on Canes of Rose Plant by Anamorph Marssonina rosae
Infestation on Canes of Rose Plant by Anamorph Marssonina rosae

The Management of Black Spot of Rose - Symptoms

The Black Spot of Rose occurs only on the overground vegetation, but not with the same intensity. Thus, the parasite mainly infects the leaves, and to a lesser extent the shoots, the flowers, and sometimes even the fruits.

Leaves

The Black Spot of Rose manifests itself mainly on the adaxial surface of the lamina and less often on the abaxial.

Initially, pustular structures, between 2 and 15 mm in diameter, appear as dark brownish-black spots -surrounded by a chlorotic yellow halo-, mostly numbering from 1 to 20 per leaf.

It takes several weeks for the spots to reach their final size, while their color is not due to the fungus itself, but to the disorganization and death of the cells of the affected leaflet.

The spots have an almost circular shape that progressively – as they join with their neighbors – changes to irregular or like that of aggregated snowflakes.

The margins of all spots are feathery, radiate, and fimbriate, reflecting the growth pattern of under the cuticle mycelium hyphae. The fimbriate appearance of the margin is an important diagnostic character for the identification of the parasite.

As a result of the infestation, the leaflets are abscise and fall, in combination with both the reduced presence of auxin and the contribution of ethylene and abscisic acid.

Depending on the degree of sensitivity of the varieties or cultivars to the pathogen, defoliation can be total or partial. Rose plants that experience total defoliation happen to emerge new leaves, which also lose them, and it is not unusual to attempt the same a third time – by late summer.

This process weakens the plants to such an extent that in the coming winter the completely exhausted bushes may die. However, even those that survive show dead stems as well as a reduction in the size and number of roses.

The difference in sensitivity to the parasite does not only concern varieties but also the various organs of the plant, such as the leaves. It has been observed that M. rosae preferentially infects the youngest of them, aged up to about 14 days. On the contrary, the older leaves, even if infected, show only smal number of spots.

Twigs, Canes, Petioles, Flowers & Fruits

Mainly the tops of the twigs and the immature wood of first year canes are affected by the disease. Its presence is noticed by the appearance of spots of irregular shape without fimbriate margins. The color of the spots is initially purplish-red to change progressively to black, while pustular structures containing the acervuli develop on them.

Inconspicuous spots, similar to those of the leaves, appear on the stipules, penducles, sepals and fruits.

The same happens with the petioles, but in addition, an abscission zone is created, but without their abscising.

On the petals, the disease manifests itself as red or black spots along with slight distortion.

The Management of Black Spot of Rose

Domestic Control of Black Spot of Rose

The management of Black Spot of Rose can be carried out in various ways and means, both preventive and suppressive. But while for all forms of commercial plant cultivation, the use of special fungicidal formulations is necessary, for house roses the treatment of the disease, with relatively satisfactory results, can be obtained by using simple home materials in various combinations. Some of them – which are presented in the imaginative form of recipes – are discussed immediately below.

1st Recipe

Mix 2 parts distilled water with 1 part whole milk. The plants are sprayed once a week, and subsequent cleaning of the leaves with water spray is carried out 24 hours after each application.

With this recipe, in addition to black spot, rose powdery mildew is also controlled.

2nd Recipe

Dissolve baking soda in an amount of 1 tablespoon as well as ½ teaspoon of grated green soap in 2 liters of water. This recipe is applied preventively by spraying the roses twice a month.

3rd Recipe

Cut into very small pieces 3 cloves of garlic. They are allowed to soak in 1 liter of water for 2 days, and then, after the water is strained, 1 to 2 tablespoons of baking soda and 1 teaspoon of liquid dish soap are added. The preparation is mixed until all the ingredients are well dissolved and then sprayed, at a rate of 1 time every 7 to 10 days.

With this recipe, which is applied preventively and therapeutically, the powdery mildew of the rose is also controlled

The Management of Black Spot of Rose

Leaflets Displaying Response to Races 3 and 8 of Diplocarpon rosae for 3 Roses of Knock Out® Cultivars
Leaflets Displaying Response to Races 3 and 8 of Diplocarpon rosae for 3 Roses of Knock Out® Cultivars

The Management of Black Spot of Rose - Preventive Treatment

The preventive treatment mainly refers to the identification of the suitable planting areas, the proper infrastructure of the crop, the correct ways of plants establishment as well as the selection of those varieties of Rose which show resistance or complete immunity to the parasitic fungus.

Areas, Conditions & Ways of Plants Establishment

For the correct establishment of a rose plantation in order to avoid plant infestation by the ascomycete Diplocarpon rosae (Marssonina rosae), both in the field and in the greenhouse, the commercial grower must take into account, among other things, that:

  • Areas with warm summers and relatively cold winters are selected, which do not promote infestations by the parasite and/or limit the possible development of epidemics, even if frequent rainfall is observed there
  • In coastal areas, moist and cool winds favor the occurrence of the disease, and therefore such areas should be excluded
  • The installation of an over head irrigation system with sprinklers should be avoided, because this method of watering is beneficial not only for Diplocarpon rosae, but also for other plant pests
  • Dense plantings are not recommended, because they contribute to poor air circulation aroud plants, increase humidity and make it difficult to control the rose black spot disease – if it occurs

Resistant Cultivars & Varieties

Although Rose Black Spot is extremely widespread in its plants, there are nevertheless several types of which include cultivars and varieties with satisfactory and/or high resistance to the disease. These include the following:

Buck Roses Type

Amiga Mia, Applejack, Carefree Beauty, Country Dancer, Goldenwings, Prairie Wren, Prairie Princess

Canadian Explorer® Type

Alexander Mackenzie, Captain Samuel Holland, Charles Albanel, David Thompson, Henry Charles Albanel, David Thompson, Henry Hudson, John Cabot, John Davis, Lambert Close, Will Aldermann, William Baffin

Easy Elegance Garden Path® Type

Golden Eye, Sunrise Sunset, Great Wall, My Hero, Tahitian Moon, Paint the Town

Explorer Type

Champlain, Charles Albanel, , Henry Kelsey, John Davis, Simon Fraser, William Baffin

Griffith Buck Type

Carefree Beauty, Carefree Delight, Carefree Sunshine, Carefree Wonder, Prairie Sunrise, Prairie Harvest, Distant Drums

Knock-Out® Type

Knock-out, Double Knockout, Pink Knock Out, Pink Double Knock Out, Rainbow Knock Out, Blushing Knock Out, Sunny Knock Out

Parkland Type

Morden Sunrise, Morden Snowbeauty, Prairie Joy

Pavement Type

Pink Pavement, Purple Pavement, Snowy Pavement

Rugosas Type

Belle Pointevine, Blanc Double de Coubert, Dart’s Dash, Frau Dagmar Hartupp, Magnifica, Pavement Series, Pink Grootendorst, Therese Bugnet

Various Varieties

Bonica, Flower Carpet, Haidee, Home Run, Konigin von Danemark, Lavender Lassie, Sir Thomas Lipton, The Fairy, Two Sisters

Note

Related research, of great interest to breeders, showed that the species Rosa carolina, various lines of the R. multiflora-hybrid, as well as the variety Rosa ‘President de Seze’ are completely immune to Rose Black Spot.

The Management of Black Spot of Rose - Cultural Practices

For the cultural treatment of Rose Black Spot, it is recommended:

  • Aggressive spring pruning, much more severe than normal, which reduces the amount of inoculumon infected rose stems
  • With the appearance of symptoms, remove and burn in spring all affected leaves, shoots and canes if possible, but in dry weather and dry plants, so that there is no further spread of the disease
  • Avoide to racking up fallen and infected leaves, as well as pruned infected parts of the rose bush in autumn – although it is still recommended – because it has been shown to be completely ineffective in stopping the spread of the parasite, while instead it can be applied mulch in the garden or the field over the fallen plant parts, which actually works as a deterrent
  • If -incorrectly- watering is carried out with a sprinkler irrigation system, then the watering must be done in the early hours of the morning, so that the entire canopy of the bushes can then dry in time
  • Avoid overfertilising

The Management of Black Spot of Rose

Biological Control of Black Spot of Rose

Biological control of the Diplocarpon rosae fungus can be carried out with biofungicide products containing strains of the bacterium Bacillus subtilis.

It is a fact that the control of the disease achieved by the use of formulations containing Bacillus subtilis is not particularly satisfactory, at least for commercial rose growers.

However, they do not cease to be a worthy alternative choice of biofungicide products for amateur growers and plant lovers against Rose Black Spot.

In addition to Bacillus subtilis, encouraging results have been obtained from the possibility of utilizing various bacterial strains of the species Brevibacillus sp. and Lysinibacillus fusiformis, which have been isolated from the rhizosphere of the rose cultivar cv. Edward.

But for now they are in the research stage.

The Management of Black Spot of Rose

The Management of Black Spot of Rose - Chemical Control

The Black Spot of Rose is effectively treated with preventive or therapeutic sprays.

Treatment of the parasite can begin in winter – although the effectiveness of a dormant spray against Diplocarpon rosae in particular has little value – and continue in spring, before the leaves develop, during the bud brake phase, using the appropriate fungicides.

Sprays are carried out once every fifteen days, and from the appearance of the leaves, then once a week.

If fungicidal preparations are used in the form of wettable powder together with surfactants, then their application can be done at the rate of one spray every 14 days.

Listed immediately below are the active substances that can be used to control Diplocarpon rosae (Marssonina rosae)

Those active substances with the indication [E] are authorized for use in Greece (and most European Union countries) at the time of writing this article.

Active Substances

Conventional Farming
  • Azoxystrobin [Ε]
  • Azoxystrobin + Benzovindiflupyr [Ε]
  • Captan [Ε]
  • Chlorothalonil
  • Copper [Ε]
  • Cyprodinil + Fludioxonil [Ε]
  • Fenarimol
  • Fluoxastrobin [Ε]
  • Fluxapyroxad + Pyraclostrobin [Ε]
  • D-Limonene (Orange Oil) [Ε]
  • Mancozeb
  • Mefentrifluconazole [Ε]
  • Metconazole
  • Myclobutanil [Until Stocks are Exhausted]
  • Propiconazole
  • Propiconazole + Chlorothalonil
  • Pyraclostrobin + Boscalid [Ε]
  • Sulfur [Ε]
  • Tebuconazole [Ε]
  • Thiophanate-Methyl
  • Thiophanate-Methyl + Chlorothalonil
  • Thiophanate-Methyl + Mancozeb
  • Trifloxystrobin [Ε]
  • Trifloxystrobin + Fluopyram [Ε]
  • Triticonazole [Ε]
Organic Farming
  • Bordeaux Mixture [Ε]
  • Copper Hydroxide [Ε]
  • Copper Oxide [Ε]
  • Copper Oxychloride [Ε]
  • Sulfur [Ε]

Absolutely Necessary Reminder

The reference to the active substances that can be used against Diplocarpon rosae is for purely informative reasons. Under no circumstances is their use implied or encouraged without the approval of the local agronomist.

The Management of Black Spot of Rose

Ιnfection Caused by Diplocarpon rosae on a Rose Leaflet
Ιnfection Caused by Diplocarpon rosae on a Rose Leaflet

References

The References of the – sometimes fiercely and sometimes not – spoted article entitled The Management of Black Spot of Rose are presented by the Royal Concertgebouw Orchestra, under the musical direction of conductor Riccardo Chailly, with the piece Waltz II from the work Jazz Suite No. 2 by the great Russian composer Dmitri Shostakovich.

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