Diplocarpon rosae (Black Spot of Rose) Fungus

Diplocarpon rosae (Black Spot of Rose) Symptoms on Leaves
Diplocarpon rosae (Black Spot of Rose) Symptoms on Leaves

The Diplocarpon rosae (Black Spot of Rose) is one of the worst fungal diseases of the plant and crop. It is planetary and often takes on epidemiological proportions, mainly affecting field cultivation of roses.

Although the fungus itself does not kill the plants, however under favorable conditions it can cause total defoliation, greatly weakening them and degrading the quality of the growing product.

But in order to understand the magnitude of the problem, and because misfortunes never come singly, the infested rose plants, in addition to the damage caused by the fungus, become an additional privileged target of other pests and diseases.

Taking into account all the above, for this reason the editorial team of "Kalliergeia" decided to make a privileged target of a tribute divided into two parts, this ascomycete.

In this first part, an attempt will be made to describe D. Rosae‘s detestable but – admittedly – at the same time heroic endeavor for life, and in the second part, an attempt will be made to describe the prosaic but – admittedly – at the same time epic endeavor by the rose grower to rid his plants of another huge nuisance of crop.

The asexual reproduction of the ascomycete Diplocarpon rosae takes place with the conidia.

Black Spot (D. rosae) Asexual Spores
Black Spot (D. rosae) Asexual Spores - © Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org
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Diplocarpon rosae (Black Spot of Rose) - Classification

The first scientific announcement about the Black Spot of Rose was made by the Swedish botanist and mycologist Elias Magnus Fries (1794 – 1878 CE) in 1815, and concerned the imperfect stage of the fungus. Fries named it Erysiphe radiosum, but it was the Belgian phytopathologist Marie-Anne Libert (1782 – 1865 CE) who in 1827 gave its correct description, under the name Actinonema rosae. It took almost a century  to arrive the American botanist-phytopathologist Frederick Adolph Wolf (1885 – 1975 CE), and in 1912 to identify the teleomorph state of the fungus, Diplocarpon rosae – which is its current name.

Classification

  • Domain: Eukaryota
  • Kingdom: Fungi
  • Subkingdom: Dikarya
  • Phylum: Ascomycota
  • Subphylum: Pezizomycotina
  • Class: Leotiomycetes
  • Subclass: Leotiomycetidae
  • Order: Helotiales
  • Family: Dermateaceae
  • Genus: Diplocarpon
  • Species: D. rosae

Diplocarpon rosae F.A.Wolf (1912)

Teleomorph: Diplocarpon rosae F.A.Wolf (1912)

Anamorph: Marssonina rosae Lind.

Synonyms

  • Actinonema rosae (Lib.) Fr., 1849
  • Asteroma rosae Lib., 1827
  • Dicoccum rosae Bonord., 1853
  • Dothidea rosae Schwein., 1832
  • Fabraea rosae (F.A. Wolf) Seaver, 1951
  • Marssonia rosae (Bonord.) Briosi & Cavara, 1889
  • Marssonia rosae Trail, 1889
  • Marssonina rosae (Lib.) Died., 1915
  • Phyllachora rosae (Schwein.) Sacc., 1883

Common Names

  • Greek: Μελανή Κηλίδωση της Τριανταφυλλιάς, Μαύρη Κηλίδωση της Τριανταφυλλιάς, Μαύρη Βούλα της Τριανταφυλλιάς
  • English: Black Spot of Roses, Rose Black Spot, Leaf Blotch of Roses
  • Spanish: Marsonina del Rosal
  • French: Marssonina du Rosier, Taches Noires du RosierMaladie des Taches Noires du Rosier
  • German: Sternrußtau
  • Lithuanian: Rožių Rudoji Dėmėtligė
  • Arabic :ننفساء خرامة

Diplocarpon rosae (Black Spot of Rose)

Black Spot of Rose (Diplocarpon rosae F.A. Wolf) Sexual Spores-Apothecium, Asci, Ascospores
Black Spot of Rose (Diplocarpon rosae F.A. Wolf) Sexual Spores-Apothecium, Asci, Ascospores - © Fred Brooks, University of Hawaii at Manoa, Bugwood.org

Diplocarpon rosae (Black Spot of Rose) - Biology & Ecology

The fungus Diplocarpon rosae is another cosmopolitan species, which reproduces mainly asexual and is therefore basically found in the imperfect stage of its life, in which it is recognized as Marssonina rosae.

It is a parasite monoxenous, obligatory and hemibiotrophic, which is additionally characterized as polycyclic, due to the successive secondary infections that it causes, through its asexual reproduction.

The latter takes place with the spores produced by monogony, the conidia, which need a drop of water to germinate and penetrate the host’s epidermis.

However, for all this, but also a little more for this ascomycete, reference is made immediately below.

Biological Cycle

With the onset of spring, conidia that have overwintered in fallen leaves or in infected plant parts of the rose, take advantage of the relatively low temperatures and the first rains to produce germ tubes (or sometimes to initially produce an appressorium). For the germination of conidia it is necessary to completely covered by water drops for several hours.

The emerging conidia are established and / or grown in the host tissues, mainly in the intercellular spaces between the parenchyma and the upper epidermis of the leaves -rarely in the corresponding lower part- creating a dense network of mycelial hyphae.

In the mycelium appear the fruiting bodies of the asexual reproduction of the parasite, the acervuli, which as long as the conditions remain favourable produce uninterrupted new conidia, which further spread in the end of summer.

Towards the end of autumn, in the fallen leaves or in parts of the rose, new acervuli are formed from the mycelium, from where the conidia will grow and germinate next spring.

Under ideal conditions, and in extremely rare cases, it is possible in the spring period to appear the products of the sexual fruiting bodies of D. rosae, the ascospores, which have formed in the apothecia, and through the ascospores, that are discharched by force and are airborne, to reproduce the species in its teleomorph form. The apothecia appear in autumn, on the fallen leaves of the rose.

Sexual Reproduction

The sexual reproduction of the species takes place with the ascospores, which are formed in special bowl-shaped structures, the asci. The asci are contained in the ascomata apothecia, which are compound fruiting body. The ascigerous stage is observed in spring on fallen leaves.

Asexual Reproduction

The D. rosae fungus reproduces asexual by conidia. The conidia, which form inside the acervuli (compound fruiting bodies of shallow cup-shaped form) penetrate directly through the cuticle into the host, and settle on the leaves or young shoots. There they develop the mycelium, where by producing specialized intracellular hyphae, the haustoria absorb nutrients from the subcuticular tissues of the rose.

Overwintering

The fungus overwinters through the mycelium, the conidia, or very rarely -when there is sexual reproduction- with the apothecia. The apothecia overwinter on the fallen leaves, while the mycelium and conidia both on the infected leaves and on the canes as well as on the buds of the plants. If exist, they can overwinter even on prunings.

Diplocarpon rosae (Black Spot of Rose)

Acervuli in Leaf Spot
Acervuli in Leaf Spot - © Bruce Watt, University of Maine, Bugwood.org

Diplocarpon rosae (Black Spot of Rose) - Μorphology

The basic morphological elements of the Black Spot of Rose are the mycelium, the apothecia, the asci, the ascospores, the acervuli as well as the conidia. 

Mycelium

The mycelium of D. rosae grows sub-cuticular of the affected organ. It grows radially, it is branching, and is composed of single or in strands of parallel hyphae, forming a network. The mycelial hyphae are initially hyaline, showing off-white color, which with age turns into a dark grayish.

Apothecia

The apothecia develop a sub-cuticular, circular, shield consisting of cells with a thick wall, the color of which is dark brown. When they rupture the epidermis, they appear as stellate irregular structures that radiate from the center. The diameter of the apothecia ranges between 100 and 250 μm.

Asci

The asci have a subclavate, elongated shape, the upper part of which narrows abruptly. The dimensions of the asci range in length from 70 to 80 μm, and in diameter from 12 to 15 μm. Inside them, 8 ascospores are carried.

Ascospores

Ascospores are oblong-elliptical, hyaline, constricted at the septum and 2-celled. The cells are rather gutulate and visibly unequal in size, since the upper of them is larger than the lower. Ascospores have a length ranging from 20 to 25 μm and a diameter ranging between 5 and 6 μm.

Acervuli

The acervulus is a complex fruiting-body of the asexual reproduction of the ascomycete. The acervuli are initially sub-cuticular covered by the cuticle of the affected organ, which then ruptures irregularly. They appear on the leaves mainly on the upper surface as very small black spots, having various sizes, with a diameter ranging between 50 and 400 μm.

Conidia

The conidia are two-celled (the upper cell usually thicker), hyaline in appearance, with a smooth and sticky surface, which present as a white, slimy mass when extruded from the fruit body. They are straight or subfalcate, cylindrical or deeply constricted, with dimensions ranging from 15 to 25 μm in length, and 5 to 7 μm in diameter.

Diplocarpon rosae (Black Spot of Rose)

Diplocarpon rosae (Black Spot of Rose), Vertical Section of Acervulus
Diplocarpon rosae (Black Spot of Rose), Vertical Section of Acervulus - © Bruce Watt, University of Maine, Bugwood.org

Diplocarpon rosae (Black Spot of Rose) - Hosts

The rose enjoys the honor and has the privilege, in the whole plant kingdom, to be the only host of the – in other respects so likable – ascomycete Diplocarpon rosae.

Diplocarpon rosae (Black Spot of Rose)

Diplocarpon rosae (Black Spot of Rose) - Geographical Distribution

The disease of Black Spot of Rose has spread in the last 2 centuries almost everywhere in the world, where roses exist and are cultivated.

In the history of the spread of the ascomycete Diplocarpon rosae it is reported that after being first recorded in Sweden in 1815 and in the United States in 1830, by 1844 it had already made its presence felt in England, Belgium, France, Germany and the Netherlands.

It was followed by South America in 1880, Australia in 1892, the Soviet Union in 1901, China in 1910, Canada in 1911, Japan in 1914, and parts of Africa in 1920 – 1922.

Since then it has continued to spread globally with the exception of a few areas, such as:

  • Almost the entire Middle East
  • About half of Central and North Africa
  • Of Mongolia

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