Anamorph: Oidium sp. Taxonomic Position

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DRAFT Data sheets on Quarantine Pests
Erysiphe cf. euphorbiicola (Oidium sp.)
Name: Erysiphe cf. euphorbiicola (Oidium sp.)

Anamorph: Oidium sp.

Taxonomic Position: Fungi: Ascomycota: Erysiphales

Common Name of the disease: American powdery mildew on poinsettia

EPPO listing: Alert list

EU listing: None

Special notes on taxonomy and nomenclature:

The taxonomic position of this powdery mildew is not absolutely clear because on Euphorbia pulcherrima there has been found only the anamorph Oidium sp. A teleomorph on poinsettia is not known. According to BRAUN (pers. communication 2001) Oidium sp. from E. pulcherrima is not different in morphology from the anamorph of Erysiphe euphorbiicola U. Braun & S. Takamatsu (= Microsphaera euphorbiae Berk. & M.A. Curtis) occurring in North America (Braun and Takamtsu, 2000). BRAUN suggests the preliminary name Erysiphe cf. euphorbiicola (Oidium sp.).

Hosts of Oidium sp. from Euphorbia pulcherrima

Euphorbia pulcherrima is the only reported natural host of the anamorph Oidium sp.

Euphorbia heterophylla, E. helioscopia, E. exigua and E. marginata are experimental hosts for the Oidium sp. isolate from poinsettia (Brielmaier-Liebetanz, 2003, unpublished data).
Hosts of Erysiphe euphorbiicola U. Braun & S. Takamatsu

Euphorbia (brasiliensis, corollata, dentata, heterophylla, hypericifolia, hyssopifolia, maculata, marginata, presleyxi, spec.), Manihot esculenta, Euphorbiaceae (Braun, 1987).
Africa: no record

America: Reports of Oidium sp. on poinsettia from USA, Mexico and Puerto Rico (Daughtrey and Hall, 1992; Koike and Saenz, 1998).

Distribution of Erysiphe euphorbiicola, the supposed teleomorph of Oidium sp.: North America (USA widespread), Central America and South America (e.g. Puerto Rico, Venezuela), Cuba, Dominican Republic (Braun, 1987)

Asia: no record

Australia: no record

Europe: Denmark, Germany and UK (Motte and Unger, 1995; Gerlach, 2001). In rare instances outbreaks under protected conditions on poinsettia plants produced from cuttings with origin in countries where the pest is known to occur. Not established in Europe. The outbreaks were subject of eradication.
Infection of plants takes place by conidia germinating on the surface of poinsettia leaves. Mycelium develops from one or several secondary germ tubes and, dependent on temperature and light conditions, conidiophores and conidia are generated. When sporulation starts, powdery mildew infection gets visible to the naked eye. 15-23°C are favourable to sporulation. The fungus produces large numbers of conidia which easily can be spread by air within the crop and secondary infection follows. A teleomorph has not been observed.

Inoculation experiments in a growth chamber had shown that the incubation period for an infection with Oidium sp. on poinsettia lasts 7-12 days at temperatures from 15 to 23°C. At lower temperatures conidia were able to germinate but growth of hyphae and sporulation was delayed. At 28°C the infection was inhibited. (Brielmaier-Liebetanz, 2002; Shaw and Hausbeck, 1995). Microscopical observations from Celio and Hausbeck (1998) revealed that conidia germinate and form appressoria at 20°C within 6 h, followed by the development of one or several secondary germ tubes and formation of a haustorium within 24 h. At 30°C conidial germination was significantly reduced in comparison to 20°C and neither secondary germ tubes nor haustoria developed. That means that at permanent high temperatures infection will not be successful.

Symptoms: Symptoms can develop on the upper and the lower leaf surface. Early signs of infection are either yellow spots or pale white spots especially on the older lower leaves. In this stage hyphae growing across the leaf can be seen with a hand lens. Once colonies are producing conidia they get the typical whitish powdery mildew appearance. In a progressing stage of infestation colonies flow together. On severely diseased plants powdery mildew colonies develop also on stems, petioles and bracts and leaves get twisted.
Morphology: The conidia of Oidium sp. from poinsettia are formed singly. They are cylindric and measure about 24-35 x 12-18 µ (length x width). Fibrosin bodies are lacking. Conidiophores are erect, straight, with cylindric, sometimes slightly flexuous foot-cells of about 19-45 x 4,5- 8µ, followed by predominantly 2, sometimes up to 4 shorter cells.

Oidium sp. can be easily distinguished from Podosphaera euphorbiae which forms conidia in chains and contains fibrosin bodies (Brielmaier-Liebetanz and Merz, 2002) as well as from Leveillula clavata and L. taurica with non cylindrical conidia (Nour, 1957).
Detection and inspection methods:
Inspection of unrooted cuttings:

In a very early stage powdery mildew infection can be detected microscopically at a magnification of 50x. Mycelium is growing radially, even across the leaf surface and looks web like. It can be well distinguished from mycelium of Cladosporium sp. In case of doubt it is recommended to incubate leaves in a moist chamber and observe for sporulation. Disadvantage: The microscopical examination is time consuming, its reliability depends on the sample size.

Inspection during cultivation process:

Plants should be scouted weekly for early signs of powdery mildew infection. First of all the older leaves should be controlled on the upper as well as on the lower leaf side. In case of suspicion the leaves should be checked microscopically for growth of powdery mildew mycelium on the leaf surface. If powdery mildew infection has been proven, confirmation of the genus of the powdery mildew is necessary. This is only possible in case of sporulation.

Means of movement and dispersal of the pathogen
Dispersal within the greenhouse via conidia may be by air currents, man and tools. Conidia adhering to clothings remain infectious for a minimum of 24 hours. Spread of the pathogen over long distances occurs mainly by movement of diseased plants. There is no information on the dissemination of the conidia of this powdery mildew over long distances. From Erysiphe graminis it is known that conidia are still infectious after crossing the North Sea from Britain to Denmark (Spencer, 1978). Oidium sp. can survive as mycelium on the host. Survival of conidia in absence of the host is possible for several days, depending on climate conditions (unpublished experimental observation). A long term survival of the conidia is unlikely.
Economic impact
The economic impact can vary considerably. If powdery mildew symptoms are detected in an early stage of infection the pest can be well controlled by fungicides. This will be the case, when plants are regularly checked on first signs of powdery mildew. Left unchecked, the disease can spread very quickly across the crop and as in a progressed stage of disease development chemical control is no longer efficient the economic loss would be high.

Powdery mildew found in an early stage of development can be well controlled by fungicides. The whole crop should be treated, not only plants with symptoms. Fungicides with active ingredients like bitertanol, triadimefon and myclobutanil are efficient against powdery mildew on poinsettias as well as strobilurin fungicides. In few cases injury to bracts caused by fungicide application was observed (Böhmer, 1994). The actual situation of the registration of pesticides for use on poinsettias has to be taken into account. In case of progressed powdery mildew infestation affected plants have to be destroyed in order to reduce inoculum levels.

Phytosanitary risk

Erysiphe cf. euphorbiicola (Oidium sp.) does not occur in Europe. There have been few outbreaks in Europe most propably due to the import of latently infected cuttings from a third country. The risk for an outbreak depends on the amount of cuttings imported from countries where the pest is known to occur. Discovered in time the powdery mildew can be well controlled by fungicides. Establishment of the pest is very unlikely in countries where poinsettia are grown only seasonally and under protected conditions because a survival of the Oidium sp. from one season to the next in absence of poinsettia is not possible.

Poinsettia cuttings imported into the EU from third countries must be accompanied by a Phytosanitary Certificate stating that the plants have been inspected for quarantine diseases and pests in the country of origin and are substantially free from other harmful organisms. Plants of Euphorbia pulcherrima originating from countries where Erysiphe cf. euphorbiicola (Oidium sp.) is known to occur (USA, Mexico, Puerto Rico) should be thoroughly inspected at import for symptoms of powdery mildew, preferably by means of microscopy in order to detect early stages of infection. During the whole cultivation period plants should be regularly monitored for powdery mildew symptoms. Especially the older leaves must be checked carefully.

Böhmer, B. 1994. In Deutschland noch nicht aufgetreten: Poinsettien-Mehltau. Deutscher Gartenbau 48 (29), 1704-1705.

Braun, U. 1987. A monograph of the Erysiphales (powdery mildews). Beih. Nova Hedwigia 89, 302-303.

Braun, U. & Takamatsu, S. 2000. Phylogeny of Erysiphe, Microsphaera, Uncinula (Erysipheae) and Cystotheca, Podosphaera, Sphaerotheca (Cystotheceae) inferred from rDNA ITS sequences – some taxonomic consequences. Schlechtendalia 4, 1-33.

Brielmaier-Liebetanz, U. 2002. Eingeschleppt aus Drittländern? Echter Mehltau an Poinsettien. TASPO 136 (40), 2.

Brielmaier-Liebetanz, U. and Merz, F. 2002. Echten Mehltau erkennen und bekämpfen. Deutscher Gartenbau 56 (4), 14-16.

Celio, G.J. and Hausbeck, M.K. 1998. Conidial germination, infection structure formation, and early colony development of powdery mildew on Poinsettia. Phytopathology 88 (2), 105-113.

Daughtrey, M. and Hall, J. 1992. Powdery mildew- a new threat to your poinsettia crop. Grower Talks 58 (4), 23-31.

Gerlach, W. 2001. Echter Mehltau an Poinsettien. Deutscher Gartenbau 55 (45), 11.

Koike S.T. and Saenz, G.S. 1998. First report of powdery mildew caused by an Oidium sp., on poinsettia in California. Plant Disease 82 (1), 128.

Motte, G. and Unger, J.-G. 1995. Zum Auftreten von Echtem Mehltau (Oidium sp.) an Poinsettien (Euphorbia pulcherrima) in Dänemark. Nachrichtenblatt des Deutschen Pflanzenschutzdienstes 47(1), 22.

Nour, M.A. 1957. Leveillula clavata sp.nov. Trans. Brit. mycol. Soc. 40 (4), 477-480.

Shaw, B. and Hausbeck, M. 1995. Epidemiology of Powdery Mildew on Poinsettia. Phytopathology 85 (2), 1170.

Spencer, D.M. (ed.) 1978. The Powdery Mildews. Academic Press, London, 565 pp.
Symptoms of powdery mildew on older leaves of poinsettia (natural infection)
Symptoms of powdery mildew on leaves and bracts (experimental infection)

Mycelium of powdery mildew 7 days after germination of conidia

(observed at magnification 50 x )

Conidia of Oidium sp. from poinsettia ( observed at magnification 1000 x )

Photos: Federal Biological Research Centre for Agriculture and Forestry, Braunschweig

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