04/11350
DRAFT Data sheets on Quarantine Pests
Erysiphe cf. euphorbiicola (Oidium sp.)
IDENTITY
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
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 isolate from poinsettia (Brielmaier-Liebetanz, 2003, unpublished data).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).
GEOGRAPHICAL DISTRIBUTION
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.
BIOLOGY
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.
DETECTION AND IDENTIFICATION
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.
PEST SIGNIFICANCE
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.
Control
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.
PHYTOSANITARY MEASURES
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.
REFERENCES
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, 302303.
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