New Oidium sp. on Euphorbia pulcherrima: Addition to the EPPO Alert List
On poinsettias, powdery mildew can be caused by different fungi: Oidium sp. (whose identity remains unclear as only the anamorph is observed, present in America on poinsettia only), Leveillula taurica (present in Africa) and Leveillula clavata (present in the Mediterranean region on many different hosts). Since the 1990s, powdery mildew caused by Oidium sp. is causing problems in USA on poinsettias. It appears that recent outbreaks observed in different countries in Europe are caused by this Oidium sp. which is a new pathogen for the region. In addition, the NPPO of Sweden wrote to the EPPO Secretariat and ask for more information on this disease on poinsettias. Therefore, the EPPO Secretariat considered that it could be added to the EPPO Alert List.
Oidium sp. on Euphorbia pulcherrima (a new powdery mildew of poinsettias)
Why: The NPPO of Sweden asked the EPPO Secretariat for more information of Oidium sp. on Euphorbia pulcherrima, as several countries in Europe have recently reported new outbreaks, and in addition since the 1990s this disease is causing problems in USA.
America: Mexico, Puerto Rico, USA (California, Georgia, Illinois, Kansas, Kentucky, Maine, Maryland, New Hampshire, North Carolina, Ohio, Pennsylvania, Tennessee).
Europe: Denmark (one outbreak found in 1995 and eradicated), Germany (found in autumn 2001, measures are being taken), Sweden, United Kingdom.
On which plants: According to the American experience, Euphorbia pulcherrima is the only host of this Oidium sp.
Damage: White mycelium is observed on stems, petioles, mature and immature leaves, and bracts. Severely diseased leaves become twisted, and prematurely senescent. Powdery colonies are produced on both leaf surfaces. In USA, the disease often remains unnoticed until late in the season when bracts are beginning to colour. Earlier in the season, it may remain undetected because it mostly occurs on the under surface of the older, lower leaves. It is stated that in USA, Oidium sp. on poinsettia has become an economically significant problem for Poinsettia growers in the Midwest and northern USA.
Dissemination: The fungus produces large numbers of dry, powdery spores which are easily spread by air currents. They are also dispersed by man and tools within the glasshouse.
Pathway: Plants for planting, pot plants of Euphorbia pulcherrima from countries where it occurs
Possible risks: Euphorbia pulcherrima is an important glasshouse crop in Europe, with substantial movement of planting material between countries. This Oidium sp. has already shown its ability to move undetected in trade. Chemical control is possible but data is lacking on its efficacy. Data is also lacking on the identity of the pathogen and, despite its rather long presence in the USA, it has not been possible to make progress on this. So far, in Europe, poinsettia crops are not affected by powdery mildew, the introduction and establishment of this Oidium sp. would indeed cause problems to growers.
EPPO RS 2002/021
Panel review date - Entry date 2002-02
Celio, G.J.; Hausbeck, M.K. (1998) Conidial germination, infection structure formation, and early colony development of powdery mildew on Poinsettia. Phytopathology, 88(2), 105-113.
Koike, S.T.; Saenz, G.S. (1998) First report of powdery mildew, caused by an Oidium sp., on poinsettia in California. Plant Disease, 82(1), p 128.
Motte, G.; Unger, J.G. (1995) [Appearance of powdery mildew (Oidium spp.) on poinsettias (Euphorbia pulcherrima) in Denmark]. Nachrichtenblatt des Deutschen Pflanzenschutzdienstes, 47(1) p 22.
NPPO of Germany, 2002-02.
NPPO of Sweden, 2001-11.
ADAS Bedding and Pot Plant Technical Notes (UK) http://www.adas.co.uk/horticulture/HONSNOTES/Bpn1100.PDF
Bureau of Plant Industry in Pennsylvania (US) Emerging Plant Diseases. http://sites.state.pa.us/PA_Exec/Agriculture/bureaus/plant_industry/pests/disease/diseases/emerging.html
DEFRA web site – A new Poinsettia powdery mildew http://www.defra.gov.uk/planth/poinset.htm
North Carolina State University (US) New, emerging, and re-emerging plant disease in the United States. http://www.ces.ncsu.edu/depts/ent/clinic/Emerging/fpm2.htm