Rapid spread of soybean rust (Phakopsora pachyrhizi) in the Americas: addition to the EPPO Alert List
Two distinct pathogens are involved in soybean rust: Phakopsora pachyrhizi and P. meibomiae (see EPPO RS 2002/030). P. pachyrhizi is more aggressive and is considered as one of the most destructive foliar disease of soybean (Glycine max). P. pachyrhizi originates from Asia (hence its common name: Asian soybean rust) but in recent years, it has spread to other continents. The first confirmed report of P. pachyrhizi on the African continent was made in 1996 from Kenya, Rwanda, and Uganda, and the disease then continued to spread to other African countries. P. pachyrhizi was first reported in Hawaii in 1994, but until very recently it was still absent from the continental part of USA. In South America, P. pachyrhizi was first detected in Paraguay in 2001, in a limited number of fields in the Paraná River basin bordering Brazil. By 2002, soybean rust was widespread throughout Paraguay and in limited areas of Brazil bordering Paraguay, with reports of severe disease in some fields in both countries. During the 2003 growing season, the pathogen was detected in most of the soybean-growing regions of Brazil with significant yield losses (approximately 5% of the annual production). In Argentina, the pathogen was found in 2002 in a limited area in the north of the country. In 2004, the disease spread readily throughout most soybean-growing areas of northwest and northeast Argentina. In USA, P. pachyrhizi was first found in November 2004 in Louisiana, and later in other southeastern states. It is considered that the rust was transported from South America to North America by Hurricane Ivan which occurred in September 2004. There are also recent reports of the disease in Bolivia and Uruguay. Although, it is not entirely clear whether P. pachyrhizi would be able to survive in Euro-Mediterranean conditions (cold winter temperatures, lack of humidity), a CLIMEX study did not exclude the possibility that P. pachyrhizi could survive in southern Mediterranean countries. As this damaging rust of soybean is still absent from the Euro-Mediterranean region, the EPPO Secretariat decided to add it to the EPPO Alert List.
Phakopsora pachyrhizi (Asian soybean rust)
Why: The recent and rapid spread of Phakopsora pachyrhizi in the Americas attracted our attention. Although data is lacking on potential establishment in the Euro-Mediterranean region (tropical and sub-tropical pathogen), the EPPO Secretariat decided to add it to the EPPO Alert List.
Where: Asia: Cambodia, China, India, Indonesia, Japan, Korea, Malaysia, Nepal, Philippines, Russia (Far East), Taiwan, Thailand, Vietnam.
Africa: Ghana, Mozambique, Nigeria, Rwanda, Sierra Leone, South Africa, Uganda, Zambia, Zimbabwe.
North America: USA (Alabama, Arkansas, Florida, Georgia, Hawaii, Louisiana, Mississippi, Missouri, North Carolina, Tennessee).
South America: Argentina, Bolivia, Brazil (Goias, Maranhão, Mato Grosso, Mato Grosso do Sul, Minas Gerais, Paraná, Rio Grande do Sul, São Paulo), Paraguay, Uruguay.
Oceania: Australia, Papua New Guinea.
On which plants: Soybean (Glycine max) is the main cultivated host but many other Fabaceae can host this rust, for example: Lupinus hirsutus, Medicago arborea, Melilotus officinalis, Phaseolus vulgaris, P. lunatus, Vicia dasycarpa, Vigna unguiculata, and the weed Pueraria montana var. lobata (kudzu). More data is needed on the range and economic importance of P. pachyrhizi on legume hosts, other than soybean.
Damage: The most common symptoms of infection by P. pachyrhizi are tan-to-dark brown or reddish brown lesions (2 to 5 mm2) which are usually clustered along the veins. Lesions contain erumpent, globose uredinia. Urediniospores are released through the circular ostiole. The disease begins with small, water-soaked lesions, which gradually increase in size, turning from grey to tan or brown. They assume a polygonal shape restricted by leaf veins and usually coalesce to form larger lesions. As the plant matures and sets pods, the symptoms spread rapidly to the middle and upper parts of the plant. Lesions are found on petioles, pods, and stems but are most abundant on leaves. As rust severity increases, premature defoliation and early maturation of plants is common. In areas where the pathogen occurs commonly, yield losses up to 80% have been reported. Successful infection is dependent on the availability of moisture on plant surfaces. At least 6 h of free moisture is needed for infection with maximum infections occurring with 10 to 12 h of free moisture. Temperatures between 15 and 28°C are ideal for infection.
Dissemination: Over long distances, P. pachyrhizi is mainly spread by wind-borne spores (e.g. in USA, it is considered that Hurricane Ivan transported it from South America to Southern USA, see Internet animation https://netfiles.uiuc.edu/ariatti/www/SBR/Ivan.htm). Trade of host plants cannot be excluded as a pathway (e.g. leafy vegetables, ornamentals, pods).
Pathway: Plants for planting, ornamental cut foliage, vegetables of host plants may ensure dissemination of the pathogen.
Possible risks: Soybean is an important crop in the EPPO region. P. pachyrhizi is considered as a serious rust disease in countries where it occurs. Control methods are available (chemical control, destruction of weed hosts) but more data is needed on their efficacy. Preliminary CLIMEX studies have showed that low winter temperatures and lack of humidity are limiting factors for the establishment of the pathogen, and therefore in Europe, only Southern Mediterranean countries may be at risk. However, more detailed studies on its potential for establishment would be needed for the EPPO region.
EPPO RS 2005/029
Panel review date - Entry date 2005-03
Sources
Klag N (2005) Soybean Rust. NAPPO Newsletter, March, p 4.
Pivonia S, Yang XB (2004) Assessment of the potential year-round establishment of soybean rust throughout the World. Plant Disease, 88(5), 523-529.
INTERNET
ProMed postings. http://www.promedmail.org
Soybean rust – USA: 1st report (2004-11)
Soybean rust, Asian strain – Arkansas: 1st report (2004-11).
Soybean rust, Asian strain - Brazil (2004-12).
Soybean rust, Asian strain – Argentina (2005-01).
Soybean rust, Asian strain – USA (Florida) : 1st report 2005 (2005-03).
USDA-APHIS. Pest Alert. Soybean Rust. http://www.aphis.usda.gov/ppq/ep/soybean_rust/
Plant Management Network. Soybean Rust. http://www.plantmanagementnetwork.org/infocenter/topic/soybeanrust/