May 27, 2005
Source: European Plant Protection Organization (EPPO), Reporting Service, 2005 No. 2 [edited]

Rapid spread of soybean rust (_Phakopsora pachyrhizi_) in the Americas: addition to the EPPO Alert List

2 distinct pathogens are involved in soybean rust: _Phakopsora pachyrhizi_ (Pp) and _P. meibomiae_ (Pm) (see EPPO RS 2002/030). Pr is more aggressive and is considered as one of the most destructive foliar diseases of soybean (_Glycine max_). Pp originates from Asia (hence its common name: Asian soybean rust [ASR]) but in recent years, it has spread to other continents.

The 1st confirmed report of ASR on the African continent was made in 1996 from Kenya, Rwanda, and Uganda, and it then continued to spread to other African countries. ASR was first reported in Hawaii in 1994, but until very
recently it was still absent from the continental part of USA.

In South America, ASR was first detected in Paraguay in 2001, in a limited number of fields in the Parana River basin bordering Brazil. By 2002, ASR 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, ASR was detected in most of the soybean-growing regions of Brazil with significant yield losses (approximately 5
percent of the annual production).

In Argentina, ASR was found in 2002 in a limited area in the north of the country. In 2004, it spread readily throughout most soybean-growing areas of northwest and northeast Argentina. In USA, ASR 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 in September 2004. There are also recent reports of the
disease in Bolivia and Uruguay.

Although it is not entirely clear whether ASR would be able to survive in Euro-Mediterranean conditions (cold winter temperatures, lack of humidity), a CLIMEX study did not exclude the possibility that ASR 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 ASR 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, Maranhao, Mato Grosso, Mato Grosso do Sul, Minas Gerais, Parana, 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 Pp.on legume hosts, other than soybean.

Damage

The most common symptoms of infection by ASR are tan-to-dark brown or reddish brown lesions (2 to 5 mm square) 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 percent 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 deg C are ideal for infection.

Dissemination

Over long distances, ASR 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. ASR 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.

Source(s):

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:

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/>

EPPO RS 2005/029

ProMED-mail
<promed@promedmail.org>

[ASR was reported on late crops of soybean and on kudzu (_Pueraria lobata_) in several southern states in 2004. In 2005, it has so far been reported only from Florida and Georgia. -Mod.DH]