SeedQuest - Central information website for the global seed industry

News Page

The news
and
beyond the news

Index of news sources

Topics

Species

Archives

News archive 1997-2008

1st report of lethal necrosis on maize in the Northern Province of Rwanda

A ProMED-mail post

<http://www.promedmail.org>

ProMED-mail is a program of the

International Society for Infectious Diseases <http://www.isid.org>

Date: June 2014

Source: New Disease Reports [edited]

<http://www.ndrs.org.uk/article.php?id=029022>

[Ref: IP Adams et al (2014): First report of maize lethal necrosis disease in Rwanda. New Disease Reports 29, 22; DOI:

10.5197/j.2044-0588.2014.029.022]

----------------------------------------------------------------------

In March and June 2013 samples of maize showing signs of maize lethal necrosis (MLN) symptoms from the Northern Province of Rwanda were tested for _Maize chlorotic mottle virus_ (MCMV) and _Sugarcane mosaic virus_ (SCMV) by real-time PCR. All samples tested positive for MCMV and negative for SCMV. In order to determine if MCMV was responsible for the symptoms, or if another potyvirus was present, deep sequencing of total RNA from four of the maize samples was performed.

Complete genomes of MCMV were recovered from all 4 samples and shown to have a very close relatedness with MCMV from Kenya and China (99 percent homology), and some separation from MCMV from the United States (97-96 percent homology). Complete SCMV genomes were found in 3 of the 4 samples. The SCMV isolated from Rwandan samples is distinct from that isolated in Kenya (87 percent identity) and most closely related (95 percent identity) to strain SCMV-DMB. A high degree of divergence at the primer binding regions of the respective SCMV genomes explains the negative results of real-time PCR designed for the SCMV sequence from Kenya.

The data would support a shared origin for MCMV in Rwanda and Kenya, but different sources for SCMV. This is not unexpected as SCMV has been observed for many years in East Africa, whereas MCMV is believed to be new to the region. MCMV would now appear to be spreading across the East Africa region, potentially in seed, and causative of MLN where populations of SCMV are already endemic.

These findings have implications for the spread of MLN throughout Africa and for the testing for MCMV and SCMV. Control of the disease may prove more effective by initiating quarantine measures to counter the movement of MCMV, rather than attempting to control the endemic SCMV. This report confirms that MLN, caused by MCMV and SCMV, is now present in Rwanda and that this is most probably due to the recent introduction of MCMV.

Communicated by:

ProMED-mail

<promed@promedmail.org>

[Maize lethal necrosis (MLN) caused by co-infection of _Maize chlorotic mottle virus_ (MCMV, genus _Machlomovirus_), transmitted by chrysomelid beetle vectors, and either _Maize dwarf mosaic virus_ (MDMV, genus _Potyvirus_) or _Wheat streak mosaic virus_ (WSMV, genus

_Tritimovirus_) has been reported from the Americas and some locations in Europe.

MLN as well as MCMV were reported for the 1st time in Africa from Kenya in 2012 (ProMED-mail post 20130123.1510727) and the disease is spreading in the region. WSMV has not been reported from Africa, but MDMV is known to occur in the region. MDMV is closely related to _Sugarcane mosaic virus_ (SCMV, genus _Potyvirus_), also present in the area, and both are transmitted by a number of aphid species. SCMV has been reported as the co-infecting potyvirus in Kenya, but transmission in maize seed was previously only reported for MDMV, and not SCMV. However, seed transmission of MLN in Africa is being claimed and mandatory testing of maize seed imports has recently been introduced in Kenya (see link below). This would imply that at least one of its viruses is seed transmitted, and that infected seedlings then acquire the 2nd virus from infectious insect vectors present locally. Thus, the 2 viruses involved in Africa may be MCMV, which is not seed transmitted, and either seed transmitted MDMV (undifferentiated from SCMV in the diagnosis) or a new strain of SCMV that can be seed transmitted in maize. The high genetic divergence of African SCMV strains reported above may support the latter hypothesis.

Symptoms of the individual viruses are synergistically enhanced in MLN and may include leaf mottling and necrosis, distortion of ears, absence of kernels, failure to produce tassels, as well as stunting, premature aging, and death of plants. Symptoms may disappear during the growing season leaving plants with latent infections but reduced yield as virus reservoirs and making disease monitoring difficult.

Infectious vector insects may be carried by wind over long distances.

Disease management may include crop rotation, certified clean seeds, control of vector species and weedy reservoir hosts, and use of crop cultivars or hybrids with reduced sensitivity to the viruses. A resistance breeding programme has been set up in Kenya screening a large set of diverse pre-commercial hybrids from public and private institutions.

MCMV had been detected in the Northern Province of Rwanda in October

2013 (ProMED-mail post 20131004.1983210) and the country put on alert for MLN (see link below).

Maps

Rwanda:

<http://www.welt-atlas.de/datenbank/karten/karte-2-789.gif> and <http://healthmap.org/promed/p/39457>

Rwanda provinces:

<http://www.mapsofworld.com/rwanda/rwanda-political-map.html>

Africa, overview:

<http://worldatlas.com/webimage/countrys/africa/maps/africa.htm>

Pictures

Maize lethal necrosis on field maize:

<http://www.flickr.com/photos/cimmyt/4926986403>