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Julie 2016
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MAIZE LETHAL NECROSIS:
Possible threat to local maize production
M
aize (
Zea mays
L.) is the most important cereal crop in
sub-Saharan Africa, covering over 25 million hectares,
largely in smallholder farming systems that produce
over 38 million metric tons (MMT) of grain.
This represents 34% of cereal production and is 8% of the value of
all crops in the region. Maize is critical for food security in sub-
Saharan Africa – eastern and southern Africa use 85% of the maize
produced as food, while Africa as a whole use 95% as food.
Maize production in South Africa is largely rain dependent as 60%
of maize is cultivated on dry land while only 20% is irrigated. Maize
is planted in seven of the nine provinces of South Africa and in-
cludes the Free State, Mpumalanga, North West Province, Gauteng,
KwaZulu-Natal, Limpopo and the Northern Cape provinces on a
total area of 2,78 million ha. A total of 11,72 million tons of maize
in the 2012/2013 season were produced that increased to a total of
13,029 million tons of maize from 2,68 million ha in the 2013/2014
season.
History of maize lethal necrosis
A serious disease outbreak in Kenya, later diagnosed as maize
lethal necrosis, was first reported in September 2011 in the Longisa
division of the Bomet district. By 2012, symptoms consistent with
maize lethal necrosis were observed in a number of districts in
the Central, Nyanza, Western and Rift Valley provinces of Kenya.
Since then, the disease has been reported by Rwanda and Demo-
cratic Republic of Congo. Similar symptoms on maize have been
reported by Uganda and Tanzania, and more recently by South
Sudan and Ethiopia. Recent discussions with pathologists in Tanzania
have raised suspicions that the disease may already be in Tanzania
in the Mtwarao region which borders on northern Mozambique.
Maize lethal necrosis has previously been reported in Peru, United
States of America, Argentina, Mexico, Thailand, Brazil and China.
Crop losses of up to 100% have been reported. The potential av-
enues of reaching South Africa are from Tanzania into Mozambique
and into the sub-tropical areas of South Africa where subsistence
maize is often grown continuously throughout the year.
Viruses involved in maize lethal necrosis
disease
Maize lethal necrosis disease symptoms (
Photo 1a
and
Photo 1b
) is
caused by the synergistic co-infection of maize with maize chloro-
tic mottle virus symptoms (
Photo 2
) and any virus from the family
Potyviridae
, these include wheat streak mosaic virus, maize dwarf
mosaic virus symptoms or sugarcane mosaic virus symptoms (for-
merly maize dwarf mosaic virus-B) (
Photo 3
).
Singular infections of each virus alone produces milder symp-
toms but when combined infections of maize chlorotic mottle virus
and any of the potyviruses produce a synergistic reaction that re-
sults in plant death. In many cases maize chlorotic mottle virus
infections alone under abiotic stress conditions, such as drought
and low nitrogen have also been known to develop maize lethal ne-
crosis symptoms.
Maize chlorotic mottle virus has not yet been reported in South
Africa, however, maize dwarf mosaic virus infects maize plantings
on a localised scale but can be very severe in certain areas. Sugar-
cane mosaic virus has been reported to occur on sugarcane in South
Africa and is widespread throughout the sugarcane industry.
Sugarcane is often grown in close proximity to maize fields both
commercially and by subsistence farmers. It is highly possible
that maize grown in these areas may be infected with sugarcane
mosaic virus. It is critical that a full survey on maize viruses be con-
ducted throughout the South African maize industry to determine
what viruses do occur in South Africa.
Maize dwarf mosaic virus and maize streak virus have been reported
on maize in South Africa. The results of the survey will allow an ob-
jective assessment as to the potential risk we have of maize lethal
necrosis becoming an epidemic in South Africa.
Maize streak virus is common throughout Africa and is restricted
to sub-Saharan Africa where it has been known to result in major
yield losses. The possible synergistic action of maize chlorotic mot-
tle virus and maize streak virus has to date not been examined.
In a presentation in Naivasha in Kenya Dr Anne Wangai highlighted
that the possible synergistic action of maize chlorotic mottle virus
and other viruses outside the
Potyviridae
needs to be determined.
Host plants
The experimental host range where maize lethal necrosis disease
is concerned, is restricted to the grass (
Poaceae
) family with maize
being the primary and natural host. A number of natural and planted
grass species have been infected with either or both viruses us-
ing mechanical inoculations and include:
Bromus
spp.,
Digitaria
sanguinalis, Eragrostis trichodes, Hordeum
spp.,
Panicum
spp.,
Setaria
spp.,
Sorghum
spp. and
Triticum aestivum
.
Research on potential alternate host plants is ongoing in Kenya
and the list of hosts is increasing. Locally we need to determine
which of these alternate hosts grow naturally or are grown as crops
in South Africa and where.
This will enable us to determine the extent of a potential inoculum
reservoir for the different viruses involved in the possibility of a
maize lethal necrosis outbreak in South Africa. This will require a
survey of grasses in high risk areas and screening of the grasses for
any of the previously reported maize viruses.
Transmission of maize lethal necrosis
Transmission is largely mechanical, by insect vectors and/or seed-
borne. Mechanical transmission is illustrated by the ease at which
plants can be inoculated using a blower to blow a fine mist virus
suspension onto plants to screen them for maize lethal necrosis.
Mechanical transmission will also include movement of people,
animals and implements through infected fields. Vector transmis-
sion of the primary viruses has been studied to a point but more
research on the interactions between the vector, virus and host is
required to determine latent periods for feeding to carry over the
various viruses that are involved in this complex.
Seed transmission of maize chlorotic mottle virus was originally
shown to be very low with maize chlorotic mottle virus transmis-
ON FARM LEVEL
MLN symptoms / Control
Integrated pest control
BRADLEY FLETT
and
KINGSTONE MASHINGAIDZE,
ARC-Grain Crops Institute, Potchefstroom