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A maize-free period has been employed in certain production areas
in Kenya to reduce the rate of the maize lethal necrosis epidemic
with reasonable success (in combination with other practices). The
efficacy of this maize-free period has shown that it can reduce the
rate of the maize lethal necrosis epidemic.
In South Africa, particularly on the Highveld, the cold maize-free
winters should have the same effect on slowing the epidemic.
Vectors
The insects that are regarded as being involved in maize lethal
necrosis transmission include: Corn thrips (
Frankliniella williamsi
),
corn flea beetle (
Chaetocnema pulicaria
), southern corn rootworm
(
Diabrotica undecimpunctata
), northern corn rootworm (
D. loni-
cornis
), western corn rootworm (
D. virgifera
), flea beetle (
Systena
frontalis
) and cereal leaf beetle (
Oulema melanopa
).
The poty viruses are primarily spread by aphids and pests believed
to transmit maize chlorotic mottle virus which include
Peregri-
nus maidis, Sardia pluto, Empoasca solana, Adoretus sinicus
and
Tetranychus
sp.
Many of these vector x virus relationships need to be confirmed and
properly researched to determine which of these occur in South
Africa and if they don’t what potentially related species occur in
South Africa and what would their ability be to transmit the viruses
involved in the complex.
Plant host studies
Different types of resistance have been described which include
antibiosis, non-preference, escape and mechanical barriers. The
possible vectors have been discussed above and research on identi-
fying other potential vectors should continue.
The interactions between the suspected vectors, the various virus-
es involved in the disease complex and various alternate hosts as
well as maize need to be studied intensively to improve our under-
standing of priority vectors and alternate hosts, the mode of virus
transmission and feeding period required by each vector on a maize
plant to successfully transmit the viruses.
Virus testing
Various methods are available for detection of plant viruses which
include visual symptomology, use of indicator plants, electron mi-
croscopy, serological methods (most commonly used in Africa)
and molecular techniques (these being most expensive, but also
most accurate).
Surveying in SADC – diversity and
diagnostics
The disease is most likely to reach South Africa through two pos-
sible routes. Once it spreads to southern Tanzania, the disease is
expected to spread to South Africa via Mozambique and/or eventu-
ally through Zimbabwe.
Thus maize lethal necrosis is a potential threat to food security in
southern Africa. The Southern African Development Community
(SADC) countries cannot afford to be caught unprepared to tackle
this disease.
Both ARC and IIAM (Mozambique) need to be proactive and develop
multi-stress resistant and tolerant maize inbred lines and hybrids
with resistance/tolerance to maize lethal necrosis.
To date more than 1 000 ARC hybrids have been screened by the
International Maize and Wheat Improvement Centre (CIMMYT) at
Naivasha and they have all been found to be highly susceptible to
maize lethal necrosis. Maize lethal necrosis resistance breeding is
therefore essential.
CIMMYT has identified a number of sources of tolerance but these
still need to be tested over various climatic zones for adaptation
to local production areas before being included into local breeding
programmes.
In addition, a survey needs to be carried out to determine viruses
and vectors in maize crops along the borders of northern Mozam-
bique and southern Tanzania; and along the borders of South Africa
and Mozambique (in Mpumalanga and KwaZulu-Natal provinces);
and South Africa and Zimbabwe (in Limpopo).
In most of these areas, smallholder farmers grow both maize and
sugarcane, and they are therefore potential hot spots for sugarcane
mosaic virus (presently) and/or maize chlorotic mottle virus (in fu-
ture) which cause maize lethal necrosis.
Epidemiology
Epidemiology involves the holistic understanding of all interactions
included in the disease cycle of maize lethal necrosis. This includes
understanding all possible interactions between the viruses, vectors
and other modes of transmission, hosts and environmental factors
influencing the entire disease.
There is still a lot of confirmatory work to be done, surveys and
basic agronomical research on this disease and/or its potential to
infect maize in South Africa. Once all this information has been
obtained, we shall be able to understand the maize lethal necrosis
epidemic and identify points where integrated control measures can
be implemented.
This information will be used in a systems analysis and once we
understand the dynamics of the entire disease, we shall confidently
be able to assess the potential risk this disease will have to maize
production in South Africa and how we and neighbouring countries
can successfully manage the disease to reduce its impact.
Control
To date the most significant control measure is to prevent maize
chlorotic mottle virus coming into South Africa, which is why the
phytosanitary regulations have been put in place by DAFF. Contin-
ued breeding efforts are critical to reduce the potential impact of
maize lethal necrosis.
Agronomic practices which may slow down the epidemic need to
be determined and packaged together with resistance into an inte-
grated disease management system.
For further information, contact Prof Bradley Flett at
018 299 6362 or Dr Kingstone Mashingaidze at 018
299 6356.
ON FARM LEVEL
Integrated pest control
Julie 2016
80
MAIZE LETHAL NECROSIS