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What is maize lethal necrosis

Evidence of a new and devastating disease in maize

Top: Maize field infected by maize lethal necrosis
Lower: Discoloured, mottled and dying leaves of maize plant infected by maize lethal necrosis

Maize lethal necrosis (MLN) is a new, devastating disease in East Africa that leads to severe production losses and, in many cases, complete crop failure. The disease prevents the plants from growing tall, causes yellowing and death of the leaves, and stops the ears from growing and setting seeds. Infected plants die prematurely. In addition, diseased plants develop secondary fungal and bacterial infections, which means that the few grains that may be formed may contain toxins and therefore should not be eaten.

In Kenya alone, MLN affects nearly a quarter of total maize production, with yearly losses estimated at $US110 million. Most farmers in the Western region – the hardest hit by the disease – are affected. All commercial maize varieties available in East Africa are affected.

Maize is the most important cereal and staple crop in East Africa, and also the main livelihood of many smallholder farmers. MLN could therefore lead to food shortages and even famines if left uncontrolled which has lead to a substantial international effort to curb the spread of the disease and to develop resistant varieties.

When and where was maize lethal necrosis first reported in East Africa?

Distribution and severity of maize lethal necrosis is highest in tropical, humid zones, followed by the highlands and coastal areas, and lowest in drylands

MLN was first reported in the Bomet and Naivasha Districts in the South Rift Region of Kenya in 2011. Just a year later though the disease had spread not only to other maize growing parts of the country, but also to Tanzania, Democratic Republic of the Congo and Uganda, and more recently, to South Sudan and Northern Rwanda. The disease is now endemic to large parts of East Africa, and if left unchecked will continue to spread.

What causes maize lethal necrosis 

MLN arises when maize plants are infected at the same time by two viruses:

  1. Maize chlorotic mottle virus (MCMV)
  2. Sugarcane mosaic virus (SCMV; other related cereal viruses can also cause MLN in combination of MCMV)

It has been proposed that MCMV is new to East Africa, while the viruses in the second group are not. Infection by MCMV alone can cause serious damage to the crop but in combination the viruses are devastating and quickly kill infected plants.

How does maize lethal necrosis spread

Although still a matter of debate, MLN is believed to spread mainly through the seed (transmission of the disease from infected seeds to seedlings can be as high as 1 in 5). Once the disease is established in a field it is spread further by insects and by contaminated farm equipment. In areas where maize is grown all year around, the viruses always have a host to survive and infect the following crop.

Mounting a defence against maize lethal necrosis: key steps

MLN is a major concern because it can completely destroy a key staple and food security crop, and because it spreads extremely fast. How can it be stopped? The response to new plant disease epidemics always includes the key steps shown below.

  1. Know your enemy: identification, monitoring and surveillance

When a new, serious disease is reported in farmers’ fields, the first response is to try to answer the following questions:

  • What causes the disease? Is it due to viruses? Or bacteria? Or fungi? Or insects? Is the cause of the disease changing over time?
  • How many commercial varieties of the affected crop are susceptible to the disease?
  • How can the disease be confidently (and cheaply) detected and monitored in the field?
  • How many areas/regions are already affected?
  • How (and by what) is the disease transmitted?
  • How fast is it spreading?
  • How can the spread of infection be stopped or at least slowed?

The outbreak of MNL has lead to the formation of a task force and the establishment of on-going monitoring and surveillance efforts of the spread of MLN.

For diseases that affect several countries, development of appropriate diagnostic kits and harmonisation of detection protocols is important. Several international workshops have been carried out to build capacity in the region in this area. In early 2015, a workshop in Kenya trained sixty phytosanitary (seed health) regulators and seed industry scientists from 11 countries in eastern and southern Africa on MLN diagnostics, MLN-free seed production and MLN screening. And Zimbabwe recently announced the establishment of a MLN quarantine facility.

  1. Establish and disseminate information on good management practices to limit the damage and spread of the disease

Good management practices are critical because they can limit the damage the disease may cause to a field and help slow down its spread. Importantly, these measures can be applied immediately. The development and release of varieties resistant to the disease by the scientific community and the seed industry – the ultimate solution to the problem – takes a few years.

To contain the damage and spread of MLN farmers are advised to:

  • Avoid growing maize after maize in the same field as this provides the disease with a constant reservoir (MLN affects maize but no other crop). Planting different crops each season will reduce the infection rates in your field.
  • Do not plant a new maize crop near an infected field – insects and the wind will spread disease from the infected plants to the new crop.
  • Plant maize at the start of the main rainy season, rather than during the short rain season. A break between crops interrupts the disease cycle.
  • Weed fields regularly to eliminate alternate hosts for insect vectors.
  • Keeping unnecessary machines and people out of the field.
  • Controlling insect-vectors by using an appropriate insecticide (at weekly intervals)
  • Immediately remove diseased plants from the fields and burn them.
  • Human or animals should not eat infected grains: MLN makes the plant very susceptible to fungal diseases so they may be poisonous.

A list of ASARECA’s recommendations to affected, non-affected countries, and for seed companies in affected countries can be found here.

  1. Develop and disseminate maize lines resistant to the disease

The long-term solution to a disease is the development of resistant varieties although this takes typically many years. In response to the outbreak, a screening facility for MLN, the biggest of its kind, was established in Naivasha. Operations at the facility include maintaining pure forms of the MLN viruses MLN, and testing maize hybrids and inbred lines for resistance to MLN. Another important task is to train scientists, technicians, farmers and extension workers about how to handle the disease.

The evaluation of tropical maize varieties from several sources has identified resistant lines that can be used as parents in breeding programmes. In October 2015 the International Centre for the Improvement of Maize and Wheat (CIMMYT) announced the release of 16 new MLN-resistant maize lines.

This analysis has also shown that genetic resistance to MLN is complex: at least 20 genes are involved.

Top photo CIMMYT 2014