Maize seed can transmit Maize Lethal Necrosis Disease (MLN)

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Research has shown that maize seeds may contain the viruses that cause Maize Lethal Necrosis (MLN) disease and can transfer the disease through seeds to farms. This poses great danger to food security in the country and neighbouring countries.

As the Maize Lethal Necrosis Disease (MLN) spreads across the country, the government, including research institutions, are working to understand how the disease came into Kenya and how it spread rapidly to almost all maize growing areas within a short period.

Efforts to combat the Disease

The government has set up a task force to carry out a survey in all maize growing areas across the country and recommend the best ways of controlling the disease. The task force is working with the Ministry of Agriculture, Livestock and Fisheries (MOALF), Kenya Plant Health Inspectorate Service (KEPHIS), the Kenya Agricultural and Livestock Research Organization (formerly KARI), the University of Nairobi (UON), The International Centre for Maize Improvement (CIMMYT) and the International Centre of Insect Physiology and Ecology (ICIPE) to map the extent of the disease and come up with ways to combat it.

Although pests are known to be the major carriers of viruses causing plant diseases, there is no any conclusive study that can show that they are responsible for the rapid spread of MLN. Scientists are working to find out the extent of the damage caused by the disease. Long term solutions are awaited to protect the future of maize, a crop that more than 90 per cent of Kenya’s population depends on for food.

Infected commercial seeds may be a source of disease

As research continues, it is important to look at other possible causes of the fast spread of this disease. For example, there are two interesting studies, which can perhaps shed some light on one of the likely causes of transmission of the MLN disease in the country.

The studies - Effects of Seed Quality on the Proportion of Seed Transmission for Sugarcane Mosaic Virus in Maize (by Li, X.F Wang and G.H Zhou), and Seed Transmission of Maize Chlorotic Mottle Virus (by Jensen S.G and David S. Wysong, and others) show that the quality of seed can contribute to the spread of MLN. This disease is caused by a combination of both Maize Chlorotic Mottle Virus (MCMV) and Sugarcane Mosaic Virus (SCMV).

 

According to the study on transmission of MCMV through seeds, four seed samples of maize containing 11,000 seeds supplied by two seed companies had 17 infected seeds. One sample with 3,000 seeds had one infected seed. However, three seed samples with 8000 seeds were found to have 16 infected seeds. The remaining 21 seed samples with 31,000 seeds from other seed companies were free of the virus.

Percentage of diseases Determined

To find out the potential of the diseased seed in each seed sample to pass on the disease wherever they were planted, a simple calculation was done to determine the percentage of diseased seeds that were likely to pass on the disease. In the seed samples free of the viruses the seed transmission rate was 0.0%, while among the seed samples with MCMV, the least was in the first seed sample i.e. 1 out of 3,000 seeds indicating a seed transmission rate of 0.03% (i.e. 1 divided by 3,000 and multiplied by 100) and the highest was in the second seed lot at 0.33% (i.e. 10 infected seeds divided by 3,000 and multiplied by 100).

Estimated number of diseased seeds in one acre

 One acre of maize has approximately 42,000 seeds. Therefore, to get the number of seeds per acre that are likely to be infected and have the potential to pass on the disease, we have to go by what Jensen and his group found out. In the case of the first seed sample case scenario, 13 seeds per acre are likely to be infected (i.e. 0.03 multiplied by 42000 and divided by 100). In the second lot case scenario with 0.33% transmission, 139 seeds per acre are likely to be infected (i.e. 0.33 multiplied by 42,000 seeds and divided by 100). Farmers need to know that the likelihood of MCMV spreading among seed shows that seed contamination depends on how the seed farms are maintained and certified.

Seed with 10% rate of MLN infection certified

 Several technical staff in a number of seed companies in Kitale and Nairobi interviewed by TOF indicated that seed farms with up to 10 per cent infestation levels were being certified by the Kenya Plant Health Inspectorate Service (KEPHIS) to sell the maize as seed. This essentially means that in nominal seed crop yield of 1.7 tonnes per hectare, 170kg of seeds are at the risk of carrying some seeds with the virus. At a 1000 seed weight of 240gm, this 170kg of seeds are likely to have 0.7 million seeds and at a maximum seed transmission rate of 0.33% nearly 2310 seeds from among seeds produced in one hectare are likely to be infected and have the ability to spread the disease where such seed is planted. It is important to note that the seed transmission rates of 0 – 0.33% discussed above are for MCMV one of the two viruses responsible for MLN disease.

Seed transmission of SCMV virus

The second study that focused on transmission of SCMV virus showed that for every 42,000 seeds per acre, approximately 966 to 1,638 seeds could be infected and can transmit the disease where the seed is planted. This means that nearly 27,300 seeds from one hectare are likely to be infected and pass the disease if used as seed. The distribution and marketing of these seeds needs to be carefully monitored and controlled to prevent fast spread of the disease in the farms.

Insects pests spread the viruses

 The number of infected seeds per acre could look small compared to the large number of seeds sown in an acre. However, in a scenario where these spreads in the whole farm if there are pests like thrips and aphids, which act as carriers; this can cause major damage to the entire maize field in areas where such seed is planted.

In this regard, the level of disease infestation in the seed farms has to be revisited and it is therefore, recommended that more stringent measures put in place by Kenya Plant Health Inspectorate Service (KEPHIS) to ensure only clean maize seed is certified and sold to farmers. As the regulatory authority mandated to control the quality of seed in the country, KEPHIS should ensure that all maize seed it certifies is of the highest quality and does not contribute to spread of all diseases including MLN. KEPHIS should ensure that there is a high level of management in seed farms so that certified seed is not contaminated.

Already KEPHIS has taken measures to reduce the possibility of seed to transmit the disease. In a meeting with seed companies and other stakeholders held at KEPHIS headquarters on October 9 2014, the regulatory authority agreed that any seed with more than 1 percent infection will not be certified for sale to farmers. All imported seed would not be certified if it has any traces of both MCMV and SCMV pathogens.

Maize seed companies and contracted producers need to a high level of crop management to reduce the risk of spreading the disease. This may have implications on the seed cost but it will go a long way in reducing the potential losses incurred by farmers.

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