Targeting virus transmission in a vital crop for African food security

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In East and Central Africa beans are a vital crop that naturally enriches the soil with nitrogen, providing natural fertilizer for other important crops such as maize and cassava. In the regional diet beans are important because they are rich in protein and critically important micronutrients. Since beans are mostly grown and traded by women, this crop provides important direct economic benefits to families & children. Unfortunately, a number of viruses attack bean plants, causing severe crop losses from disease.Although some bean varieties have resistance to one of these viruses, bean common mosaic virus, a closely related virus that occurs widely in sub-Saharan Africa (bean common necrotic mosaic virus) causes plants of these 'resistant' lines to die. Thus, many farmers prefer to plant susceptible bean lines despite the risk of crop loss and it is important to develop new strategies to defend this vital crop.The viruses are transmitted on the needle-like mouthparts of aphids. Importantly, the virus particles are lost by the aphid as soon as it feeds on a healthy plant; a weak link in the infection chain that we can exploit. Aphids are attracted to volatile chemicals released by plants. Virus infection can change these plants 'odours' so that aphids are encouraged to land on infected plants, acquire virus particles and then transport them to healthy plants. However, some bean varieties emit odours that are more attractive to aphids even when the plants are healthy. This means that if we plant mixtures of bean plants that include 'attractive' varieties we could decoy aphids away from the rest of the crop and slow disease transmission.Even better, if we can decoy virus-bearing aphids to land on plants that are resistant to the virus, then as soon as the aphid feeds it will lose the virus particles. This would effectively 'sanitize' the aphids and stop them spreading the virus, as well as dumping the virus into plants in which it cannot grow. An additional benefit of using a mixture containing a limited number of resistant plants is that viruses will be under less pressure to evolve new strains that can overcome resistance and, in cases where virus infection kills resistant plants, only a small proportion of the crop will be lost. Since aphids and related insects transmit the majority of plant viruses the method could be applied to a wide range of crops. Experiments under controlled conditions using varieties of a plant called Arabidopsis and an aphid transmitted virus called cucumber mosaic virus showed that aphid and virus spread can be altered using combinations of plants with different levels of attractiveness to aphids. A pilot field experiment in Uganda using mixtures of bean varieties with different degrees of attractiveness to aphids showed that aphids can be detained on more attractive varieties and inhibited from moving to other plants until close to harvest time, showing that our approach can work under farm conditions. Our multinational team will:A. Screen bean varieties that are popular in bean-growing regions in Kenya & Rwanda with respect to their attractiveness to aphids. We will identify the chemical odours emitted by plants that are most attractive to aphids. It is important to work with local varieties since farmers prefer certain bean types for hardiness under local conditions, cooking properties and local marketability. We will focus particularly on identifying aphid-attractive varieties carrying one or more virus resistance genes. B. Mathematically model the consequences of different arrangements and mixtures of attractive/repellent/susceptible/resistant plant lines on aphid and virus spread in fields. This will not only help us design our field and lab experiments but also to analyze the data we obtain. C. Test our models and lab experiments in Kenya & Rwanda to devise the most effective & practical field designs that minimize crop losses due to viral diseases.