Bean Golden Mosaic Virus
First symptoms generally appear on trifoliate leaves. Bright yellow, chlorotic veins appear on newly emerged leaves. The venial chlorosis spreads further and gives the leaf a characteristic net-like aspect, with yellow veins contrasting sharply with dark green tissues. Later on, the chlorosis starts to expand and covers the rest of the leaf with a mottled pattern in different shades of yellow. Leaves that emerged after the appearance of symptoms might be distorted, curled, stiff and leathery. Pods fail to expand and can also curl down. Plants infected at a very early stage have fewer pods and poor seed production as well as poor seed quality.
The virus is transmitted in a persistent manner by the whitefly Bemisia tabaci. Plants can also be infected via mechanical injury during field work. The virus is not passed from plant to plant in a systematic manner, nor is it seed- or pollen-borne. Beans usually get infected when volunteer plants or host weeds are present in the field. The virus multiplies in the transport tissues of plants, which explains why veins are affected first. The appearance of visible symptoms and their severity is favored by elevated temperatures around 28°C. Cooler conditions (around 22°C) may delay the multiplication of the virus and the development of symptoms.
Application of leaf extracts of Iresine herbstii (Herbst's bloodleaf) and Phytolacca thyrsiflora can partly inhibit virus infection and results in less incidence in the field. Extracts of the beneficial fungus Beauveria bassiana have insecticide properties against the adults, eggs and nymphs of bemisia tabaci.
Always consider an integrated approach with preventive measures together with biological treatments available. Chemical control of viral infections is not possible. As for the control of whiteflies, very few treatments are effective.