Infection starts in lower parts of the plant and then moves upwards, affecting mainly young leaves. First symptoms appear around flowering stage in the form of minute, brick-red specks on the underside of leaves, often along the veins. Later, these spots increase in size and number and become reddish brown or black. As the disease progresses, they become covered by raised, pale-brown fungal pustules visible to the naked eye. Some of them coalesce and form irregular dark brown spots surrounded by a yellowish halo. They are now present on both sides of the leaf, occasionally also on the petioles and stems. Premature defoliation of plants is possible.
Soybean rust is an aggressive disease caused by the fungus Phakopsora pachyrhizi. It is not seed borne and needs green living tissue to live and complete its life cycle. When there are no soybean plants around, it needs an alternative host to survive. The spores produced in the pustules fly from plant to plant and penetrate the plant cells directly, rather than through pores or wounds in the leaf tissue. The development of the disease is favored by 6 to 12 hours of continuous leaf wetness, cool to moderate temperature (16 to 28°C) and high humidity (>75%).
Apply products containing essential oils of Corymbia citriodoria at 1 %, Cymbopogon nardus at 0,5%, and Thymus vulgaris at 0,3 % to reduce the severity of the infection.
Always consider an integrated approach with preventive measures together with biological treatment if available. Selecting the right fungicide and applying it at the right time is crucial. Apply fungicides based on hexaconazole (2 ml/l of water) and propiconazole (1 ml/l of water). Apply formulations of zinc iron-maneb complex periodically throughout the growing season.