Infection starts in lower parts of the plant and then moves upwards. First symptoms appear around flowering stage in the form of small, grey spots on the underside of leaves and along the veins. Later, these spots increase in size and number, coalesce and turn reddish brown or black, often surrounded by a yellowish halo. They are now present on both sides of the leaf, the petioles, and sometimes the stems. As the disease progresses, they become covered by raised, pale-brown fungal pustules visible to the naked eye.
Soybean rust is an aggressive disease caused by the fungus Phakopsora pachyrhizi. It needs mild winters and alternative hosts to survive when there are no soybean plants around because it can't live and reproduce without green living tissue. Rust spores can penetrate the plant cells directly, rather than through pores or wounds in the leaf tissue. The development of the disease is favoured by 6 to 12 hours of continuous leaf wetness, moderate temperature (15 to 25°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 of the chloronitriles, strobilurins, and triazoles groups. Apply formulations of zinc ion-maneb complex periodically throughout the growing season.