Peanut rust appears as minute circular orange brown (rusty) pustules, often on the underside of the leaves. These are often surrounded by a halo of yellow chlorosis. This reduces the growth of the leaves and the plant substantially. As the disease progresses, severely infected leaves become covered with rust pustules on both sides, turn yellow and “rusty", and finally shrivel. Elongated, reddish brown (and later black) pustules may also appear on pegs, stems and petioles. Defoliation can ensue. The disease can significantly reduce pod and fodder yield and oil quality.
Peanut rust survives in crop debris in the soil or in other legume plants that act as alternative hosts. Primary infection is from spores produced during this -borne stage that land onto the lower leaves. Secondary spread occurs through spores transmitted by wind. The infected spots can expand quickly during environmental conditions that favor the growth of the fungus, for example, warm temperatures (21 to 26°C) and wet, cloudy weather (fogs or heavy overnight dews). It also suppresses shoot and root growth of the plant, resulting in stunted growth. High concentrations of phosphorus fertilizer in the soil seems to slow down the development of the rust.
Biological agents can help to control the infection. Plant extracts of Salvia officinalis and Potentilla erecta have a protective effect on leaves against fungal growth. Other plant extracts such as flaxseed oil and peanut oil were also effective at reducing the incidence of the disease.
Always consider an integrated approach with preventive measures together with biological treatments if available. Chemical treatment may be inviable at later stages of the development of the disease. If fungicides are needed, spray products containing mancozeb, propiconazole or chlorothalonil (3 g/l of water). The application should be started straight after the first appearance of the infection and should be repeated after 15 days.