In the beginning orange-brown powdery masses can be found on the leaf underside. Small yellowish to brown lesions are later seen on both leaf sides. As the diseases develops the orange masses become dark-brown to almost black and form elongated lesions. Heavy infestation will cause the whole tree to appear yellow or brown and eventually causes premature defoliation. In the following growing season shoots will have poor growth causing stunted growth of vines. The disease can cause poor shoot growth, reduction of fruit quality and yield loss.
The symptoms are caused by the fungus Phakospora vitis. The fungal spores survive on plant debris and alternate hosts and are dispersed via wind. The pathogen of rust is developes in the spots which are present on the lower surface of leaf in the form of orange colored granules. Yellowish orange masses of uredinospores are produced on the lower leaf side, with dark necrotic spots on the upper surface. Temperatures above 20°C and moist weather conditions favor the disease development. The spores can easily be transported by wind and air currents.
Apply a foliar spray with sulfur containing fungicide. Avoid spraying during rainy weather, to ensure the fungicide can act against the pathogen.
Always consider an integrated approach with preventive measures together with biological treatments if available. Use fungicides containing Bordeaux mixture, captafol, difolatan, propiconazole, tebuconazole or azoxystrobin, which are said to significantly reduce the pathogen incidence. Rust can be controlled in vineyards by applying 3-4 sprays of Baycor (0.1%) at fortnight intervals during the subsequent growing seasons.