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On young developing leaves, diffuse yellowish spots of different sizes appear on the upper surface. As the disease progresses, these spots enlarge and become angular, delimited by veins. Their center becomes necrotic, with different shades of brown, and they can be surrounded by a yellow halo. Usually after a series of warm humid nights, a dense, white to grayish cottony layer develops beneath these spots. The fungus extracts nutrients from the plant and causes a stunting of growth. Fruits and other plant parts can also be affected. The defoliation and the dwarfing or killing of young shoots, flowers or fruits lead to stunted growth and poor yields. In contrast to powdery mildew, the coating appear exclusively on the underside of leaves and its growth is limited by the main veins. Moreover, it cannot be removed easily.
The symptoms are caused by fungi of the group of the Peronosporalesa and can be very destructive in shaded areas with frequent rainfalls and warm temperatures (15-23°C). These fungi have adapted pretty well to their hosts, meaning that each major crops harbors its own species of fungus. The fungus overwinters in infected plant debris or shoots, in the soil or on alternative hosts (crops and weeds). Wind and rain splashes spread the spores during favorable conditions. The spores germinate and produce structures that enter the leaf through natural pores on the underside of leaves. There it starts to spread through tissues, eventually outgrowing the internal tissues and forming the characteristic mildew coating outside.
Commercial biological treatments for fighting downy mildew are available. In mild cases, it is often better not to do anything and wait until the weather improves. In some cases, organic pre-infection fungicides can help to avoid contamination of plants. These include copper-based fungicides, such as Bordeaux mixture.
Always consider an integrated approach with preventive measures together with biological treatments if available. Protectant fungicides can help to prevent the contamination of plants but they have to be sprayed properly to the underside of leaves. Fungicides of the family of the dithiocarbamates can be used. Post-infection fungicide must be applied immediately after the detection of the first symptoms. Commonly used post-infection fungicides include fosetyl-aluminumazoxystrobin and phenylamides.
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