Yellowish spots of different sizes appear on the surface of young developing leaves. The spots later enlarge and become angular and delimited by veins. Their center becomes necrotic with different shades of brown. A dense white to grayish cottony layer develops beneath the spots after a series of warm humid nights, and they disappear as soon as it becomes sunny. Young shoots defoliate or experience stunted growth. The disease also affects the fruits and other plant parts.
The symptoms are caused by fungi of the group of the Peronosporales 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 harbor 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 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-aluminum, azoxystrobin and phenylamides (ex. metalaxyl-M).