This can be done by venting and heating, which fills the greenhouse with warm, drier air. Keep relative humidity below 85% to inhibit sporulation on infected plants and stop germination of spores on healthy plants. Others may survive in weed hosts near greenhouses.Ĭultural management: Cultural management practices for downy mildew diseases consist of excluding the disease from the greenhouse, managing relative humidity and strict sanitation. Some downy mildews produce long lasting survival spores called oospores that can persist in the soil indefinitely. The period between infection and symptom development can vary from days to weeks and is dependent largely upon environmental conditions. Air currents and splashing water dislodge these sporangia and spread the pathogen to nearby healthy plants. There may be a decrease in the quality and quantity of flowers.ĭisease cycle: Downy mildews reproduce through special structures called sporangia that develop on leaf undersides. As the disease progresses, lower leaves often wither and fall off. Symptoms: Under humid conditions, the fungus sporulates on leaf undersides, producing a downy growth which may be white, tan, gray or purple. In addition to roses, other hosts of this pathogen include caneberries (blackberries and raspberries), dewberries (Rubus spp.) and cherry laurel (Prunus laurocerasus). Downy mildew is also caused on rose by Peronospora sparsa. All types of roses are susceptible: wild roses and all cultivated roses including shrub roses. Perennial hosts include aster, buddleia, coreopsis, geranium (not Pelargonium), geum, gerbera, lamium, delphinium, veronica and viola. Most of the downy mildew fungi are very host specific and infect only one plant family. Downy mildew diseases are caused by oomycetes, a group of fungus-like organisms that also includes Pythium and Phytophthora species. They attack different plants under different environmental conditions and are controlled by different classes of fungicides. Downy mildew pathogens are very different from powdery mildews. The pathogens need wet leaves and high relative humidity (greater than 85%) to cause disease under these conditions, disease can occur rapidly and is difficult to control. Downy mildews thrive under the cool, wet conditions of spring and fall.
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