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Downy mildew refers to any of several types of oomycete microbes that are
obligate parasite An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasi ...
s of plants. Downy mildews exclusively belong to the Peronosporaceae family. In
commercial Commercial may refer to: * a dose of advertising conveyed through media (such as - for example - radio or television) ** Radio advertisement ** Television advertisement * (adjective for:) commerce, a system of voluntary exchange of products and s ...
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
, they are a particular problem for growers of
crucifer A crucifer or cross-bearer is, in some Christian churches (particularly the Roman Catholic Church, Anglican Communion, Lutherans, and United Methodist Church), a person appointed to carry the church's processional cross, a cross or crucifix wi ...
s,
grape A grape is a fruit, botanically a berry, of the deciduous woody vines of the flowering plant genus '' Vitis''. Grapes are a non- climacteric type of fruit, generally occurring in clusters. The cultivation of grapes began perhaps 8,000 years a ...
s and
vegetable Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the edible flower, flowers, ...
s that grow on vines. The prime example is '' Peronospora farinosa'' featured in NCBI-Taxonomy and HYP3. This pathogen does not produce survival structures in the northern states of the United States, and overwinters as live mildew colonies in Gulf Coast states. It progresses northward with cucurbit production each spring. Yield loss associated with downy mildew is most likely related to soft rots that occur after plant canopies collapse and sunburn occurs on fruit. Cucurbit downy mildew only affects leaves of cucurbit plants.


Symptoms

Initial symptoms include large, angular or blocky, yellow areas visible on the upper surface. As lesions mature, they expand rapidly and turn brown. The under surface of infected leaves appears watersoaked. Upon closer inspection, a purple-brown mold (see arrow) becomes apparent. Small spores shaped like footballs can be observed among the mold with a 10x hand lens. In disease-favorable conditions (cool nights with long dew periods), downy mildew will spread rapidly, destroying leaf tissue without affecting stems or petioles.


Treatment and management


Cultural options

Because the downy mildew pathogen does not overwinter in midwestern fields, crop rotations and tillage practices do not affect disease development. The pathogen tends to become established in late summer. Therefore, planting early season varieties may further reduce the already minor threat posed by downy mildew.


Chemical control

Fungicides applied specifically for downy mildew control may be unnecessary. Broad spectrum protectant fungicides such as
chlorothalonil Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) is an organic compound mainly used as a broad spectrum, nonsystemic fungicide, with other uses as a wood protectant, pesticide, acaricide, and to control mold, mildew, bacteria, algae. C ...
, mancozeb, and fixed
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
are at least somewhat effective in protecting against downy mildew infection. Systemic fungicides are labeled for use against cucurbit downy mildew, but are recommended only after diagnosis of this disease has been confirmed. In the United States, the
Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale ...
has approved
oxathiapiprolin Oxathiapiprolin (trade names Orondis, Zorvec, and Segovis) is a fungicide. In the United States, the Environmental Protection Agency has approved it for use against several fungal diseases including downy mildew and various ''Phytophthora'' spec ...
for use against downy mildew. In Canada, a mixture of zoxamide and mancozeb was registered for control of the mildew under the trademark Gavel (fungicide) as early as 2008.


Organic control

One way to control downy mildew is to eliminate moisture and humidity around the impacted plants. Watering from below, such as with a drip system, and improve air circulation through selective pruning. In enclosed environments, like in the house or in a greenhouse, reducing the humidity will help as well.


Resistant cultivars

Recent breeding efforts have produced resistant basil cultivars. Choosing resistant cultivars, in combination with other prevention and control measures, can help growers with high downy mildew pressure still produce a viable crop.


Plant-specific mildews


Basil

Downy mildew of basil caused by '' Peronospora belbahrii'' has been a huge problem for both commercial producers and home growers. The disease was first reported in Italy in 2004, was reported in the U.S. in 2007 and 2008 and has been steadily increasing in prevalence, distribution, and economic importance since then.


Cucurbitaceae

Cucurbitaceae The Cucurbitaceae, also called cucurbits or the gourd family, are a plant family consisting of about 965 species in around 95 genera, of which the most important to humans are: *''Cucurbita'' – squash, pumpkin, zucchini, some gourds *'' Lagen ...
downy mildew (caused by '' Pseudoperonospora cubensis'') is specific to cucurbits (e.g., cantaloupe (''
Cucumis melo ''Cucumis melo'', also known as melon, is a species of '' Cucumis'' that has been developed into many cultivated varieties. The fruit is a pepo. The flesh is either sweet or bland, with or without a musky aroma, and the rind can be smooth (such a ...
''), cucumber ('' Cucumis sativus''),
pumpkin A pumpkin is a vernacular term for mature winter squash of species and varieties in the genus ''Cucurbita'' that has culinary and cultural significance but no agreed upon botanical or scientific meaning. The term ''pumpkin'' is sometimes use ...
, squash, watermelon (''
Citrullus lanatus Watermelon (''Citrullus lanatus'') is a flowering plant species of the Cucurbitaceae family and the name of its edible fruit. A scrambling and trailing vine-like plant, it is a highly cultivated fruit worldwide, with more than 1,000 vari ...
'') and other members of the gourd family). The disease is one of the most significant diseases of cucurbits worldwide.


Grapes

'' Plasmopara viticola'' is the causal agent of grapevine downy mildew.


Hops

Hop Downy Mildew (caused by '' Pseudoperonospora humuli'') is specific to
hops Hops are the flowers (also called seed cones or strobiles) of the hop plant '' Humulus lupulus'', a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to w ...
(''Humulus lupulus''). The disease is the single most devastating disease in Western United States hopyards, since the microbe thrives in moist climates. Infected young hop bines become stunted with thickened clusters of pale curled leaves. These spikes have a silvery upper surface, while the undersides of leaves become blackened with spores. These dwarfed spikes are called "basal spikes". 'Lateral' or 'terminal' spikes occur further up the vine. An entire hop crop could be devastated in only a few days.


Ornamentals

A new and particularly aggressive form of impatiens downy mildew has recently emerged as a major threat to the cultivation of ornamental
impatiens ''Impatiens'' is a genus of more than 1,000 species of flowering plants, widely distributed throughout the Northern Hemisphere and the tropics. Together with the genus '' Hydrocera'' (one species), ''Impatiens'' make up the family Balsaminacea ...
in the United States, where they are one of the most popular ornamental plants.


Soybeans

''
Peronospora manshurica ''Peronospora manshurica'' is a plant pathogen. It is a widespread disease on the leaves of soybeans and other crop plants. The fungi is commonly referred to as downy mildew, "leafspot", or "leaf-spot". Symptoms The most visible symptoms of ''P ...
'' infects soybeans, reducing photosynthetic activity, yield, and quality. The fungus spreads by oospores on diseased leaves and/or on infected seed. The disease spreads in environments with high humidity and favors temperatures between 20-22 °C. Tufts of grayish to pale-colored
sporangiophore {{Short pages monitor