| Downy mildew | |
|---|---|
 Example of downy mildew (left) along withpowdery mildew on a grape leaf | |
| Causal agents | oomycetes |
| Hosts | plants |
Downy mildew refers to any of several types ofoomycete microbes that areobligate parasites of plants. Downy mildews exclusively belong to thePeronosporaceae family. Incommercialagriculture, they are a particular problem for growers ofcrucifers,grapes andvegetables that grow on vines. The prime example isPeronospora farinosa featured in NCBI-Taxonomy[1] and HYP3.[2] 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 withcucurbit 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.
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Initial symptoms include large, angular or blocky, yellow areas visible on the upper surface.[3] They can also be distinguished by their sporadic yellow patch appearance. As lesions mature, they expand rapidly and turn brown. The under surface of infected leaves appears watersoaked. Upon closer inspection, a purple-brown mold (conidiophores) becomes apparent. Small spores shaped like footballs can be observed among the mold with a 10x hand lens. As a result of numerous infectious sites, leaves might show a blighted appearance if the disease continues to spread. In disease-favorable conditions (cool nights with long dew periods), downy mildew will spread rapidly, destroying leaf tissue without affecting stems or petioles.[4]
One monophyletic clade ofPeronospora, known as thefloricolous downy mildews, only cause symptoms in the flowers of their hosts.[5] Infection is systemic,[5] withhyphae produced at low densities in the host xylem.[6] This clade is known from hostplants in theAsteraceae,Campanulaceae,Dipsacaceae,Lamiaceae, andOrobanchaceae.[5]
One floricolous species,Peronospora violacea, causes its hostSuccisa pratensis to produce longer, pinker petals,[7] and abort itsanthers.[6] Other authors report that on the same host it causes the hostcorollas to be brownish and dead-looking.[8]
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.[4] When downy mildew does pose a threat, the removal and destruction of plants displaying symptoms is good practice.
Fungicides applied specifically for downy mildew control may be unnecessary. Broad spectrum protectant fungicides such aschlorothalonil,mancozeb, and fixedcopper are at least somewhat effective in protecting against downy mildew infection. Systemic fungicides are labeled for use againstcucurbit downy mildew, but are recommended only after diagnosis of this disease has been confirmed.[4] In the United States, theEnvironmental Protection Agency has approvedoxathiapiprolin for use against downy mildew.[9] In Canada, a mixture ofzoxamide and mancozeb was registered for control of the mildew under the trademarkGavel (fungicide) as early as 2008.[10]
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.
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.[11]
Downy mildew ofbasil caused byPeronospora belbahrii has been a huge problem for both commercial producers and home growers. The disease was first reported in Italy in 2004,[12] was reported in the U.S. in 2007 and 2008[13][14] and has been steadily increasing in prevalence, distribution, and economic importance since then. The highest disease occurrence in basil leaves occurs at temperatures around 68°F.
Cucurbitaceae downy mildew (caused byPseudoperonospora cubensis) is specific to cucurbits (e.g., cantaloupe (Cucumis melo), cucumber (Cucumis sativus),pumpkin,squash, watermelon (Citrullus lanatus) and other members of the gourd family). The disease is one of the most significant diseases of cucurbits worldwide.
Plasmopara viticola is the causal agent of grapevine downy mildew.
Hop Downy Mildew (caused byPseudoperonospora humuli) is specific tohops (Humulus lupulus). The disease is the single most devastating disease in Western United States hopyards, since the microbe thrives in moist climates. Infected young hopbines 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.
A new and particularly aggressive form ofimpatiens downy mildew has recently emerged as a major threat to the cultivation of ornamentalimpatiens in the United States, where they are one of the most popular ornamental plants.
Peronospora sparsa attacks rose bushes (includingRosa canina) andRubus species.[15]
Peronospora manshurica infects soybeans, reducing photosynthetic activity, yield, and quality.[16] The fungus spreads byoospores 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-coloredsporangiophores on the underside of leaves easily distinguish the infection from other foliar diseases.[17] The disease is often controlled using the fungicidesmancozeb,maneb, orzineb.[16]
Downy mildew onspinach is caused byPeronospora effusa, an oomycete pathogen that poses a challenge to spinach production worldwide, especially in organic production.[18]
Plasmopara halstedii infectssunflowers, producingoospores which can remain dormant in the soil for many years.[19]
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