| Pseudoperonospora humuli | |
|---|---|
Scientific classification  | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | SAR |
| Clade: | Stramenopiles |
| Phylum: | Oomycota |
| Order: | Peronosporales |
| Family: | Peronosporaceae |
| Genus: | Pseudoperonospora |
| Species: | P. humuli |
| Binomial name | |
| Pseudoperonospora humuli (Miyabe & Takah.) G.W. Wilson, (1914) | |
| Synonyms | |
Peronoplasmopara humuliMiyabe & Takah., (1905) Contents | |
Pseudoperonospora humuli is a plant pathogen that causes downy mildew on hops.
Downy mildew on hops is caused by the pathogen Pseudoperonospora humuli, an oomycete protist. P. humuli is an obligate biotrophic pathogen, meaning that it can only live and grow in living host tissue. P. humuli, like most downy mildews, is highly host-specific and thus will only infect hop (Humulus lupulus) and also Japanese hop (Humulus japonicas). [1]
The most characteristic symptom of hop downy mildew are the “basal spikes” that form on the plant. These structures result from systemically infected shoots and are “stunted and have brittle, downward-curled leaves from which masses of purple to black sporangiophores and sporangia are produced.” [2] These spikes can exist as three distinct types:
1) “Primary spikes”, which arise from infected hop crowns
2) “Secondary spikes”, which arise from infected apical meristems
3) “Aerial spikes”, which occur on lateral branches or trained vines
Primary and secondary spikes result in stunting shoots and brittle leaves. Primary spikes also cause shortened internodes. Aerial spikes adversely affect the development of the plant, resulting in bines (the long, flexible stem the plant uses to climb) falling away from the string on which hops is allowed to climb. Infected leaves and cones show symptoms as well. Infected leaves will change color, ranging from purple-gray to black downy growth. Infected cones become hard and brown and display disrupted development. [1]
Downy mildew on hops has a polycyclic disease cycle. [3] The pathogen overwinters as mycelium in hop crowns. The pathogen infects crown buds, resulting in the emergence of infected shoots and primary basal spikes in the spring under correct conditions. This initiates the disease cycle. [4] Infected crowns may also produce uninfected shoots. The cycle becomes polycyclic as sporangiophores with sporangia emerge on the underside of infected leaves. Mature sporangia are dispersed via wind and release zoospores to infect leaves, cones, and shoots. This secondary cycle or sporulation and infection persists throughout the season. Mycelia grow systemically throughout the plant, leading to the infection of the crown and buds in which the pathogen will overwinter. [3]
A sexual stage also exists for P. humuli, in which an antheridium fertilizes an oogonium to produce a recombinant oospore. While oospores are classically thought to be the chief survival structure of oomycetes, their role in primary infection in downy mildew of hops is uncertain. [5]
The environmental conditions favored by P. humuli are consistent with other oomycetes and fungal-like pathogens. High levels of moisture and warmer temperatures facilitate the germination of many pathogens. [6]
The development and severity of the disease is dependent on a number of environmental factors. Studies have found that hours of relative humidity >80%, degree-hours of wetness, and mean night temperature are paramount in predicting model plants' susceptibility to infection. [2] The pathogen favors extended periods of wetness, high humidity, and mildly warm temperatures ranging from 15.5–21 °C (59.9–69.8 °F). [3] Leaf infection can occur at temperatures as low as 5 °C if wetness persists for 24 hours or longer, indicating the primary role of moisture level in infection. [1] [3]
Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables 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.
Peronosporaceae are a family of water moulds that contains 21 genera, comprising more than 600 species. Most of them are called downy mildews.
Peronospora sparsa is an oomycete plant pathogen that causes downy mildew in berry producing plants; especially in the genus's Rubus and Rosa. Downy mildew plant pathogens are often host specific and cause problems in cloudberries, blackberries, boysenberries, strawberries, and arctic bramble. Since they are host specific, Peronospora sparsa will not cause downy mildew in grapes because a different plant pathogen causes downy mildew in grapes; Plasmopara viticola. Although it depends on the cultivar, symptoms do not normally start until later stages of disease and can look different on different plants. The most common symptoms include red lesions in the veins of leaves, with dry and deformed berries.
Phytophthora erythroseptica—also known as pink rot along with several other species of Phytophthora—is a plant pathogen. It infects potatoes causing their tubers to turn pink and damages leaves. It also infects tulips (Tulipa) damaging their leaves and shoots.
Pythium aphanidermatum is a soil borne plant pathogen. Pythium is a genus in the class Oomycetes, which are also known as water molds. Oomycetes are not true fungi, as their cell walls are made of cellulose instead of chitin, they are diploid in their vegetative state, and they form coenocytic hyphae. Also, they reproduce asexually with motile biflagelette zoospores that require water to move towards and infect a host. Sexually, they reproduce with structures called antheridia, oogonia, and oospores.
Sclerophthora macrospora is a protist plant pathogen of the class Oomycota. It causes downy mildew on a vast number of cereal crops including oats, rice, maize, and wheat as well as varieties of turf grass. The common names of the diseases associated with Sclerophthora macrospora include "crazy top disease" on maize and yellow tuft disease on turf grass. The disease is present all over the world, but it is especially persistent in Europe.
Podosphaera macularis is a plant pathogen infecting several hosts including chamomile, caneberrie, strawberries, hop, hemp and Cineraria. It causes powdery mildew of hops.
Albugo occidentalis, the causal agent of spinach white rust, is an oomycete plant pathogen, although some discussions still treat it as a fungal organism. Albugo occidentalis is one of the most important spinach diseases in North America, found throughout the United States east of the rocky mountains.
Albugo is a genus of plant-parasitic oomycetes. Those are not true fungi (Eumycota), although many discussions of this organism still treat it as a fungus. The taxonomy of this genus is incomplete, but several species are plant pathogens. Albugo is one of three genera currently described in the family Albuginaceae, the taxonomy of many species is still in flux.
Bremia lactucae is a plant pathogen. This microorganism causes a disease of lettuce denominated as downy mildew. Some other strains can be found on 36 genera of Asteraceae including Senecio and Sonchus. Experiments using sporangia from hosts do not infect lettuce and it is concluded that the fungus exists as a quantity of host-specific strains. Wild species, such as Lactuca serriola, or varieties of Lactuca can hold strains that infect lettuce, but these pathogens are not sufficiently common to seriously infect the plant.
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".
Plasmopara halstedii is a plant pathogen infecting sunflowers. The species is one of many pathogens commonly referred to as downy mildew. P. halstedii originated in North America.
Plasmopara viticola, the causal agent of grapevine downy mildew, is a heterothallic oomycete that overwinters as oospores in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet condition release zoospores. Zoospores are splashed by rain into the canopy, where they swim to and infect through stomata. After 7–10 days, yellow lesions appear on foliage. During favorable weather the lesions sporulate and new secondary infections occur.
Pseudoperonospora cannabina is a plant pathogen that causes downy mildew, which is a fungal-like disease caused by an oomycete.
Peronosclerospora sorghi is a plant pathogen. It is the causal agent of sorghum downy mildew. The pathogen is a fungal-like protist in the oomycota, or water mold, class. Peronosclerospora sorghi infects susceptible plants though sexual oospores, which survive in the soil, and asexual sporangia which are disseminated by wind. Symptoms of sorghum downy mildew include chlorosis, shredding of leaves, and death. Peronosclerospora sorghi infects maize and sorghum around the world, but causes the most severe yield reductions in Africa. The disease is controlled mainly through genetic resistance, chemical control, crop rotation, and strategic timing of planting.
Pseudoperonospora cubensis is a species of water mould known for causing downy mildew on cucurbits such as cantaloupe, cucumber, pumpkin, squash and watermelon. This water mould is an important pathogen of all these crops, especially in areas with high humidity and rainfall, such as the eastern United States. In most years the disease is an annual, late-season problem on squash and pumpkin in the eastern and central United States, however, since 2004 it has become one of the most important diseases in cucumber production. Considered a highly destructive foliar disease of cucurbits, successful breeding in the mid-twentieth century provided adequate control of downy mildew in cucumber without the use of fungicides. The resurgence in virulence has caused growers great concern and substantial economic losses, while downy mildew in other cucurbit crops continues to be a yearly hindrance.
Peronospora is a genus of oomycetes that are obligate plant pathogens of many eudicots. Most species in this group produce a downy mildew disease, which can cause severe damage to many different cultivated crops, as well as wild and ornamental plants. There are 19 genera that produce downy mildew, and Peronospora has been placed alongside Pseudoperonospora in the group of downy mildews with coloured conidia. Peronospora has far more species than any other genus of the downy mildews. However, many species have been moved from this genus to be reclassified to other or new genera. Among these was the most famous Peronospora species, formerly known as Peronospora parasitica, and now known as Hyaloperonospora parasitica. Now, the Peronospora species of most importance is likely the Peronospora tabacina. Peronospora tabacina causes blue mold on tobacco plants and can severely reduce yields of this economically important crop to the point where it has been classified as a bioweapon.
Peronospora destructor is a plant pathogen. It causes downy mildew on leaves of cultivated and wild Allium. Allium cepa is most often affected, while Allium schoenoprasum (chives) and Allium porrum (leek) are only occasionally affected.
Buckeye rot of tomato is caused by three species of pathogens in the genus Phytophthora: P. nicotianae var. parasitica, P. capsici, and P. drechsleri. It is an oomycete that thrives in warm, wet conditions and lives in the soil. It is characterized by a bull’s eye pattern of dark brown rotting on the tomato fruit, and affects fruit that is close to, or lying on the soil. The easiest management is to keep the plant out of contact with the soil, although other chemical methods can be very effective. This disease commonly occurs in the southeast and south central areas of the United States. The disease has affected a large portion of crop yield in the United States as well as India. The relatively small genome size of Phytophthora parasitica compared to Phytophthora infestans gives researchers the unique ability to further examine its ability to cause disease.
| Authority control databases: National |
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