- Morphological characteristics of oomycetes
- Oomycete genetics and genomics : diversity, interactions and research tools
- Oomycete Genetics and Genomics: Diversity, Interactions and Research Tools
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Of more academic interest is Hyaloperonospora arabidopsidis previously known as Peronospora parasitica that has become a model pathogen because it infects the model host plant Arabidopsis. This is the pathogen that caused the Irish potato famine in the mid th century. It was first reported in the eastern United States just prior to reports of its presence in Europe. Prior to that time, it was not known to western science.
Morphological characteristics of oomycetes
However, its devastating impact on potatoes and the terrible misery it has caused have made it infamous. Foliage, stems and tubers are susceptible Figure 15, It is a heterothallic species, with only one mating type the A1 historically dominating the worldwide population with the exception of populations that existed in Mexico; both mating types have existed in central Mexico for a very long time. However, in the late 20th century migrations from Mexico distributed a very complex and diverse population containing both A1 and A2 mating types to Europe: subpopulations were later distributed from Europe to other locations.
As an asexual organism in nature or in agriculture, P.
Oomycete genetics and genomics : diversity, interactions and research tools
However, the relatively recent migrations of the A1 and A2 mating types increases the chances of sexual reproduction and the production of oospores that would represent a long-term survival mechanism for this devastating pathogen. Phytophthora infestans is unusual for a Phytophthora in that it is an aerial pathogen. That is, it infects and reproduces mainly on the above ground portions of its host. Sporangia are dehiscent detach easily when mature and under cloudy conditions can survive transit sufficiently long to travel many kilometers in moving bodies of air.
Under favorable conditions, the asexual life cycle sporangium germination, infection, lesion growth, sporulation can be completed within as few as four days, but symptoms may not be visible for the first days after initial infection. The dominant influence of weather on the infection and sporulation process of P. These investigations have resulted in algorithms Dutch Rules, Beaumont periods, rain favorable days, severity values, etc. The explosive potential of this pathogen is legendary, dramatic and real. When the disease is uncontrolled and when environmental conditions are favorable to the pathogen, fields of acres will succumb to the disease within just a few days.
The general susceptibility of potatoes and tomatoes has stimulated much effort to develop resistant plants as well as to understand the pathogenicity of P. Single large-effect genes for resistance R genes have been identified and deployed.
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Unfortunately, because of variation in the pathogen population, the effect of these genes has not been long lasting. R genes recognize specific components of pathogen proteins effectors that are injected into the host cell. Mutation in these effectors can enable the pathogen to escape recognition and avoid the resistance mechanisms. There have also been efforts made to create resistant plants based on a less well-understood mechanism that may involve many genes. However, the most popular cultivars of potatoes and tomatoes are quite susceptible, necessitating the use of fungicides to protect plants.
Effective disease suppression requires a strategy integrating several tactics. Because infected seed tubers can be a source of the pathogen, it is important to plant only healthy seed tubers. It is also important to eliminate any tubers that might have survived from one cropping season to the next, whether these tubers survived in soil after harvest or were discarded after storage.
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Some Solanaceous weeds can also harbor the pathogen, and any infected weeds in or near the crop need to be eliminated. Fungicides are used in connection with a good scouting monitoring program to learn if the pathogen is present and application timing and rates are often aided by an appropriate forecast. This pathogen was introduced to Europe from North America in the late 19 th century. It accompanied wild grape plants imported for their resistance to the sap-sucking insect pest Phylloxera.
Oospores germinate to produce sporangia with zoospores, which can be splash-dispersed to cause lesions. Sporangia from primary lesions Figure 17, 18 can also be wind-dispersed.
Symptoms on leaves are small yellow lesions also known as oil spots. European grape varieties are susceptible to P. Forecast systems are used to improve the efficiency of disease suppression. This devastating, omnivorous pathogen was first isolated in the early 20 th century, and is thought by some to have originated in Papua New Guinea, but it now has a worldwide distribution.
Its host range is thought to include more than species of plants. It is heterothallic with A1 and A2 mating types, but sexual recombination is not thought to have a significant role in population diversity. Often, populations consist of a single mating type. This pathogen infects fibrous roots and can also survive and grow saprophytically in soil. It produces chlamydospores so that even in the absence of sexual reproduction, it can survive for long periods in soil.
The asexual cycle can be very rapid during wet conditions and is described elegantly by Hardham Molecular Plant Pathology 6: ; only a summary is presented here.
Oomycete Genetics and Genomics: Diversity, Interactions and Research Tools
Sporangia are produced on sporangiophores and the sporangia release zoospores. Zoospore formation is triggered by a decrease in temperature, resulting in the change of expression of a large number of genes. Morphologically, the cytoplasm becomes delimited into uninucleate compartments, with membranes forming around each and with each developing flagella and a water expulsion vacuole. The vesicle is ephemeral and the zoospores are quickly released into the environment. The zoospores may travel distances of several centimeters and are attracted to potential infection sites where they encyst.
A mucilaginous material is secreted over the surface and they become affixed to the host surface encyst within minutes upon arrival and quickly form a cell wall.
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Germination occurs rapidly after encystment and the germ tubes penetrate the root epidermis. Colonization of host tissue follows and in susceptible tissues, sporulation may occur within three days. This pathogen is a threat to both agricultural and native plants. One of the most seriously affected areas is in Australia Figure 19 where the pathogen was introduced in the early 20 th century.
In addition to Eucalyptus , many other native species are also susceptible to this pathogen and the disease remains severe to the present time. Unfortunately, construction of logging roads has proven to be a mechanism for transport of this pathogen throughout the forest.
Disease is most severe under wet and warm conditions, and disease abatement is associated with cooler drier weather. However, investigators are expecting the range of this pathogen in the northern hemisphere to extend further north in response to global climate changes. For agricultural plants, phosphonate fungicides effective against oomycetes, but ineffective against fungi have been particularly helpful to suppress the disease and are used extensively.
Host resistance has also been investigated with some limited success. It also causes Ramorum blight and shoot dieback on many ornamental plants. The disease was simultaneously discovered in Europe and in California in the s affecting oak and nursery crops such as rhododendron and viburnum. In the United States the sudden oak death disease gained instant notoriety because it led to extensive death in coast live oak and tan oak in California Figure One aspect of this pathogen that differentiates it from many other oomycetes is that P.
Symptoms differ on various hosts Figures and can be confused with diseases caused by other organisms or even environmental factors. Although a center of origin has not been found, scientists currently believe that P. In US nurseries this pathogen is currently managed through quarantine, eradication Figure 24 and exclusion: nurseries are trying to avoid establishment of the pathogen by excluding it from their operations and the Oregon Department of Forestry is trying to eradicate P. Like P. Lineage EU1 was first found in Europe but has now also been found in the US and Canada in select nursery environments.
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NA1 was the lineage found to cause significant mortality on coast live oak and tanoak forests in California and Oregon and has been spread throughout North America. The list is composed of plant pathogens not present in the US, but deemed to be a potential bioterrorism threat to US agriculture. Sclerophthora rayssiae var. Survival of this pathogen is via oospores in infected seeds and soil or plant debris. The disease cycle involves both sexual and asexual reproduction.
Oospores germinate to produce sporangia, which then release zoospores that penetrate leaf tissue.
Lesions are initially interveinal and appear as chlorotic, brownish or reddish stripes on the leaves Figure Sporangia are dispersed short distances via wind or rain splash, and germinate to produce zoospores or, less commonly, to produce a germ tube to repeat the cycle. Oospores are produced in necrotic tissue and can survive for years in soil or in plant debris. This oomycete does not produce zoospores, but rather the sporangia germinate directly and have been referred to as conidia. Initial infections of roots are thought to result from oospores in soil.
Most infections are initiated by sporangia conidia produced from infected foliage that can be distributed to other plants where they germinate directly and initiate local lesions. To further confirm the ploidy inferences, we classified 40, SNPs as derived or ancestral based on information from the outgroup species P.