The Plant Health Instructor. Fry and Niklaus J. The diseases they cause include seedling blights, damping-off, root rots, foliar blights and downy mildews.
Some notable diseases are the late blight of potato, downy mildew of grape vine, sudden oak death, and root and stem rot of soybean. Because of their filamentous growth habit, nutrition by absorption, and reproduction via spores, oomycetes were long regarded by plant pathologists as lower fungi.
However, as our understanding of evolutionary relationships has grown, it is now clear that "Oomycetes asexual reproduction advantages" group of organisms is unrelated to the true fungi. Indeed, fungi appear more closely related to animals than to oomycetes, and oomycetes are Oomycetes asexual reproduction advantages closely related to algae and to green plants Figure 1. Phylogenetic analyses using genes and intergenic regions have confirmed the assertions of earlier systematists that the oomycetes are different from fungi.
The data from these molecular analyses have been particularly convincing to non-systematist plant pathologists. There are many features distinguishing oomycetes from fungi. Septa cell walls in the hyphae are rare, resulting in a multinucleate condition termed coenocytic. The nuclei of vegetative cells are typically diploid. Many species produce wall-less, biflagellated swimming spores zoospores in structures called sporangia.
One of the most distinguishing characteristics is the production of zoospores produced in sporangia. The anterior flagellum of a zoospore is a tinsel type, while the posterior flagellum is a whiplash type; both are typically attached in a ventral groove Figure 2.
Although wall-less, zoospores retain a consistent but flexible shape. Zoospores can swim in water films on leaf surfaces, in soil water, in hydroponic media and in natural bodies of water. After a time of free swimming the zoospores settle on a surface, retract their flagella, and secrete a mucilaginous matrix which affixes them to the surface.
Sporangia of different taxa within the group are of diverse shapes and characteristics Figures They may be terminal or intercalary within a hyphal filamentbulbous or not, and if terminal, caducous sporangia detach readily or not.
In some species, the ability to Oomycetes asexual reproduction advantages zoospores has been lost, and sporangia are thought to have evolved into structures that germinate directly to produce germ tubes. Sexual reproduction
Oomycetes asexual reproduction advantages via the production of gametangia: Because meiosis does not occur until the formation of gametangia occurs, the vegetative nuclei are diploid.
The morphology of antheridium attachment has been an important feature in morphological taxonomy of some genera. In some genera the antheridium is attached to the side of the oogonium paragynous, Figure 8but in other genera, the antheridium surrounds the base of the oogonium amphigynous, Figure 9. Typically each individual produces both antheridia and oogonia. In some species, two distinct mating types occur and both are required for sexual reproduction these are heterothallic as opposed to homothallic species.
In heterothallic oomycetes, the gametangia are produced only in the presence of both mating types due to the fact that a hormone produced by one thallus stimulates the other to produce gametangia. In other species, sexual reproduction occurs within a single individual these are homothallic individuals.
Unlike the heterothallic species, homothallic individuals do not require distinct mating types, but can reproduce sexually by selfing. All Pythium and some Phytophthora species are homothallic. The fertilized oogonium develops into a thick-walled oospore Figure When the oospores are produced in plant tissue, they may occupy a large portion of the tissue Figure Oomycetes asexual reproduction advantages of many species have been shown to be able to survive for years in soil.
After a period of dormancy often of apparently diverse and undefined durations oospores germinate to produce hyphae, which may immediately produce a sporangium Figure Oospore germination is often asynchronous; that is, some oospores germinate while others do not. Germination and survival of oospores is dependent on environmental conditions: Some species produce thick-walled survival structures called chlamydospores Figure Our understanding of the relationships among oomycetes is evolving rapidly as we gather additional information, particularly from molecular analyses.
The techniques have evolved rapidly and analysis "Oomycetes asexual reproduction advantages" DNA sequence provides a common criterion for assessing relationships. The analysis of probable relationships among the major genera of oomycetes is depicted in Figure While PythiumPhytophthora and Peronospora appear related, the relationship of these organisms with the grass downy mildews remains problematic.
Systematics of the Peronosporomycetes including accounts Oomycetes asexual reproduction advantages the marine straminipilous protists, the plasmodiophorids and similar organisms.
Dordrecht, Boston, Kluwer Academic Publishers. The table below identifies some of the genera in the various orders:. PythiumPhytophthora Subclass: Downy Mildews of the Poaceae, such as: Several recent discoveries and important developments have taken the oomycete research community by storm.
An important development is that the genomes of several oomycete species have been sequenced Hyaloperonospora arabidopsidisPhytophthora sojaeP. One discovery is of a group of proteins effectors that are secreted by oomycetes and delivered into host cells; these proteins specifically aid pathogenicity.
Effectors are now recognized by "Oomycetes asexual reproduction advantages" amino acid motifs RxLR-dEER, where R, L, and E stand for the amino acids arginine, leucine and glutamic acidand it is now demonstrated that these motifs are required for secretion and delivery into the host cell. Each of the sequenced genomes contains hundreds of predicted effectors.
The availability of the genome sequence also facilitates investigations into the genes involved in basic developmental biology. Already, genes specific to sporulation and zoosporogenesis have been predicted and their function is now being tested.
Finally, there are several developments at the population level. Phytophthora ramorum has emerged almost overnight as a very important pathogen causing sudden oak death and other diseases with a surprisingly large host range. Recent global migrations of P. Of more academic interest is Hyaloperonospora arabidopsidis previously known as Peronospora parasitica that has become a model Oomycetes asexual reproduction advantages 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. However, "Oomycetes asexual reproduction advantages" 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: As an asexual organism in nature or in agriculture, P. 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 Oomycetes asexual reproduction advantages transit sufficiently long to travel many kilometers in moving bodies of
Oomycetes asexual reproduction advantages. The pathogen is favored by moist, cool environments: 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.
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, "Oomycetes asexual reproduction advantages" 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. 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. Oomycetes asexual reproduction advantages 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.