Plasmopara halstedii
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''Plasmopara halstedii'' is a plant pathogen infecting
sunflower The common sunflower (''Helianthus annuus'') is a large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), as ...
s. The species is one of many pathogens commonly referred to as downy mildew. ''P. halstedii'' originated in North America. ''Plasmopara halstedii'' oospores produce a thin wall which are resistant structures, sexually produced that are essential for its continuation. After entering an area, the eradication of the pathogen is difficult due to the formation of oospores, which can remain viable in soil for many years.


Hosts and symptoms

''Plasmopara halstedii'' is an obligate
biotroph Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
that attacks the flowering plants of the family
Asteraceae The family Asteraceae, alternatively Compositae, consists of over 32,000 known species of flowering plants in over 1,900 genera within the order Asterales. Commonly referred to as the aster, daisy, composite, or sunflower family, Compositae w ...
, found to infect the genus’ ''
Helianthus ''Helianthus'' () is a genus comprising about 70 species of annual and perennial flowering plants in the daisy family Asteraceae commonly known as sunflowers. Except for three South American species, the species of ''Helianthus'' are native to ...
, Bidens,
Artemisia Artemisia may refer to: People * Artemisia I of Caria (fl. 480 BC), queen of Halicarnassus under the First Persian Empire, naval commander during the second Persian invasion of Greece * Artemisia II of Caria (died 350 BC), queen of Caria under th ...
, and
Xanthium ''Xanthium'' (cocklebur) is a genus of flowering plants in the tribe Heliantheae within the family Asteraceae, native to the Americas and eastern Asia and some parts of south Asia . Description Cockleburs are coarse, herbaceous annual plants ...
.'' The pathogen has the strongest impact on ''
Helianthus ''Helianthus'' () is a genus comprising about 70 species of annual and perennial flowering plants in the daisy family Asteraceae commonly known as sunflowers. Except for three South American species, the species of ''Helianthus'' are native to ...
,'' degrading flower yields in the species '' H. argophyllus, H. debilis, H. petiolaris and H. annuus''. ''H. annuus'', the common sunflower, is also the most common host of ''P. halstedii.'' '' Xanthium strumarium'', the common cocklebur, and ''
Ambrosia artemisiifolia ''Ambrosia artemisiifolia'', with the common names common ragweed, annual ragweed, and low ragweed, is a species of the genus '' Ambrosia'' native to regions of the Americas. Taxonomy The species name, ''artemisiifolia'', is given because the le ...
'', or ragweed, have been shown to act as significant wild hosts. ''Plasmopara halstedii'' causes significant yield losses due to the production of infertile sunflowers. Infertility due to P. halstedii is a result of sporulation on the flowering bodies as well as seed damping off due to root infection.Sackston, W.E. (1981) Downy mildew of sunflower. In: The Downy Mildews (Spencer, D.M., ed.), pp. 545–575.London: Academic Press. Other symptoms include plant stunting, chlorosis, root browning and alteration of secondary metabolism of infected plants. As an oomycete, ''P. halstedii'' releases motile
zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or mo ...
s as secondary inoculum. In these cases, secondary infection symptoms are much less severe than primary symptoms.Gulya, T.J., Tourvieille de Labrouhe, D., Masirevic, S., Penaud, A., Rashid, K. and Viranyi, F. (1998) Proposal for the standardized nomenclature and identification of races of ''Plasmopara halstedii'' (sunflower downy mildew). In: ''Sunflower Downy Mildew Symposium, Proceedings of Sunflower Downy Mildew Symposium, International Sunflower Association Symposium III, Fargo, ND, USA'', pp. 130–136. A sign of the disease is the “typical downy appearance” resulting from sporulation on the bottom surface of the leaves.


Detection

Systemically infected sunflower plants may have some degree of stunting and the leaves show pale green or chlorotic mottling which spreads along the main veins and over the lamella.


Disease cycle

''Plasmopara halstedii'' is a plant pathogenic oomycete, capable of overwintering in soil due to survival structures called oospores. For this reason, ''P. halstedii'' is a soil borne pathogen infecting the roots of the host plant. Oospores have the potential to live in soil up to 10 years, while oospore germination takes 10–30 days. Germination length depends on environmental condition and typically occurs in the spring.Spring, O. and Zipper, R. (2000) Isolation of oospores of sunflower downy mildew, Plasmopara halstedii, and microscopical studies on oospore germination. J. Phytopathol. 148, 227–231. The germinating oospores form
zoosporangia A zoospore is a motility, motile asexual spore that uses a flagellum for motion (physics), locomotion. Also called a swarm spore, these spores are created by some Protist, protists, bacteria, and fungi to propagate themselves. Diversity Flagella t ...
that release motile
zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or mo ...
s that germinate upon contact with root exudates. After primary infection,
zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or mo ...
s serve as a main source of inoculum throughout the rest of the season.
Zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or mo ...
s germinate in about two hours and have two means of infection: direct penetration of the root through the use of an
appresorium An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal "pressing" organ, from which a minute infection peg grows and enters the host, using turgor pressure capable ...
or infection due to an already present wound.
Hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...
e extend throughout the intercellular space, forming parasitic haustoria. The pathogen breaks through the surface of leaves and flowering structures through
stoma In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...
ta. Asexual reproduction structures, called zoosporangiaphores, form at these sites. These structures release
zoosporangia A zoospore is a motility, motile asexual spore that uses a flagellum for motion (physics), locomotion. Also called a swarm spore, these spores are created by some Protist, protists, bacteria, and fungi to propagate themselves. Diversity Flagella t ...
containing
zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or mo ...
s to other plants, the primary means of dissemination. Sexual reproduction be either homothallic or heterothallic. Homothallic reproduction is characterized by the fusion of the asexual
oogonium An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes. In the mammalian fetus Oogonia are formed in larg ...
and antheridium. This fusion leads to the formation and release of sexual oospores, the primary inoculum for the next season. Heterothallic sexual reproduction is the fusion of sexual cells from two separate organisms, leading to "
outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity. Outcrossing can be a useful ...
".


Management and control

Once the pathogen has been detected in an area, management is essential, as ''P. halstedii'' is nearly impossible to eradicate. Between long-surviving resting spores and high levels of secondary inoculum, ''P. halstedii'' can infect from 50% to 95% of sunflower yields in a single season. Resistant sunflower strains are available, as two types of dominant major resistance genes have been identified, denoted as ''Pl''. Type 1 resistance lacks infection above the base of the
hypocotyl The hypocotyl (short for "hypocotyledonous stem", meaning "below seed leaf") is the stem of a germinating seedling, found below the cotyledons (seed leaves) and above the radicle (root). Eudicots As the plant embryo grows at germination, it send ...
. Type 2 is characterized by a weak infection, with sporulation never reaching the upper region of the host. Resistant genes have been overcome in the past as the pathogen evolves and mutates, suggesting that resistance may not give the best results.Gulya, T.J.; Sackston, W.E.; Virányi, F.; Masirevic, S.; Rashid, K.Y. (1991) New races of the sunflower downy mildew pathogen (Plasmopara halstedii) in Europe and North and South America. Journal of Phytopathology 132, 303-311. However, due to the constant search for resistant sources from wild
Helianthus ''Helianthus'' () is a genus comprising about 70 species of annual and perennial flowering plants in the daisy family Asteraceae commonly known as sunflowers. Except for three South American species, the species of ''Helianthus'' are native to ...
, the gene pool of cultivars is frequently being used to produce new, commercially resistant strains.Sackston WE, 1992. On a treadmill: breeding sunflowers for resistance to disease. Annual Review of Phytopathology, 30:529-551; 123 ref. Seed treatment has been shown to be effective in controlling the disease, as the establishment of ''P. halstedii'' in an area of soil is nearly irreversible. The compounds metalaxyl and oxadixyl have been shown to protect seeds in the case of infection, and treatments containing these compounds are commercially available. Fungicides seed dressings of this nature have also been shown to give significant control over the pathogen. However, some strains of ''P. halstedii'' have begun to show resistance to metalaxyl-based fungicides, reported in multiple countries. Alternate forms of resistance are being developed for the future, including the use of biological antagonists and defense-related proteins. Australia is the leader in pest control of ''P. halstedii.'' In Australia, any imported seed is subject to hot-water treatment, fungicide dusting and monitoring for up to 3 years.Anon. (1981) Sunflower downy mildew. Plant Quarantine Leaflet No. 13. Commonwealth Department of Health, Canberra, Australia.


References


Further reading

* *—diagnostic protocol {{Taxonbar, from=Q3906344 Water mould plant pathogens and diseases Sunflower diseases Peronosporales Species described in 1888