''Plasmopara halstedii'' is a plant pathogen infecting

sunflower The common sunflower (''Helianthus annuus'') is a species of large annual forb of the daisy family Asteraceae. The common sunflower is harvested for its edible oily seeds, which are often eaten as a snack food. They are also used in the pr ...

s. The species is one of many pathogens commonly referred to as

downy mildew 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 c ...

. ''P. halstedii'' originated in North America. ''Plasmopara halstedii''

oospore An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically induced stimulation of mycelia ...

s 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 (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

that attacks the flowering plants of the family

Asteraceae Asteraceae () is a large family (biology), family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the Order (biology), order Asterales. The number of species in Asteraceae is rivaled only by the Orchi ...

, found to infect the genus’ ''

Helianthus ''Helianthus'' () is a genus comprising around 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 ''Bidens'' is a genus of flowering plants in the aster family, Asteraceae.''Bidens''.
...

, Artemisia, 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 grow ...

.'' The pathogen has the strongest impact on ''

,'' 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 ''Xanthium strumarium'' (rough cocklebur, Noogoora burr, clotbur, common cocklebur, large cocklebur, woolgarie bur) is a species of annual plants of the family Asteraceae. Some sources claim it originates in southern Europe and Asia, but has been ...

'', 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 lea ...

'', 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 The Oomycetes (), or Oomycota, form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms within the Stramenopiles. They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction o ...

, ''P. halstedii'' releases motile

zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion in aqueous or moist environments. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Certain zoospores are ...

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 Downy, also known as Lenor in Europe, Taiwan and Japan, is an American brand of fabric softener produced by Procter & Gamble that was introduced in 1960. Brand history Lenor is a brand name of fabric softener and dryer sheets, also produced by ...

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 In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

mottling which spreads along the main veins and over the lamella.

Disease cycle

''Plasmopara halstedii'' is a plant pathogenic

, capable of overwintering in soil due to survival structures called

s. For this reason, ''P. halstedii'' is a soil borne pathogen infecting the roots of the host plant.

Oospore An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically induced stimulation of mycelia ...

s 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

s form zoosporangia that release motile

s that germinate upon contact with root exudates. After primary infection,

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 in aqueous or moist environments. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Certain zoospores are ...

s germinate in about two hours and have two means of infection: direct penetration of the root through the use of an appresorium 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 o ...

e extend throughout the intercellular space, forming parasitic

haustoria In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates th ...

. The pathogen breaks through the surface of leaves and flowering structures through

stoma In botany, a stoma (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...

ta. Asexual reproduction structures, called zoosporangiaphores, form at these sites. These structures release zoosporangia containing

s to other plants, the primary means of dissemination. Sexual reproduction be either

homothallic In fungi and algae, homothallism refers to the condition in which a single individual or thallus carries the genetic determinants (i.e., both mating types or sexes) required to undergo sexual reproduction without the need for a distinct mating partn ...

heterothallic Heterothallic species have sexes that reside in different individuals. The term is applied particularly to distinguish heterothallic fungi, which require two compatible partners to produce sexual spores, from homothallic ones, which are capable ...

Homothallic In fungi and algae, homothallism refers to the condition in which a single individual or thallus carries the genetic determinants (i.e., both mating types or sexes) required to undergo sexual reproduction without the need for a distinct mating partn ...

reproduction is characterized by the fusion of the asexual

oogonium An oogonium (: 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 large ...

and

antheridium An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. The androecium is al ...

. This fusion leads to the formation and release of sexual

s, the primary inoculum for the next season.

Heterothallic Heterothallic species have sexes that reside in different individuals. The term is applied particularly to distinguish heterothallic fungi, which require two compatible partners to produce sexual spores, from homothallic ones, which are capable ...

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 in animals Out ...

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

, 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.

Hosts and symptoms

Detection

Disease cycle

Management and control

References

Further reading