''Hyaloperonospora parasitica'' is an

oomycete Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...

from the family

Peronosporaceae Peronosporaceae are a family of water moulds that contains 21 genera, comprising more than 600 species. Most of them are called downy mildews. Peronosporaceae are obligate biotrophic plant pathogens. They parasitise their host plants as an ...

. It has been considered for a long time to cause downy mildew of a variety of species within the

Brassicaceae Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The le ...

, on which the disease can cause economically important damage by killing seedlings or affecting the quality of produce intended for freezing. ''Hyaloperonospora parasitica''http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=39723 causes downy mildew on a wide range of many different plants. It belongs to the Kingdom Chromista, the phylum Oomycota, and the family Peronosporaceae. The former name for ''H. parasitica'' was ''Peronospora parasitica'' until it was reclassified and put in the genus Hyaloperonospora. It is an especially vicious disease on crops of the family Brassicaceae. It is most famous for being a model pathogen of ''Arabidopsis thaliana'' which is a model organism used for experimental purposes. Accordingly, the former ''Hyaloperonospora'' parasitica has been split into a large number of species. For instance, the taxonomically correct name of the parasite of the well-known model organism ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...

'' is ''

Hyaloperonospora arabidopsidis ''Hyaloperonospora arabidopsidis'' is a species from the family Peronosporaceae. It is an obligate parasite and the causal agent of the downy mildew of the plant model organism ''Arabidopsis thaliana''. While ''H. arabidopsidis'' has for a long t ...

'', not ''H. parasitica'', whereas the pathogen of ''

Brassica ''Brassica'' () is a genus of plants in the cabbage and mustard family (Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, or mustard plants. Crops from this genus are sometimes called ''cole cr ...

'' has to be called ''

Hyaloperonospora brassicae ''Hyaloperonospora brassicae'', in the family Peronosporaceae, is a plant pathogen. It causes downy mildew of species of ''Brassica'', ''Raphanus'', ''Sinapis'' and probably other genera within the ''Brassicaceae''. In the past, the cause of dow ...

''.

Host range and symptoms

The infection first appears as

conidiophores A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an Asexual reproduction, asexual, non-motility, motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are al ...

covering the upper and lower leaf surfaces. Other symptoms include lesions on spikelet, fungal growth on leaves, and necrosis on stems and leaves. ''Hyaloperonospora Parasitica'' has a very wide host range. It can infect garlic mustard, horseradish, cruciferous vegetables, Shepherd's purse, Cheiranthus spp., Cucurbits, purple vein rocket, treacle mustard, wallflower, spurges, candytuft, mustard plants, radish, and sweet alyssum. Although it is not significantly damaging on any one specific plant, the damage of ''H. parasitica'' can cause major economic losses if not managed properly. The recommendation for management is weekly fungicide application.

Disease cycle

This is a polycyclic pathogen with a sexual and an asexual stage. It thrives in cool moist environments. Over winter in the soil, oospores survive and wait for spring. In warmer conditions when it gets to about 47–53 degrees F the oospores will germinate and produce an appressorium or form a short germ tube. The mycelium grows intracellularly and haustoria penetrate through the host cells when the temperature is around 60 degrees F. After about 1–2 weeks when the temperature is between 60 and 64 degrees F, conidiophores (the asexual stage) will form out of the plant stomata bearing conidia. Sporulation occurs at night. The conidia disseminate in high humidity mornings when it's about 50–60 degrees. Meanwhile, during the sexual stage, the antheridia (the male sex organ) fertilizes the

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 ...

(the female sex organ) with a fertilization tube and an oospore is developed.

Pathogenesis

Hyaloperonospora parasitica is an obligate biotroph, meaning it lives off a host obtaining nutrients. The pathogen infects about six hours after landing on a plant surface. The spores produce an appressorium which in turn produces a penetration peg. This hypha grows from bottom of the appressorium and works its way through an intercellular junction of two epidermal cells. As the penetration peg grows between cells, haustoria grow into the cells to gain nutrients. The oomycete grows until it has penetrated through the entire leaf where conidiophores are then grown out of the stomata and disseminated by wind and rain. Synonyms:Species Fungorum - Species synonymy
* ''Botrytis parasitica'' Pers., 1796 * ...

Host range and symptoms

Disease cycle

Pathogenesis

References

Further reading