''Albugo occidentalis'', the causal agent of spinach white rust, 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 ...
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
pathogen, although some discussions still treat it as a
fungal
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
organism. ''Albugo occidentalis'' is one of the most important spinach diseases in North America, found throughout the United States east of the rocky mountains.
Hosts and symptoms
The economically important host of ''Albugo occidentalis'' is
spinach
Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...
(''Spinacia oleracea''); although the oomycete has also been reported to affect plants of the genus ''
Chenopodium
''Chenopodium'' is a genus of numerous species of perennial or annual herbaceous flowering plants known as the goosefoots, which occur almost anywhere in the world. It is placed in the family Amaranthaceae in the APG II system; older classifica ...
'', the genus including the crop plant
quinoa
Quinoa (''Chenopodium quinoa''; , from Quechua ' or ') is a flowering plant in the amaranth family. It is a herbaceous annual plant grown as a crop primarily for its edible seeds; the seeds are rich in protein, dietary fiber, B vitamins, and ...
. This pathogen causes white rust or white blister of spinach. It is unrelated to the
basidiomycete
Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basi ...
rusts biologically, but appears somewhat similar on the surface of the leaf, sometimes causing the plant to form white or yellow blister-like pustules on leaves. The early stage, a milder
chlorosis
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 ...
, is found on the on abaxial face, but if the white rust is allowed to thrive, it can blister and be visible on the adaxial surface as well.
Disease cycle
Like all the other species in its genus, ''Albugo occidentalis'' is an oomycete. The survival structure is an
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, ...
, which is the result of the
karyogamy
Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In order for ...
of two haploid gametes (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 ...
and antheridium). The oospore can overwinter in the soil, and in the spring it produces
zoospores
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 mor ...
which will encyst on the surface of spinach leaves in the presence of water and germinate.
Asexual propagation
Plant propagation is the process by which new plants grow from a variety of sources: seeds, cuttings, and other plant parts. Plant propagation can also refer to the man-made or natural dispersal of seeds.
Propagation typically occurs as a step i ...
of ''Albugo occidentalis'' occurs via the production of
sporangia
A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
on sporangiophores. These sporangia disperse and form zoospores, which complete the asexual cycle by re-infecting or infecting a new spinach leaf. The asexual oomycete can overwinter in spinach debris.
Environment
Because it is an oomycete, presence of water or atmospheric moisture is essential for infection via motile zoospores. Periods of greatest risk of infection include spring and fall when cool evenings promote dew formation on leaf and days are not hot and dry enough to dry out the leaf surface. ''Albugo occidentalis'' is most successful at temperatures between 12° and 18 °C, and in this range the white rust takes about three days to germinate. It takes longer at temperatures outside of this window.
References
External links
Index FungorumUSDA ARS Fungal Database
Further reading