Black rot on orchids is caused by ''

Pythium ''Pythium'' is a genus of parasitic oomycetes. They were formerly classified as fungi. Most species are plant parasites, but ''Pythium insidiosum'' is an important pathogen of animals, causing pythiosis. The feet of the fungus gnat are frequent ...

'' and ''

Phytophthora ''Phytophthora'' (from Greek language, Greek (''phytón''), "plant" and (), "destruction"; "the plant-destroyer") is a genus of plant-damaging oomycetes (water molds), whose member species are capable of causing enormous economic losses on cro ...

'' species. Black rot targets a variety of

orchids Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Along with the Asteraceae, they are one of the two largest families of flowering ...

but Cattleya orchids are especially susceptible. ''

Pythium ultimum ''Pythium ultimum'' is a plant pathogen. It causes the damping off and root rot diseases of hundreds of diverse plant hosts including corn, soybean, potato, wheat, fir, and many ornamental species. ''P. ultimum'' belongs to the peronosporal ...

'' and ''

Phytophthora cactorum ''Phytophthora cactorum'' is a fungal-like plant pathogen belonging to the Oomycota phylum. It is the causal agent of root rot on rhododendron and many other species, as well as leather rot of strawberries. . Hosts, symptoms, and diagnosis ''Phy ...

'' are known to cause black rot in orchids. ''Pythium ultimum'' is a pathogen that causes

damping-off Damping off (or damping-off) is a horticultural disease or condition, caused by several different pathogens that kill or weaken seeds or seedlings before or after they germinate. It is most prevalent in wet and cool conditions. Symptoms There ar ...

and

root rot Root rot is a condition in which anoxic conditions in the soil or potting media around the roots of a plant cause them to rot. This occurs due to excessive standing water around the roots. It is found in both indoor and outdoor plants, although ...

on plants. Symptoms of this pathogen include stunting and

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 ...

. Identifying ''Pythium ultimum'' has traditionally been done by examining

oogonia 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 ...

antheridia 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. Androecium is also ...

, and

sporangium 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 ...

structure. Now, PCR (

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) t ...

) technology can identify the pathogen using DNA fragments. ''Phytophthora cactorum'' is a pathogen that causes root rot on many plant species. Symptoms of this pathogen include necrosis in the roots, chlorosis, and wilting. ''Phytophthora cactorum'' can be identified by examining oogonia, antheridium,

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, ...

, and sporangia structure.

Disease cycle

Pythium ultimum ''P. ultimum'' requires moist conditions to germinate. When conditions are favorable, surviving oospores in the soil produce a sporangia and zoospores which facilitate infection via germ tube. From there, mycelium will grow throughout all plant tissues. The disease is primarily a root rot that causes symptoms of foliar blight. Sporangia are produced on the mycelium and can produce zoospores for asexual reproduction or an

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 for sexual reproduction. Once the oogonium is fertilized, the oospore either infects via germ tube or produces sporangia and zoospores. Phytophthora cactorum ''P. cactorum'' requires free water to reproduce. Zoospores are released from sporangium and blown via wind or rain splash and use free water on the leaf to germinate. An appressorium is formed and a penetration peg penetrates the leaf surface. From there, hyphae grow throughout the leaf and infects all plant tissues. While this pathogen enters through the leaf, the disease is caused by root rot that causes symptoms of foliar blight. The mycelium gives rise to

chlamydospore A chlamydospore is the thick-walled large resting spore of several kinds of fungi, including Ascomycota such as '' Candida'', Basidiomycota such as ''Panus'', and various Mortierellales species. It is the life-stage which survives in unfavourable c ...

s and oospores. Oospores produce mycelium that produce sporangia. Oospores are the survival structure of P. cactorum. Chlamydospores produce mycelium that continues to infect the plant.

Environmental conditions

Pythium ultimum ''Pythium'' species rely on moist soils and cool conditions. Moist conditions allow the movement of zoospores which infect the plant. From here, the fungus penetrates the leaf surface and

mycelia Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...

l growth takes over the plant. Phytophthora cactorum ''Phytophthora'' species thrive in moist soils. When there is free water in the soil, conditions are met for

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 ...

to be produced. As the disease cycle mentioned, sporangia produce zoospores which are infectious. Spring and autumn also produce temperatures that are most favorable for zoospore production.

Control and management

Avoiding black rot in orchids is challenging. Avoiding symptomatic plants and isolating new plants from larger populations is helpful. In addition, decontaminating pots, tools, and work surfaces ensures that there are no contamination that could infect healthy plants. Potting media should also be sterilized as spores and hyphae can survive in dust or free water. Being careful not to over water, providing good drainage, air circulation, and proper potting media are helpful to prevent zoospores from having an optimal environment to move and infect. Fungicides are also used. The primary fungicide treatments to control black rot are

metalaxyl Metalaxyl is an acylalanine fungicide with systemic function. Its chemical name is methyl N-(methoxyacetyl)-N-(2,6-xylyl)-DL-alaninate. It can be used to control ''Pythium'' in a number of vegetable crops, and ''Phytophthora'' in peas. Metalaxyl ...

, fosetyl-AI, and

etridiazole Etridiazole is a fungicide and pesticide used for prevention of pythium ultimum on cotton plants. Synthesis Etridiazole can be synthesised from acetonitrile as follows: It can also be is produced by the reaction of trichloroacetamidine hyd ...

that are sprayed onto the plant. Poultices such as

copper sulfate Copper sulfate may refer to: * Copper(II) sulfate, CuSO4, a common compound used as a fungicide and herbicide * Copper(I) sulfate Copper(I) sulfate, also known as cuprous sulfate, is an inorganic compound with the chemical formula Cu2 SO4. It ...

and lime or

cinnamon Cinnamon is a spice obtained from the inner bark of several tree species from the genus ''Cinnamomum''. Cinnamon is used mainly as an aromatic condiment and flavouring additive in a wide variety of cuisines, sweet and savoury dishes, breakfa ...

can also be useful

fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

References

{{Reflist Phytophthora Pythium Fungal plant pathogens and diseases