Citrus Green Mold

Penicillium digitatum

"Penicillium digitatum" is a mesophilic fungus found in the soil of citrus-producing areas. It is a major source of post-harvest decay in fruits and is responsible for the widespread post-harvest disease in "Citrus" fruit known as green rot or green mould.
Blue Mold or Penicillium digitatum on a navel orange  Penicillium digitatum,Penicillium expansum

Appearance

In nature, "P. digitatum" adopts a filamentous vegetative growth form, producing narrow, septate hyphae. The hyphal cells are haploid, although individual hyphal compartments may contain many genetically identical nuclei. During the reproductive stages of its life cycle, "P. digitatum" reproduces asexually via the production of asexual spores or conidia. Conidia are borne on a stalk called a conidiophore that can emerge either from a piece of aerial hyphae or from a soil-embedded network of hyphae. The conidiophore is usually an asymmetrical, delicate structure with smooth, thin walls. Sizes can range from 70–150 μm in length. During development, the conidiophore can branch into three rami to produce a terverticillate structure although biverticillate and other irregular structures are often observed. At the end of each rami, another set of branches called metulae are found. The number of metulae varies with their sizes ranging from 15–30 × 4–6 μm. At the distal end of each metula, conidium-bearing structures called phialides form. Phialides can range in shape from flask-shaped to cylindrical and can be 10–20 μm long. The conidia produced, in turn, are smooth with a shape that can range from spherical to cylindrical although an oval shape is frequently seen. They are 6–15 μm long and are produced in chains, with the youngest at the base of each chain. Each conidium is haploid and bears only one nucleus. Sexual reproduction in "P. digitatum" has not been observed.

"Penicillium digitatum" can also grow on a variety of laboratory media. On Czapek Yeast Extract Agar medium at 25 °C, white colonies grow in a plane, attaining a velvety to deeply floccose texture with colony sizes that are 33–35 mm in diameter. On this medium, olive conidia are produced. The reverse of the plate can be pale or slightly tinted brown. On Malt Extract Agar medium at 25 °C, growth is rapid yet rare, forming a velvety surface. At first, colonies are yellow-green but ultimately turn olive due to conidial production. Colony diameter can range in size from 35 mm to 70 mm. The reverse of the plate is similar to that observed for Czapek Yeast Extract Agar medium. On 25% Glycerol Nitrate Agar at 25 °C, colony growth is planar yet develops into a think gel with colony size diameter ranging from 6–12 mm. The back of the plate is described as pale or olive. At 5 °C, 25% Glycerol Nitrate Agar supports germination and a colonial growth of up to 3 mm in diameter. This species fails to grow at 37 °C. On Creatine Sucrose Agar at 25 °C, colony size diameter ranges from 4 to 10 mm. Growth is restricted and medium pH remains around 7. No change on the back of the plate is noted. Growth on media containing orange fruit pieces for seven days at room temperature results in fruit decay accompanied by a characteristic odour. After 14 days at room temperature, the reverse is colourless to light brown."Penicillium digitatum" can be identified in the laboratory using a variety of methods. Typically, strains are grown for one week on three chemically defined media under varying temperature conditions. The media used are Czapek Yeast Extract Agar, Malt Extract Agar and 25% Glycerol Nitrate Agar. The resulting colonial morphology on these media allows for identification of "P. digitatum". Closely related species in the genus "Pencillium" can be resolved through this approach by using Creatine Sucrose Neutral Agar. Molecular methods can also aid with identification. The genomes of many species belonging to the genus "Penicillium" remain to be sequenced however, limiting the applicability of such methods. Lastly, "P. digitatum" can also be distinguished macroscopically by the production of yellow-green to olive conidia and microscopically, by the presence of large philades and conidia.

Habitat

"Penicillium digitatum" is found in the soil of areas cultivating citrus fruit, predominating in high temperature regions. In nature, it is often found alongside the fruits it infects, making species within the genus "Citrus" its main ecosystem. It is only within these species that "P. digitatum" can complete its life cycle as a necrotroph. However, "P. digitatum" has also been isolated from other food sources. These include hazelnuts, pistachio nuts, kola nuts, black olives, rice, maize and meats. Low levels have also been noted in Southeast Asian peanuts, soybeans and sorghum.

Evolution

"Penicillium digitatum" is a species within the Ascomycota division of Fungi. The genus name "Penicillium" comes from the word "penicillus" which means brush, referring to the branching appearance of the asexual reproductive structures found within this genus. As a species, "P. digitatum" was first noted as "Aspergillus digitatus" by Christiaan Hendrik Persoon in 1794 who later adopted the name "Monilia digitata" in "Synopsis methodica fungorum". The synonym "M. digitata" can also be found in the writings of Elias Magnus Fries in "Systema mycologicum". However, the current binomial name comes from the writings of Pier Andrea Saccardo, particularly "Fungi italici autographie delineati et colorati".

Uses

"Penicillium digitatum" is used as a biological tool during the commercial production of latex agglutination kits. Latex agglutination detects "Aspergillus" and "Penicillium" species in foods by attaching antibodies specific for the extracellular polysaccharide of "P. digitatum" to 0.8 μm latex beads"." This method has been successful in detecting contamination of grains and processed foods at a limit of detection of 5–10 ng/mL of antigen. In comparison to other detection assays, the latex agglutination assay exceeds the detection limit of the Enzyme-linked immunosorbent assay and is as effective in detecting "Aspergillus" and "Pencillium" species as the ergosterol production assay. However, the latter displays an increased ability to detect "Fusarium" species when compared to the latex agglutination assay.

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Taxonomy
KingdomFungi
DivisionAscomycota
ClassEurotiomycetes
OrderEurotiales
FamilyTrichocomaceae
GenusPenicillium
SpeciesP. digitatum