Friday Fellow: Green mold

by Piter Kehoma Boll

At least once in your life you probably saw a rotting orange with greenish and white mould growing on its peel. This unfortunate condition is caused by the species that I am going to present today.

Penicillium digitatum growing on an orange. Photo by Alison Northup.*

Known popularly as the green mold or green rot, its scientific name is Penicillium digitatum, being closely related to a similar, but slightly bluer mold that also attacks oranges, the blue mold Penicillium italicum. As a member of the genus Penicillium, this fungus is also related to Penicillium chrysogenum, the main source of penicillin, and to several fungi used to produce cheese such as Camembert (by Penicillium camemberti), Gorgonzola (by Penicillium glaucum) and Roquefort (by Penicillium roqueforti).

Infecting exclusively fruits of species in the genus Citrus, the green mold grows and feeds on the fruit’s peel, being the main source of post-harvest decay and thus of major economic concern. The optimal temperature for the development of the green mold is 20-25 °C, although it is able to grow in a range of temperatures going from 6 °C to 37 °C. The spores of the green mold are unable to germinate at the surface of the fruits, though, and they need a fissure on the peel to start growing. However, the storage and transportation of the fruits is enough to create small fissures that are rapidly filled by the growing mycelium.

Conidiophores (spore-producing structures) of Penicillium digitatum as seen with a magnification of 40 times. Photo by Wikimedia user Ninjatacoshell.**

The green mold is known to produce ethylene, an organic gas that is a plant hormone leading to fruit ripening. It is likely that this fungus synthesizes it to induce the ripening of citrus fruits, thus increasing the substrate for its development.

Currently, the main methods used to avoid the spoilage of citrus fruits by P. digitatum include the aplication of fungicides, sometimes in massive amounts. However, as such fungicides can lead to serious environmental and health issues, and sometimes increase the public rejection, there is a demand for the development of less aggressive and more environmentally friendly options.

The genome of the green mold has been recently sequenced, being the second species of Penicillium to be sequenced (after P. chrysogenum), as well as the first main plant pathogenic fungus to have its whole genome analyzed.

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References:

Chou, T. W., & Yang, S. F. (1973). The biogenesis of ethylene in Penicillium digitatum. Archives of Biochemistry and Biophysics, 157(1), 73–82. doi:10.1016/0003-9861(73)90391-3

Marcet-Houben, M., Ballester, A.-R., Fuente, B., Harries, E., Marcos, J. F., González-Candelas, L., & Gabaldón, T. (2012) Genome sequence of the necrotrophic fungus Penicillium digitatum, the main postharvest pathogen of citrus. BMC Genomics, 13, 646. doi: 10.1186/1471-2164-13-646

Plaza, P., Usall, J., Teixidó, N., & Viñas, I. (2003) Effect of water activity and temperature on germination and growth of Penicillium digitatum, P. italicum and Geotrichum candidum. Journal of Applied Microbiology, 94(4), 549–554. doi: 10.1046/j.1365-2672.2003.01909.x

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* This work is licensed under a Creative Commons Attribution 4.0 International License.

** This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

Friday Fellow: Gray Mold

by Piter Kehoma Boll

Today’s Friday Fellow will show you how beauty is only a matter of perspective. Being an ascomycete fungus, it is commonly known as gray mold and is usually found growing on decaying vegetables, especially fruits such as the strawberry in the photo below:

Gray mold growing on a strawberry. Most people would not see it as a beautiful image. Photo by Wikimedia user Rasbak.*

The gray mold has a controversial biological nomenclature, as many other fungi. The most common name is Botrytis cinerea used for its asexual stage (anamorph), which is the most common. Its sexual stage (teleomorph) is known as Botryotina fuckeliana. I guess this issue, which was common in naming fungi with rare or unknown occurrences of sexual stage, has already been settled, but as I’m not a taxonomist of fungus, I cannot speak much on the subject.

More than only having a controversial name, this fungus has also a controversial interaction with humans. It is a notable pest in wine grapes and may lead to two different infections on them. One of those is called “grey rot” and happens under wet conditions, leading to the loss of the grapes. The other is called “noble rot” and is a beneficial form of the infection that happens when the wet condition is followed by a dry one and produce a fine and sweet vine due to the concentration of sugars in the grape.

Out of the vine world, however, the gray mold is not something that you want growing on your crops. As as it attacks more than 200 species, many of them being important food crops, there is a big interest in developing strategies to reduce the damage it causes. And these strategies include the use of pesticides, plant essential oils or even other organisms that may parasitize the gray mold.

But one cannot deny that if you look closer, even the gray mold is beautiful:

A beautiful tiny forest of gray mold on a strawberry. Photo by Macroscopic Solutions.**

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References:

Wikipedia. Botrytis cinerea. Available at <https://en.wikipedia.org/wiki/Botrytis_cinerea&gt;. Access on June 2, 2017.

WILLIAMSON, B., TUDZYNSKI, B., TUDZYNSKI, P., & VAN KAN, J. (2007). Botrytis cinerea: the cause of grey mould disease Molecular Plant Pathology, 8 (5), 561-580 DOI: 10.1111/j.1364-3703.2007.00417.x

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Friday Fellow: Black Bread Mold

by Piter Kehoma Boll

Today’s Friday Fellow lives in our houses and our gardens, among our food and our crops. And every time we notice it, we get upset, because it means that something we were supposed to eat is now spoiled. Its name is Rhizopus stolonifer, or black bread mold.

The black bread mold growing on a peach. Photo by University of Georgia Plant Pathology Archive.*

Having a worldwide distribution, the black bread mold is mainly saprotrophic, growing on decaying fruits and bread. During its reproductive phase, it can be noticed as a black and hairy mold, as in the photo above. Eventually, this species can also cause an infection in human face and oropharynx, but most commonly it can be a pathogen of many plant species, thus being of economic concern.

A closer look at the sporangia of Rhizopus stolonifer. Photo by Stanislav Krejčík.*

The black bread mold is a fungus of the order Mucorales, known as pin molds because their sporangia (the structures that contain the asexual spores) remember a pin. These sporangia, which are black, are what one usually notice growing on decaying food. When the sporangia are mature, they release spores of two kinds that germinate and originate two kinds of hyphae (known as + and -) and when two hyphae of opposite type come into contact, they fuse and create a zygospore, which then grows to originate new sporangia.

Due to its importance as an economic pest, there are many studies trying to find ways to get rid of it and very few studies trying to understand the fascinating things that it hides. What a pity.

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References:

EOL – Encyclopedia of Life: Rhizopus stolonifer. Available at <http://eol.org/pages/2944808/overview >. Access on January 14, 2107.

Hernández-Lauzardo, A., Bautista-Baños, S., Velázquez-del Valle, M., Méndez-Montealvo, M., Sánchez-Rivera, M., & Bello-Pérez, L. (2008). Antifungal effects of chitosan with different molecular weights on in vitro development of Rhizopus stolonifer (Ehrenb.:Fr.) Vuill Carbohydrate Polymers, 73 (4), 541-547 DOI: 10.1016/j.carbpol.2007.12.020

Wikipedia. Black bread mold. Available at <https://en.wikipedia.org/wiki/Black_bread_mold >. Access on January 14, 2017.

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