Mucor piriformis A. Fisch. 1892

Foodplant / saprobe
Mucor piriformis is saprobic on rotting fruit of Malus domestica

Foodplant / saprobe
Mucor piriformis is saprobic on rotting fruit of Ribes uva-crispa

Foodplant / saprobe
Mucor piriformis is saprobic on rotting fruit of Prunus persica var nectarina

Foodplant / saprobe
Mucor piriformis is saprobic on rotting fruit of Prunus persica

Foodplant / saprobe
Mucor piriformis is saprobic on rotting fruit of Pyrus communis sens. str.

Foodplant / saprobe
Mucor piriformis is saprobic on rotting fruit of Prunus domestica ssp. domestica

Foodplant / saprobe
Mucor piriformis is saprobic on rotting fruit of Fragaria x ananassa

Mucor piriformis, described by Alfred Fischer in 1892 is a zygomycete from the order Mucorales. It is a saprobe soilborne fungi widely distributed around the world, being found in several countries such as Australia, Austria, Canada, Chile, France, Germany, Italy, Mexico, Rumania, Russia, United Kingdom, and United States of America. M. piriformis can produce both sexual (zygospores) and asexual (sporangiospores) spores, it is heterothallic, does not produce chlamydospore, and survives in the soil through sporangiospores. This fungi plays an important role in plant pathology because it causes postharvest rot in several fruit crops during cold storage. The most affected crops are stone and pome fruit. There are no effective fungicide treatments to manage such disease. Thus management relies on sanitation strategies during crop conduction, harvest, and fruit storage.

Mucor piriformis was first described by Alfred Fischer in 1892 and published on the “Rabenhorst's Kryptogamen-Flora von Deutschland, Oesterreich und der Schweiz”, in the fourth chapter division called Phycomycetes. Its description can also be found on Saccardo’s Sylloge Fungorum, published in 1895. This zygomycete is also known by the following names (and year of description): Mucor wosnessenskii (1898), Hydrophora fischeri (1910), and Mucor alboater (1915). M. piriformis is a fungus from the phylum Zygomycota, subphylum Mucoromycotina, class Incetae sedis, order Mucorales, family Mucoraceae, and genus Mucor. The Mucoromycotina is a subphylum that includes many of the zygomycetes. It has approximately 325 species, mostly saprobes, plant pathogens, and mycoparasites. M. piriformis was first reported causing fruit decays in 1895. From 1962 to 1990, it was found causing serious stone fruit losses. In 2013, its internal transcribed spacer (ITS) sequence containing approximately 650 bp was published on the National Center for Biotechnology Information (NCBI) and at the Persoonia Journal under the title “DNA barcoding in Mucorales: an inventory of biodiversity” (Walther et al. 2013).

Michailides and Spotts (1990) isolated Mucor piriformis from stone fruit and described the fungi grown on Synthetic Mucor Agar (SMA), which is a media used to plate Mucorales for taxonomy and identification. The fungus can present single (monopodially) or branched (sympodially) shiny sporangiophores, which can be either short or tall, and holds gray columellate sporangia. Sporangiophores are smooth subspherical (7.3-7.8 X 4.6-5.3 µm) to ellipsoidal (8.6-9.2 X 7.8-8.1 µm). Sporangia starts as white or yellowish and turns to gray as it gets older. It has a globose shape with deliquescent spiny walls and diameter ranging from 264 to 283 µm. The columellae has dimensions that ranges up to 182 X 144 µm presenting a cylindrical-ellipsoidal, pyriform, sub-globose shape. Sporangia and columellae usually appears on 10-day or older cultures, however, chlamydospore-like structure are not common. Zygospores (sexual spores) can be formed by isogametangia. They are spherical to sub-globose (151-180 X 127-156 µm). They have melanized episporal wall that allows them to be considered resistant structures. Although M. piriformis does not produce chlamydospores, it often produces gemmae. Another important feature described by Michailides and Spotts (1986 and 1990) and Kirk (1997) is that M. piriformis is heterothallic like the majority of the Mucorales.

The primary source of inoculum are infested soil, debris (decayed fruit and orchard leaves), and weeds such as ryegrass (Lolium perenne L.) and chickweed (Stellaria media L.). Decayed fruit often are incorporated to the soil during cultivation operations and serve as a host for M. piriformis, which helps the fungi to build up its population. During harvest, the fruit bins in contact with the ground carries infested soil to the dump water tank at the packinghouse, where most likely the infection will occur. Sporangiospores can then overwinter in soil and continue the cycle on the following season. Zygospores can also be formed, however, being M. piriformis heterothallic, two mating types are needed to infect the same fruit. This dispersion can be done via insects, birds and rodents. The figure showed by Michailides and Spotts (1990) illustrates the disease cycle caused by M. piriformis in a fruit tree orchard, showing the survival, dispersion, infection, and zygospores formation steps.

Mucor piriformis is a soilborne fungi that survives in the soil and is dispersed mainly as sporangiospores (Michailides and Ogawa, 1983). It is a great competitor in the soil when temperatures are under 15 to 20 oC. However, other microbes can overcome M. piriformis when temperatures are over 20 oC (Dobson and Spotts, 1989). It is a widely distributed fungi, being found in Australia, Austria, Canada, Chile, France, Germany, Italy, Mexico, Rumania, Russia, United Kingdom, and United States of America (Alabama, California, Oregon, Pennsylvania, Washington) (Michailides and Spotts, 1990; and Kirk, 1997). One importance of this fungi is its plant pathogenicity. Although it is not considered a primary plant pathogen, M. piriformis is an important secondary invader or saprobe. It has been reported affecting several fruits such as apples, gooseberry, nectarine, peach, pear, plum, strawberry, among others. However, pome and stone fruit are the major concern. The main crops affected by this pathogen are Anjou pear and apple, which has suffered serious losses during cold storage in the Pacific Northwest fruit industry (Bertrand and Saulie-Carter, 1980). The pathogen is introduced to packinghouses with the soil adhered to fruit containers and fruit due to improper harvesting and packing handling. At the packinghouse, the fruit bins are emptied by immersion. Even though the dump water is treated with chlorine, studies of dump water samples had shown the presence of viable propagules of M. piriformis (Bertrand and Saulie-Carter, 1979). It is believed that the dump tank is where the fruit infections occurs with posterior symptom development in storage. Infection site varies depending on the crop and situation. It can occur through the calyx, stem end, and puncture wounds or wounds caused by insects. The symptoms also vary among crops; however, it follows the same pattern of soft rot. Michailides and Spotts (1990) described the symptoms of Mucor rot on pome and stone fruit. Essentially, a soft watery rot starts to develop from the infection site and increases in size. The wound becomes light brown and sporangiophores project through openings in the skin or comes out of the lenticels. Complete fruit decay can occur in two months in storage at 0 oC. Rotted apples and pears stored in airtight chambers emit a characteristic alcohol odor, whereas rotted peaches and nectarines under the same circumstances release an aromatic scent. Even though rapid refrigeration and storage at temperature as low as 0 oC reduces rotting, it does not reduce the viability of the sporangiospores and neither prevent infection (Dennis and Blijham, 1980). Producers cannot rely on the use of fungicides to control such disease because of the lack of effective molecules (Li et al, 2014). Consequently, the use of sanitation strategies for management takes place. To reduce inoculum on the soil, producers can remove fallen fruit from the ground; this will help to control disease while storage. Other sanitation practices involves avoidance of soil contacting fruit bins, and treatment of dump water tank with chlorine or sodium o-phenylphenate to keep spore levels as low as possible (Spotts, 1986 cited by Michailides and Spotts, 1990).

Mucor piriformis is a plant pathogen that causes a soft rot of several fruits known as Mucor rot. Infection of its host fruits, such as apples and pears, takes place post-harvest. The fungi can also infect citrus fruits.