Sporeless Oyster Mushrooms

By Randy Strobel

Oyster mushrooms in the genus Pleurotus are the second most commonly cultivated mushrooms worldwide. They are very tasty and have a good shelf life when refrigerated. They are medicinal in that they contain lovastatin, a natural cholesterol-lowering compound. They are also the easiest mushroom to cultivate.

Growing them is easy.

They colonize substrate very quickly, grow on any cellulose-containing substrate, and are good at resisting contamination. Several cultivars are available, including pearl, pink, yellow, king trumpet, and many more. These qualities make oyster mushrooms ideal for the biology classroom and for beginning cultivators.

But there are problems with oyster mushrooms.

Oyster mushroom cultivation is a health-related concern because it produces large quantities of white spores. Breathing large amounts of any fungal spores can lead to allergy problems that can present with arthritic and respiratory symptoms. This is especially a concern for commercial operations where large quantities of oysters are grown. The term “mushroom worker’s disease” has been used to describe a syndrome that can result from inhalation of large quantities of spores. This is also a potential hazard for home cultivators growing oyster mushrooms indoors.

But you can still enjoy oyster mushrooms!

Thankfully, cultivars of oyster mushrooms that do not produce spores have been developed. This has several benefits.

• The resulting sporeless oyster mushrooms eliminate the risk of mushroom worker’s disease in both academic and commercial laboratories as well as for home cultivators.
• They reduce the risk of invasion by non-native cultivars since airborne spores are not produced.
• Additionally, they decrease the spread of mushroom virus disease since spores can spread these viruses.

This article will discuss the history and biology of these sporeless cultivars.

The first sporeless strain of oyster mushroom, ATCC 58937, was created by serendipity during 1976 oyster mushroom breeding experiments. Although the strain was sporeless, it was unsuitable for commercial applications because the fruit bodies were abnormally shaped, and the yields were small.

Then, in 1978, in the Netherlands, the academic laboratory of Johan Baars and the Sylvan Spawn company worked on a joint project to cross the original sporeless strain with other spore-producing strains. This formed the monokaryons of both strains. Then, they crossed them to produce dikaryons. The resulting sporeless dikaryons were tested by commercial growers for yield and commercial appeal. This led to the release of the first commercial sporeless oyster cultivar (Sylvan H-195) in 2004. Since then, several new cultivars have been developed of sporeless Pleurotus ostreatus.

More Varieties of Sporeless Oysters

Research has also been initiated to produce sporeless cultivars of other Pleurotus species including P. eryngii (king trumpet), P. citrinopileatus (yellow oyster), and P. pulmonaris (lung oyster). Sylvan Spawn has also produced sporeless cultivars of Agaricus bisporus (button, crimini, and portobello), the most commonly cultivated mushroom in the world.

How are the sporeless oyster mushrooms created?

Several genetic mutations can lead to the sporeless trait. One type of mutation results in no spores being formed at all. Basidia are formed but they lack spores. Another type of mutation has viable spores, but they are not released due a defect in the spore release mechanism. Other types of mutations leading to the sporeless trait are possible, but have not been studied in detail.

The molecular basis of one cultivar that does not produce spores, SPOPPO, has been determined in the laboratory of Johan Baars. SPOPPO carries two defective copies of the gene MSH4. This gene is needed for spore formation. Heterozygotes with one copy of the defective gene produce spores. Hence, SPOPPO is a homozygous recessive for the sporeless trait. When SPOPPO is self-crossed, one-fourth of the progeny are sporeless. Thus, SPOPPO can be propagated by cloning or crossing the parental heterozygotes.

The function of the MSH4 protein is well characterized. It is needed for the pairing of homologous chromosomes and recombination that occurs during meiosis I. In the absence of MSH4 function, meiosis is blocked, and spores are completely absent. However, the morphology of the fruiting bodies is not affected, and commercially viable mushrooms are produced.

Who should use sporeless oyster mushrooms for cultivation?

Everyone! Sporeless mushrooms are highly recommended for home cultivation. The author of this article has successfully grown them at home and used them for several cultivation workshops. They grow readily on toilet paper rolls and aspen wood chips, two common substrates for home cultivation. Typically, only two to three weeks elapse between inoculation and fruiting. Unfortunately, only a few vendors offer home cultivation kits using sporeless cultivars. Hopefully, more vendors will offer them in the future. It is also hoped that cultivars of species other than P. ostreatus will become available to home cultivators.