Saprophytic fungi

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Saprophytic fungi are quite common in greenhouses and seedbeds and find cultivation substrates a favourable environment for proliferation. In the literature, there are numerous publications on the subject, also by botanical gardens, where the study and classification of these organisms is easier.

A good example is the list published by the Institute of Botany in Graz (Austria) (Pidlich-Aigner et al. 2001). In the list, numerous macromycetes (Basidiomycetes), both autochthonous and exotic, are described [1].

Among the saprophytic fungi that can establish themselves in horticultural and floricultural crops, a distinction must be made between those that originate from peat or other organic components of the substrate and those that are endemic to the cultivation sites. The former are certainly the ones of most interest to this article. A rather large group of them is known.

Some species belonging to the genera Penicillium and Aspergillum, by decomposing organic matter, cause the release of usable nutrients from crops. Their presence may also be a source of secondary repressiveness against soil-borne pathogens. In peat, the presence of some strains of the genus Trichoderma is also reported. Species belonging to this genus are common rhizosphere fungi. They have been shown to be able to establish a mutualistic association with plants and, through hyperparasitism, severely limit the growth of pathogenic fungi. These fungi also produce substances that induce systemic or localised resistance (stimulate the production of flavonoids or anthocyanins). Other fairly common genera are 𝘔𝘶𝘤𝘰𝘳, 𝘙𝘩𝘪𝘻𝘰𝘱𝘶𝘴 and 𝘗𝘢𝘳𝘢𝘴𝘰𝘭𝘢.

A rather frequent species is Chromelosporium fulva (Peziza ostracoderma syn. Plicaria fulva) - Peat mould or Cinnamon Brown Mold, which grows in circular colonies with concentric rings that are initially whitish-grey in colour, evolving towards orange-yellow. These efflorescences are destined to disappear to make way for disc-shaped gelatinous structures approximately one cm in diameter. The development of mycelium to form colonies of a visually appreciable size is normally the prerogative of the spring and summer months, when temperatures are higher.

In the last decade, much evidence points to an increased frequency of saprophytic infections in protected cultures. The reasons are to be found in the spread of monoculture, the transcontinental movement of peat and propagation materials, the application of very strict hygiene protocols combined with a reduction in the use of fungicides.

The main negative of saprophytic fungi is the hydrophobicity of their mycelia, which makes it difficult for the crop to absorb water. Direct competition for nutrients can act synergistically with this effect, leading to growth delays, especially in seedbeds. A final aspect, little investigated, is the possible release of mycotoxins.

Mycelium proliferation occurs on the surface of the pots, or in the cavity between the substrate and the pot wall, especially in the basal saturated layers. Similarly, in peat and packaged growing media, the preferential place for proliferation is the gap between the plastic film of the packaging and the contents, where, due to the formation of condensation, the most favourable thermo-hygrometric conditions occur. Sometimes, it is the woody component of the composted soil improvers in the substrate that stimulates the proliferation of basidiomycetes.

Fungal development can also be responsible for a change in the odour of the substrate, or for the formation of conspicuously coloured colonies.

The presence of these organisms, as the English name of: non-parasitic fungi makes clear, should not cause concern from a phytopathological point of view. Very rarely is the proliferation of parasitic mycelium (Alternariaceae and Sclerotinaceae). However, the proliferation of saprophytes can be an indicator of the quality of the growing substrate components.

The correlation between peats that have undergone self-heating with fungal development is not scientifically proven, but seems to find consensus among many experts. It would also seem that proliferation is more frequent in young, poorly decomposed oligotrophic peats, less so in brown and black peats with higher decomposition.

However, the equation: fungal colonisation = poor peat is by no means demonstrable, precisely because of the saprophytic nature of these organisms.

Control in cultivation can be entrusted to fungicides, but above all to agronomic measures, which should be preferred where possible.

Good ventilation (which can also be achieved by increased pot spacing) and drying of the substrate surface are, in many cases, sufficient conditions to eradicate infestations. Performing predictive tests on peat and growing media is not easy. Some protocols, not yet fully validated, are based on DNA tests and the presence of ergosterol.

When receiving peat or substrates that already show the presence of saprophytes, it is advisable to decompress and aerate the product. These measures, combined with a loss of moisture, reduce the chances of subsequent establishment in the culture.

While the culture is in progress, it is useful to remove pots affected by mycelium proliferation, if numbers permit. It is also very useful to clean the potting devices of substrate residues. As soon as possible, it is advisable to sanitise cultivation surfaces (sheets, pallets, etc.) where fungal propagules could remain.

Lastly, once the phenomenon has been eliminated, it is advisable to proceed with fertilisation that restores the nutrients (particularly nitrogen) removed by the fungi

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