The cultivated mushroom industry is subjected to increasing pressure for a change in the productive systems as consumers ask more and more for healthier products with an environmentally respectful background to cut down dependence on chemical fungicides, which are used to fight fungal diseases.
Fungal diseases are among the most severe mushroom disorders in crops, damaging yield, and mushroom quality. The preventive use of phytosanitary products is routinely applied.
However, considerable evidence of resistance has been reported to different fungicide groups from the second half
of the 20th century.
Before allowing new fungicides, the effect on commercial host strains must be studied both in vitro and in vivo before approval to prevent crop damage.
Since both mycoparasite and host are fungi, proven selectivity is the crucial aspect for suitable fungicides, limiting the emergence of alternative chemicals.
These alternative fungicides introduced in the mushroom industry by the mid-2010s showed promising results.
In today’s video, I will, therefore, talk about four different diseases, the causative agent, symptoms, chemical control, and resistance, as well as alternative treatments and recommended hygiene measurements.
What is the causative agent and what are symptoms of the Dry Bubble Disease?
The causative agent of dry bubble in mushroom crops is associated with two varieties of the species Lecanicillium fungicola, var. fungicola, and var. aleophilum.
Dry bubble is a ubiquitous fungal disease reported occurring in different countries cropping edible mushrooms, parasitizing various cultivated mushroom hosts such as Pleurotus ostreatus, Agaricus bitorquis, and Agaricus bisporus.
The dry bubble disease accounts for an estimated 20% of button mushroom crop losses globally, which, for example, represents losses of approximately 300 million euros annually for the European sector.
The infected host tissue is covered by the parasite, which produces spherical masses (clusters) of conidia covered by sticky mucilage that favors the disease dispersion through watering, fly vectors, workers, or dirty machinery.
The main primary source of this disease mentioned in mushroom crops is the casing material, especially peat.
Bonnen and Hopkins, when evaluating fungicide response against isolates of Lecanicillium fungicola detected cross-resistance between benomyl and thiabendazole, negative cross-resistance between these two benzimidazole fungicides and the carbamate fungicide diethofencarb, and relatively high resistance towards chlorothalonil.
The number of biocontrol agents available to cope with mycoparasites of mushroom crops is still limited in the market.
However, a fluent scientific activity currently aims to generate alternatives for the overused and, in some cases, relatively low-efficient chemical pesticides.
The following table shows several alternative treatments such as compost teas, essential oils from lavender, thyme or citrus limon.
What is the causative agent and what are symptoms of the Cobweb Disease?
Cobweb is a globally widespread pathology whose causative agent corresponds with several Ascomycota species described as harmful in different mushroom-producing countries and all belonging to the genera Cladobotryum spp.
Symptoms of the disease include the initial development of aerial and light-whitish mycelium over the casing layer and infected carpophores that quickly evolves due to massive sporulation to a dense white-floury mass that engulfs the surface of the casing layer and the surrounding carpophores; therefore, reducing the crop area available.
Cobweb appears more often at the end of the crop cycle commonly during autumn and winter.
Although the earlier it appears, the more devastating it can be. It is also worth noting that due to the nature of the dry spores generated by Cladobotryum, they are easy to disturb from the patches of disease during cropping operations such as watering or picking, or even by low air currents in the growing facility.
A single conidium or mycelium debris mobilized from the initial infection can generate a new secondary outbreak because of their asexual nature.
Different families of active substances have been historically applied.
Still, in the last years, the license of some broadly applied active substances are not renewed. For instance, the EU Member States recently voted to ban chlorothalonil, and the expected scenario suggests increasingly restrictive legislation.
The scarce range of available products, in addition to the continuous exposure to the same active substances, has also favored the occurrence of resistance among Cladobotryum strains, which negatively affect the efficiency of treatments.
Bioactive compounds extracted from different sources are interesting alternatives with proven antifungal activity tested as useful products to manage cobweb disease.
Essential oils from cinnamon, geranium, and spearmint completely inhibited the growth of Cladobotryum dendroides. Still, low selectivity was reported, and the authors suggested the essential oils can damage the host mycelium.
The impact of cobweb disease in Agaricus and Pleurotus crops was also reduced significantly by the active component of the biofungicide Timorex 66 EC – tea tree oil; however, it showed substantially lower toxicity against Cladobotryum
dendroides mycelium than prochloraz-Mn in vitro.
What is the causative agent and what are symptoms of the Wet Bubble Disease?
Wet bubble disease, caused by the mycoparasite Mycogone perniciosa, is a worldwide disease affecting commercial cultivation of white button mushroom Agaricus bisporus and cultivated fungi such as Pleurotus citrinopileatus but also found in the wild mushrooms, parasitizing a range of basidiomycetes.
Mycogen perniciosa affects the morphogenesis of Agaricus bisporus fruit bodies but not the vegetative mycelium.
The easily recognized symptoms of Wet bubble disease include the presence of masses of deformed tissue with no signs of differentiation into stipe or cap, which can reach 10 cm in diameter.
Wet bubble disease is managed by cultural practices, sanitation, and chemical fungicides.
By the mid-1970s, the control of Wet bubble disease relied on benzimidazole fungicides, mainly benomyl, because this fungicide was toxic against a wide range of fungi but was not effective in inhibiting the growth of most Basidiomycetes.
To date, no solid evidence of resistance occurrence has been reported among Mycogone perniciosa strains, which are remarkable compared to the evidence reported for other mycoparasites.
This could be related to the nature of the organism and a limited ability to generate mutations driven toward fungicide resistance.
Some essential oils from aromatic plants can suppress the growth of Mycogone perniciosa in vitro.
Essential oil from savory expressed better antifungal activity against Mycogone perniciosa than S. pomifera oil, and those of oregano and geranium expressed the most potent antifungal activity against the mycopathogen; since essential oils are considered nontoxic and easily biodegradable, their application is also recommended for disinfection of commercial casing soil with 2% oregano oil before application on germinated compost.
Among a wide range of forty essential oils understudy, lemon verbena, lemongrass, and thyme oils selectively inhibited the growth of Mycogone perniciosa.
In addition, lemon verbena or thyme oils effectively prevented wet bubble when Mycogone perniciosa was inoculated in the casing.
What is the causative agent and what are symptoms of the Green Mold?
Green mold is a devastating disease for mushroom farmers in crops such as button mushroom, oyster mushroom, shiitake, winter mushroom, or milky mushroom.
In December 2015, massive green mold epidemics caused by Trichoderma aggressive f. aggressive were reported in Hungary, with nearly 100% crop loss in the infected button mushroom beds.
Symptoms of disease detected in compost and casing surface have been described as extensive sporulating green patches covering the substrates and generating brown spotting on mushroom caps .
Trichoderma grows well on carbohydrates, and in this sense, seed grain is an important source of food and is very vulnerable.
Critical periods are the spawning and packaging of the compost when it is necessary to take extra precautions.
The colonization of the substrate is also critical, as there must be a good control of the temperature.
The imidazole prochloraz-Mn has proven effective against both Trichoderma pleuroticola and Trichoderma pleuroti, causal agents of green mold in Pleurotus ostreatus, both in in vitro and in crop trials when artificially infected.
Besides, the Trichoderma aggressive f. europaeum strains T76, T77, and T85, isolated from green mold infecting compost in Serbia, when confronted to a range of fungicides, showed the most significant sensitivity to chlorothalonil and carbendazim and were less susceptible to iprodione, some resistant to thiophanate-methyl, and resistant to trifloxystrobin.
Biochemical substances such as essential oils from plants have shown potent activity against Trichoderma aggressive f. europaeum, particularly basil and mint oils.
However, as noted before, some substances exhibit low selectivity and may damage the crop.
In addition, commercial biofungicides such as Serenade® WP, based on Bacillus velezensis , and Ekstrasol F SC
based on Bacillus subtilis, have been noted effective to control green mold disease when T. aggressive f. europaeum T77 was artificially inoculated in mushroom plots, showing a biological treatment comparable or even better than those plots treated with prochloraz-Mn.
Due to the evidence reported, it is highly advisable to prevent early contamination at the beginning of the crop cycle during the compost production and mycelium germination when strict hygiene measures must apply to avoid contamination.
Besides applying fungicides to control these diseases, following a strict hygiene protocol is recommended.
How to reduce contamination in mushroom farming?
1) Any agronomical activity which requires entering the growing facility should always be carried out from newer crops to the elder ones.
2) Disease vectors should be avoided by controlling fly and mite populations.
3) Batches of raw casing materials should not remain near crop facilities; otherwise, sealed spaces must be designed for casing storage to prevent contamination.
4) Remove all affected mushrooms before applying agronomical actions such as harvesting or watering.
5) Disinfection of clothes, footwear, and tools in critical areas is a crucial action.
6) Boxes employed for mushroom collection must be disinfected before entering the growing facility and never come from nearby contaminated crops in the farm.
7) Do not lengthen the crop cycle.
8) Once the crop cycle is terminated, when available, a cooking-out step on the crops must be applied using steam water to kill pathogens, followed by thoroughly cleaning and disinfection of empty facilities.
More information about a cooking-out step and the comparison of two cooking-out treatments can be found in the following video (Remark: the article below).
Talk to you there.
📝Gea, F.J.; Navarro, M.J.; Santos, M.; Diánez, F.; Carrasco, J. Control of Fungal Diseases in Mushroom Crops while Dealing with Fungicide Resistance: A Review. Microorganisms 2021, 9, 585. https://doi.org/10.3390/microorganisms9030585, https://creativecommons.org/licenses/by/4.0/