Mushroom Virus X Syndrome - Patch Disease and Brown Cap Mushroom Disease
Mushroom Virus X syndrome (MVX) describes a range of symptoms including delayed opening, reduced yields, distorted mushrooms and cap browning which express on cultivated Agaricus bisporus crops. MVX syndrome is now known to be a combination of two separate virus diseases – Patch Disease and Brown Cap Mushroom Disease – which can occur together and express a variable and complex symptomology. In some cases, one or two symptoms may predominate.
MVX syndrome first emerged in Britain in 1990 when mushroom crops began expressing symptoms resembling a virus infection. But established techniques used previously to detect the La France virus were unable to detect a virus responsible for the novel infection, leading researchers to investigate other causes.
Internal Stipe Necrosis
Internal Stipe Necrosis was recognised in the late 1980s and early 1990s as a significant emerging threat to the United Kingdom mushroom industry.
With the A. bisporus wholesale market at the time valued at around £250 million, losses of up to 10% were reported by some United Kingdom mushroom producers with one grower estimating the incidence of Internal Stipe Necrosis on his farm totalled 0.3% of mushrooms harvested at the button stage and 5.6% of mushrooms harvested at the open, flat cap stage.
Internal Stipe Necrosis has been observed sporadically on Australian mushroom farms over the past 10 years or so and its appearance is consistent with the widespread adoption of wetter and heavier black peat as a significant component of mushroom casing.
The disorder expresses as an unsightly browning and necrosis of internal stipe tissue which cannot be detected before harvest, resulting in significant loss of quality.
Lecanicillium fungicola– Dry Bubble disease
Dry Bubble is the common name given to a serious fungal disease affecting cultivated Agaricus bisporus crops. It is caused by the soil-borne mycoparasites Lecanicillium fungicola var fungicola (syn: Verticillium fungicola; Verticillium malthousei) which is found in Europe and Lecanicillium fungicola var aleophilum which is more common in North American Agaricus crops, including Agaricus bitorquis. Dry Bubble is consistently the most significant problem facing growers wherever button mushrooms are grown, including Australia.
Lecanicillium fungicola was first identified as a mushroom pathogen in France in 1892 and was described as the causal agent of ‘La mole’ disease, the French term for what we now refer to as Dry Bubble. But despite more than 130 years since its first appearance, Dry Bubble continues to have a detrimental impact on mushroom production. The disease causes significant losses estimated at 2-4% of total revenue annually and poor control of the disease may result in losses approaching 20-25% or more, while uncontrolled disease can result in farm closure.
Getting the best from your cookout
As mushroom crops mature, pest and pathogen levels increase so that by the end of the crop, the pathogen population reaches its maximum (Fletcher & Gaze 2008). Effective crop termination is essential to reduce the pathogen population, allowing the next crop to ‘start clean’ and to break the cycle of diseases, such as Dry Bubble, which are perpetuated by continual on-farm re-infection.
By far the most effective termination procedure is cookout in situ, where the crop is treated undisturbed in the grow room with steam. An effective cookout prevents contamination of subsequent and adjacent crops which occurs when spent substrate contaminated with viable pathogens, pests and their larvae is removed from a grow room (Beyer 2018).
Cookout must kill pests and pathogens within the compost and netting on shelf farms and within the compost and tray timbers on tray farms. Cookout must also kill Agaricus mycelium and spores within the compost and tray timbers to prevent spread of virus diseases.
Cladobotryum spp. – Cobweb disease
Cobweb is the common name given to a fungal disease affecting Agaricus bisporus crops in mushroom growing regions worldwide.
It is caused by species of the genus Cladobotryum (formerly Dactylium), primarily Cladobotryum mycophilum and Cladobotryum dendroides. The pathogen grows rapidly over the casing surface and colonizes mushrooms at all stages of development with a white aerial mycelium, causing a destructive soft rot. Cladobotryum sporulates heavily and the spores are easily spread around the farm causing secondary infections. Spores landing on mushroom caps incite browning, causing loss of quality.
Until the early 1990s, cobweb outbreaks had little impact and were easily controlled by available fungicides and routine hygiene practices. But during the early 1990s, the incidence and severity of cobweb on British mushroom farms increased and in 1994/95 the disease reached epidemic proportions, regularly causing up to 40% crop loss.
Penicillium hermansii – Smoky mould
Smoky mould is a destructive compost infection first recognized in the Netherlands nearly 30 years ago. Despite being identified as various Penicillium species over the years, the true identity of the causal organism has only recently been confirmed through molecular analysis as Penicillium hermansii.
Penicillium species produce long chains of very small, lightweight, dry spores which are around 0.002mm in diameter (Fig. 1) and become airborne very easily. A single Penicillium colony (Fig. 2) will produce 400,000,000 spores per day. Although P. hermansii is very slow growing, it is problematic because of the large number of spores it produces and its short generation time. Penicillium hermansii will sporulate only three or four days after infection, producing thousands of daughter colonies which in turn sporulate rapidly.
Actions points to control Sciarid & Phorid flies
Flies are effective vectors of disease because the sticky disease spores attach themselves to the flies legs and the flies transport them from crop to crop.
Sciarid and phorid flies can breed in bushland, ‘waste’ or ‘spent’ compost in the farm environment and, most efficiently of all, in growing rooms.
The odour associated with the Phase 3 compost arriving on a farm either in bulk or blocks acts like a strong magnet to attract flies to the new crop.
The fact that a female sciarid can produce around 100 offspring and a female phorid can produce around 50 offspring means that new crops need to be strongly protected from invasion by adults and a holistic and integrated approach to fly control is needed.
Fact sheet: Syzygites megalocarpus – Troll doll
Troll doll is caused by Syzygites megalocarpus a Zygomycete which is ubiquitous in nature, colonizing a diverse variety of dead or moribund fleshy mushrooms. Syzygites (pronounced “size‐a‐guy‐tees”) was initially observed on cultivated mushrooms between 2004 and 2007 in crops of Agaricus blazei (Sun mushroom) in Brazil.
It was first recorded in Pennsylvania in August 2011 and has since become widespread on commercial beds throughout North America.
Confined initially to late flushes of brown portobello strains of Agaricus bisporus, Syzygites has since been observed on earlier flushes and on white strains of Agaricus bisporus. Due to the mould’s tolerance to low temperatures, it has also been observed in postharvest packaged product, the mould appearing while on the store shelf.