After Verticillium, cobweb disease is the most important parasitic mould infection in the Belgian mushroom growing industry. Until about a decade ago, English researchers ranked the cobweb problem in third place, behind Verticillium and Mycogone. During the period 1994-1995 the UK mushroom growing sector was hit by an epidemic of cobweb disease, which rocketed the previously rather overlooked disease into the centre of attention because of the disastrous financial consequences it caused.

Various moulds can cause infection with ‘Cobweb disease’. The best-known (and probably most common) cobweb mould is Hypomyces rosellus, formerly usually referred to as Cladobotryum dendroides or Dactylium dendroides. In literature, Hypomyces aurantius and Hypomyces odoratus are also mentioned as well as Hypomyces rosellus. Both species cause more or less similar symptoms to H. rosellus (see further). It is unclear to which extent the last mentioned species also occurs in Belgium and the Netherlands. Apart from that mentioned above and a few facts about the situation in the UK, there are no further details about these two moulds from the field in Belgium and the Netherlands. In practice all three species are referred to by the same name.

Dutch               spinnewebschimmel     

French              toile

English             cobweb disease - cobweb mould

German            Spinnwebschimmel

Symptoms

‘Classic’ cobweb mould symptoms of Hypomyces rosellus

Cobweb disease has a fairly typical pattern of symptoms so is usually difficult to confuse it with other parasitic moulds. Contrary to Verticillium and Mycogone, this mould is visible on both fruit bodies and the casing soil.

Problems with cobweb mould often start with small, more or less circular patches of infection on the casing soil. At first sight, the diameter of the infection is usually no bigger than 3 to 4 centimetres. Infection can spread from dead pinheads and stalks left behind on the beds, but this is not always the case: the mould can still develop on casing soil without the immediate presence of any dead fruit bodies or discarded stalks.

Cobweb disease has fine, greyish white mycelia that rapidly develop. Certain sources indicate a rate of development of several decimetres a day, although observations in practice and research suggest this is on the high side. Widespread patches of infection lose their circular pattern and adapt random shapes. The mycelium can quickly overwhelm dead pinheads, stalks and fruit bodies. Infected mushrooms colour brown and start to decay.

If infected mushroom are left untreated, the mycelium threads of the cobweb disease will start to trail over the mushroom cap.  After a while, cobweb mould mycelium on the casing soil can change from a fine, cobweb-like mycelium to a firmer, floury or powder-like structure. In such cases, the mould is sometimes confused with white plaster moulds. Normally the greyish white colour changes into a reddish or even yellow colour. This last colour is however rarely observed in practice, possibly because the infection has already been treated at an earlier stage.

Cobweb mould spots of Hypomyces rosellus

As well as the ‘classic’ symptoms of cobweb disease, there are certain cases where cobweb mould spots or patches are visible on the caps of the fruit bodies. The infected patches are brown or reddish brown with a fuzzy outline.

If the mould infection occurs in this form, it may also be confused with, for example, Pseudomonas tolaasii (Bacterial blotch), Verticillium fungicola var. fungicola (Dry bubble), Trichoderma and Verticillium fungicola var. aleophilum .

Hypomyces aurantius and Hypomyces odoratus

According to the authors who mention these species, the symptoms are largely similar to H. rosellus. Hypomyces aurantius does not exhibit the typical pinkish red colouration of the mycelium like Hypomyces rosellus and Hypomyces odoratus has a typical camphor odour.

Life cycle

 

Spore production

The conidiophores or spores are produced in massive colonies by cobweb disease creating a floury or patchy appearance. Estimates of how many spores are produced are unavailable. The conidiophores are produced at the ends of the upright conidiophore carriers. The conidiophore carriers are branched into three to four phialides. The conidiophores are hyaline, initially single-celled and later have 1 to 3 septae. The spores are approximately 20.6 x 9.6 µm and have a symmetrically placed basal scar where they previously joined the phialide.

In laboratory conditions on an agar nutrient medium Hypomyces mycelium produces spores just four days after inoculation. The development is characterised by typical colour changes from creamy white (+ 4 days) to yellow (+ 9 days) to pink (+ 12 days) and finally to red.  This changing pattern of colour is also visible on beds with older infections.

Microsclerotia

In certain cases cobweb disease starts to produce microsclerotia. The vegetative hypha that make up these microsclerotia are enlarged and irregular from an early stage in their development. The microsclerotia are dark in colour with a thickened cell wall.

Until present, microsclerotia have only been observed in axenic (sterile) cultures, and not in actual situations on working farms. However the importance of their role in the life cycle of the mould cannot be dismissed, as ordinary spores cannot survive between two growing cycles.

Survival of spores and microsclerotia

Ordinary Hypomyces rosellus spores can survive for a maximum of seven days in sterile water. Saved at 0 % relative humidity (RH) their germinating power drops from 64.3 % initially to 44.7 % after being kept for 49 days. Data relating to storage at RH levels customary in mushroom growing is not available.

The survival rate of microsclerotia is considerably higher. Saved in sterile water, 100 % still have germinating power after four months Saved at 0 % RH the germinating power after 14 days is 100 % and after 56 days 76.9 %.

In light of the high survival rate of microsclerotia it’s plausible that they contribute to the survival chances of the mould in adverse conditions. Their significance in mushroom growing is however still not entirely clear. According to some researchers it’s possible that similar microsclerotia are present in raw materials used in casing soil.

Spread of the spores

The sources of cobweb mould spores can be sought on the farm as well as elsewhere. Hypomyces spp. are micro-organisms also found in places such as soil, on moss (Polytrichum sp.), on dead wood and wild fungi.

It is evident that the majority of spores are dislodged and carried by air movement. However, regarding Hypomyces spp. certain nuances apply. Cobweb mould spores are quite large and aerodynamic and easily disintegrate. In order to transport spores a minimum airspeed is necessary. Adie & Grogan (2000) stated that airspeed of 0.1 m/sec was insufficient to dislodge cobweb mould spores and transport them over any great distance. Dar (1997), who in details of his experiments, worked with a higher airspeed, did however identify airborne spore transport. Higher airspeeds (> 0.1 m/sec) occur in and around farms. The normal airspeed over beds in Belgian-Dutch systems is approximately 0.2 m/sec; the airspeed in air ducts in the growing rooms is 4 m/sec in the duct and 8 m/sec at the air outlet. These airspeeds are considerably higher than those used in the experiments run by Adie & Grogan. This suggests that airborne spore transport cannot be excluded.  Continued research confirms this view.

The work of Adie & Grogan (2000) illustrates that certain actions can create clouds of spores. After artificial infection of the casing soil with Hypomyces odoratus mycelium during pin heading, the first cobweb mould colonies were observed when the 1st flush started. Despite the early appearance of infected patches, the first ‘major’ clouds of spores were only noted after the first flush. These clouds of spores appeared to have been caused by physical disturbances to the infected patches. This disturbance can be traced to watering and covering the infected patches with salt as a preventative measure. In the 2-hour peak period of the spore cloud, the number of spores amounted to 29,000 parts per m³ air. During the first flush no large amounts of spores worth mentioning were recorded. The presence of spores in the air, however small, does indicate that factors other than watering and applying salt can dislodge spores. The authors mentioned above do not prohibit a similar influence of picking on creating spore movement, although the study does not demonstrate this. During further crop development from the 2nd flush, massive amounts of spores were noted in the air without their being any human intervention, which emphasizes the importance of spread by air.

Spread of cobweb disease via insects (Phorids and Sciarids) is not proven and the presence of Hypomyces in spawn is improbable.

Spore germination

Spore germination depends on a number of factors. The most important for the mushroom industry are RH and temperature.

At a high RH (97-100 %) the spores germinate very easily, if the RH drops the number of germinating spores drops accordingly. At an RH lower than 85 % very few spores germinate. The RH level where little or no spores germinate is so low that using this technique is difficult. However, slightly lower levels of RH (-1 to -2 % compared to usual levels) in growing rooms could be an important tool in preventing the spread of cobweb disease.

The optimal temperature for spore development is 25 degrees Celsius. If treated for 30 minutes the temperature required to destroy Hypomyces rosellus spores is 43 degrees Celsius.

Mycelium growth and spread

The mycelium grows on nutrient mediums at a speed of 7.5 mm per day. The optimal temperature for mycelium growth is 20 degrees Celsius.

As well as spores, mycelium can also been seen as a carrier of cobweb disease.  Other vectors include pickers and equipment and material. Spread via mycelium usually leads to rapid development of symptoms.

Symptom development

Several sources demonstrate that infection via mycelium has more disastrous consequences than infection via spores. If the casing soil is infected via spores at casing, the symptoms will only appear in later flushes (from 3rd to 4th flush). Only with a very persistent infection will symptoms be observed earlier. If mycelium fragments cause infection, the first symptoms may occur in the first flush.

Both RH and temperature affect the degree of infection. On artificially infected cultures carried out with RH intervals of 96-100 %, 91-95 % and 85-90 % and at a fixed temperature (16-18°C) the number of infected mushrooms fell from 64 % to 50 % and to 30 %. In the same experiments (Seth & Dar, 1987) carried out at temperatures of 19-22°C, 16-18°C and 12-15°C and at a fixed RH (90-95 %) the number of infected mushrooms fell from 54 % to 37 % and to 21 %. Considering the importance of a high RH and a high temperature for the development of the infection, the autumn in particular seems to be the high-risk period for cobweb mould infections, a theory confirmed by practical observation.

The development of spots can already occur at an early stage after the initial infection and at low spore pressure. Experiments by Adie & Grogan (2000) showed spot development on the mushrooms in the first flush after infection of the casing soil with Hypomycesmycelium during pin heading. Spots development even with fairly low spore concentrations in the air.

There is no data available regarding the actual infection mechanisms of cobweb disease in Agaricus bisporus.

Prevention and control

Recently there have been many problems with cobweb disease, particularly in the autumn. Relatively high temperatures, a high humidity and beds that dry with difficulty encourage development of the mould. Normally, the switch to winter weather would seem to reduce the risks in this situation. In the winter the outside air has to be heated up considerably, which usually results in a lower RH in the growing rooms. This inhibits the development of cobweb disease.

We have also noticed that cook out is being used less and less by growers and that on some farms drastic savings are being made on the use of disinfectants - both matters that are detrimental to proper crop protection. In the long term, any immediate savings achieved by these measures, will be negated by the need to use extra disinfectant, production losses caused by the use of fungicides, clearing away infected mushrooms during or after the 2nd flush, etc. A number of tips for prevention and treatment are listed below.

Causes

Internal infection: Air movement (caused by ventilating in the growing rooms, spreading salt, harvesting); Watering (splashing); pickers; machines; packaging and containers.

External infection: Infected packaging and containers; external visitors (advisors, traders, drivers, etc.); infected vehicles; infected casing soil (after unloading at infected site).

Encouraged by 

Mushrooms and stalks left on the beds; high temperatures; high RH and too moist casing soil layer.

Prevention

Prevent disturbance of spores by: sprinkling salt (sprinkle from just above the bed!); watering (first apply salt, then water). 

Climate: work at lower temperatures (e.g.17°C instead of. 18°C);work with lower RH (e.g. 2% lower than normal).  

Spraying: do not spray if casing soil fails to dry sufficiently; do not spray on infected patches.      

Disinfect machinery, equipment, etc.

Ensure employees wear clean clothes at all times.        

Cook out.

Treatment

No efficient agents allowed in Belgium (carbendazim is NOT permitted in Belgium).         

Efficiency of prochloraz (Sporgon and Sporex) is very low; Cover infected patches with salt as quickly as possible; Cook out early.