Penicillium must be one of the best known of moulds. But how many know there are over 1000 Penicillium species and that some of these have many varieties.
At least one, that happens to be particularly drab and insignificant (even under the microscope) is nevertheless of great interest to mushroom growers. This is because of its ability to colonise compost and greatly reduce yields.
The main symptoms, other than reduced yield, are a tendency to increased spawn run temperatures with later, poor mushroom quality, particularly in the third flush. The crop essentially becomes miniature flats, usually with no value. Finally, at emptying the compost is extremely dusty and this is likely to affect ease of breathing.
Variable yield losses, up to 90%
There may be more than one Penicillium species that can cause such problems. However, only one has provisionally been identified, this is P. implicatum (P.i. for short) At HRI, in Wellesbourne, an experiment indicated that P.i. could reduce yields by up to 90%.
However, this trial had a very high level of inoculum, over 3 million spores per kilogram of compost, applied at the beginning of spawn run.
Field experience in Northern Ireland suggests losses are typically very much less than this although occasionally they have been high, particularly with phase three compost. With phase two it was usually in the range 5-15%.
Even so, the reduction in quality after the first flush might double such losses. Comparing average percentages of the final yield taken in first, second and third flushes the change was from 42%, 38% and 20% to 47%, 39% and 14%.
Some life cycle observations
Observations by the author at Newforge Lane, several years ago, may help explain why P.i. can induce such a wide range of losses, 5 - 90% and why it is usually only the third flush that is badly affected.
Another mystery was, how does such a slow growing mould cause such high losses on occasions?
Its colonies expand extremely slowly, particularly if compared with the rapid growth rate of Trichoderma (Th2). P.i’s main ’weapon’ and one that Th2 lacks is that colonies start producing large numbers of tiny, easily airborne spores after only 3-4 days.
A single colony, growing in compost (provided it is disturbed now and then; to detach and disseminate spores) could produce thousands of ‘daughter’ colonies in only a couple of weeks.
Similarly, each daughter colony could then produce more colonies to produce huge numbers by the end of cropping.
A novel method of detecting spore smoke Nevertheless, examining affected compost, even under a low power microscope, it is often difficult to find a single colony. This is because they are so insignificant.
However, when compost is disturbed, by first removing the casing and then lifting a handful, this will appear to be smoking, hence the common name ‘smoky mould’.
To increase the chances of seeing such ‘smoke’ the house lights are best turned off and a torch placed so that the beam illuminates the sample from the side. This is then gently broken apart.
To concentrate the ‘smoke’ a device was made that would have put the Vatican cardinal’s signalling system to shame!
Two 12mm copper tubes were joined in parallel about 10-15 cm apart and were provided with sharp points. Each tube also had a long slit that faced the other slit. In use, the pair of tubes was gently pushed into the side of a bag full of cropping mushroom compost.
Then, using a flexible plastic tube connected to one of the copper tubes, air was blown into one tube so that it moved into the bag via one slit, through the compost and out via the other slit, carrying P.i. spores in a stream along the beam of a small torch attached to the device.
The beam was normally almost invisible from the side in the darkened ‘house’, but it briefly and brightly lit up if P.i. spores were present. To prove it was P.i. spores that caused this, one or more petri dishes of sterile media were briefly held open, facing the spore stream, these later grew a multitude of P.i. colonies.
Alternatively, empty plastic petri dishes were charged up electrostatically, by rubbing the backs of them with dry cotton wool and then individually held facing a spore stream. The electric charge in the dish trapped the typical chains of small, round P.i. spores on the surface of the clear plastic. These could then be identified directly under a microscope, using 400 magnification.
Only occasionally was the spore ‘smoke’ found to comprise species other than P.i., in particular a heat loving actinomycete was sometimes found. It flourishes in spawn run compost that has been allowed to overheat and this is also associated with reduced and/or delayed cropping. The actinomycete’s spores were sometimes joined in chains, but its individual spores were brown and angular not round, pale blue, green or yellow, as in the case of P.i.
Why is this weed mould not more common?
The reason why P.i. is not more commonly found causing problems appears to be because a large number of spores must get into compost early on for it to cause noticeable losses. Certain circumstances favour this.
For example, if phase three compost is being produced in bulk, it is being spawn-run with air ‘blowing’ through it intermittently. These conditions are ideal to encourage P.i. to produce several generations of daughter colonies, thus astronomically increasing spore release during phase three’s bagging/blocking.
Under such circumstances, if phase two is produced under the same roof, it may also become sufficiently contaminated to show a marked yield reduction.
In normal circumstances where phase two is not produced alongside either phase three or near to cropping houses, that could release spores, it is very unlikely to be affected, except at a level too low to noticeably reduce yield.
The normal sites for P.i. survival are in the soil and possibly also in straw bales.
A Warning
A warning for growers using Dutch shelving, if not cooking out, is to watch out for dustiness at emptying, this might be the first sign of P.i. cycling from crop to crop, severe yield and quality reductions could follow.
However, if using phase three this might equally well be the source of the P.i. problem.
