Growers trying to keep their heads above water, not thinking enough about farm hygiene details, or not having time for them, are the main reasons why Verticillium gets out of control.
This would be the short answer to the grower that phoned The Mushroom People and asked for an article on Verticillium. The fact that a previous article was quoted as being helpful in combating cobweb is gratifying. Any feedback from articles, complimentary or critical is most welcome.
The term Verticillium, as used by growers, usually lumps two quite distinct diseases together. They are commonly also known as wet and dry bubble. Both produce grossly distorted mushrooms and can have a similar looking spore producing stage in common, but they are best dealt with separately.
Wet bubble caused by the fungus Mycogne perniciosa is termed ‘wet’ because it produces brown droplets on the surface of affected ‘mushrooms’. If these are infected early, they cease to have a typical mushroom shape with cap and gills and instead become potentially huge, irregular lumps.
This disease is usually not serious, but lumps should be removed from the beds as early as possible and disposed of in sealed polythene bags. At very least, the damage caused is that lumps weighing a pound mean a pound less of saleable mushrooms.
Mycogne produces two types of spores. One type is easily airborne, after becoming detached by pickers or by watering. Infection is usually through contaminated casing, although this is just as likely to occur on farm, as elsewhere, particularly if there is any farm dust blowing about.
If casing is known to be likely to be infected at source, as indicated by a number of comrade loads being simultaneously affected. According to ‘Fletcher*’ such casing at risk should be treated with 1 % formalin at a rate of 25 litres per square meter of bed. However, it is no longer permissible for growers to attempt this treatment.
Prochloraze manganese (Sporgon, for example) can be applied to casing by growers to help with control.
Dry bubble is a much more common cause of serious losses. It is caused by the fungus Verticillium fungicola, of which there are several varieties. Under the microscope each sporing structure looks like a miniature Christmas tree that has lost its needles. It has instead sticky spores at the tip of each ‘twig’.
Mushroom flies very easily spread these spores, however splash dispersal during watering and movement of pickers are the most likely causes of widespread dispersal. Faint cap spotting, or dark brown spots with irregularly shaped edges are a common cause of loss of quality. Such marks indicate a late infection. Early infection of pinheads leads to various bizarre distorted mushrooms. These become dry, dusty looking and brown as spores are produced on them.
Good illustrations of wet and dry bubble are to be found in posters produced by the spawn companies, or in a useful book to have, Mushrooms – Pest and Disease Control by Fletcher* White and Gaze.
A new edition is due out soon. One of the authors, Gaze can be contacted, c/o The Mushroom Journal 2 St. Paul’s Street, Stamford, PE 9 2EB, for further information.
The value of cooking out
If you have a house full of flies, have just lost yet another third flush to Verticillium don’t take this lying down, get angry, beg borrow or steal a steamer and cook the lot, anything to help break the cycle.
Feeling vindictive about flies helps stimulate ideas on how to get rid of them. Farm hygiene is extremely important because of the vast numbers of easily distributed spores produced by Verticillium.
I have seen houses with many clumps of brown distorted mushrooms in the third flush. With no cook out, this is likely to lead to a succession of infected crops, if only because the spores are likely to become so widely distributed around the farm on particles of dust. Any alterations or new building works are likely to lead to an outbreak.
Losing a third flush means losing one fifth of the crop. Perhaps a fourth flush is never grown. However if it were, it would have meant on average another 10% addition to yield. Consequently, if compost is capable of producing 6501b/t (295kg) from 4 flushes on bags, or 3 on shelves, which bunker compost often is, then the total loss is 30%, or 3900 lb in a house containing 20 t of compost.
The value of such a weight of third and fourth flush mushrooms is likely to be hotly debated, especially at the present time. (Some of the factors that control quality were outlined in a previous article.)
Nevertheless, a few growers consistently produce such high yields and also have very low disease levels. This proves it can be done, even without cook out facilities available. However, sufficient attention must be given to detail.
Farm hygiene details
A list of hygiene details has been given several times before, however I suspect most growers only have time to read the occasional article that catches their eye, so these may be worth repeating.
All distorted pins should be removed by picking them as early as possible, certainly before they start to be a source of disease spores. It is most important not to water over infected mushrooms that are producing Verticillium spores.
Even if distorted ‘pins’ are not infected, misshapes are a hazard to quality and a source of loss if they end up low priced or dumped. The nutrients could instead have been available to ‘feed’ perfectly formed mushrooms.
Designated pickers should be instructed to go round every day and certainly before watering. Good lighting will help them spot distorted mushroom ‘pins’ at an early stage. Pickers themselves are likely to carry spores on clothing so if outer clothing is frequently laundered, this helps.
Get rid of those flies
Because flies are a means of spore dispersal, fly control is doubly important. Go into each house in turn, close the door, turn off the lights and see where the sun shines in. An hour or two with builder’s gap-filling foam plus some mesh, fine enough to keep flies out, will soon pay high dividends.
I favour the use of light traps for reducing the number of surviving flies if any manage to get in. However if such traps are home-made to reduce cost they should contain fluorescent, low wattage, bulbs as being most attractive to mushroom flies and being least expensive to run.
Traps should be sited in corners, each with a dark cover above and the light should shine out through a slit 2-3 cm wide. A plastic bucket of water plus a few drops of detergent in it placed just below the bulb will drown most of the flies. Traps can also, or instead, have sticky yellow strips, either attached to them, or sufficiently close to be well illuminated.
Traps should be on all the time, from even before the house is filled. However traps should not illuminate the compost. This could help surviving flies find each other and find suitable sites for egg laying! It is thus possible that lighting the compost in a house with light from a trap could make the fly problem worse, instead of better. A design for an effective trap was given previously.
Casing should be protected from dust when being stored and/or from dust landing on the ‘apron’ of the house.
Before a batch of casing arrives, this apron should be disinfected and as much as possible of the surrounding area wetted as method of slaying the dust. Sporgon can be applied in the last watering of the casing, or best use might be in a split application.
If there is very little disease on farm it may be better to keep Sporgon in reserve, rather than using it routinely. This latter has some low risk of making it lose its effectiveness.
Another stitch in time ...
If houses become severely affected they must be terminated early, preferably by cooking out. Old houses already cropped should be emptied as soon as possible to prevent further production of Verticillium and flies on farm, potentially able to infect the next crop.