Ask any grower, read any book on mushrooms, consult any adviser and the message is same. They will all agree that water can spread mushroom diseases. But the option of preventing spread by using less or no water is ridiculous. After all water makes up about 90% of the fresh weight of the mushroom and is absolutely essential for crop production. In recent years with new casing materials, the amount of water applied to a crop has considerably increased together with yields. A part of the increase in yield is probably an increase in water content of the mushroom which could explain why it is no longer possible to fry mushrooms; they can only be stewed in the frying pan.
So watering is an essential process and pathogen spread will continue to happen. Taking all the usual precautions, such as removing disease before watering and effectively treating disease patches, will minimise water spread but is unlikely to eliminate it as with the best will in the world complete removal of disease, especially towards the end of the crop, is very difficult.
Water is also used as part of the hygiene programme. Power washing can be a very effective way of cleaning up after a crop and water is also used to wash down pathways after casing, after picking or after making any sort of mess on the floor. It is this latter use of water that may be overlooked as a means of pathogen spread.
Verticillium and Water
Some years ago, and again more recently, I investigated a persistent Verticillium problem. The distribution of disease was significant although it took a long time to understand why. Both crops were layered, one was trays 4 high and the other shelves, also 4 high although the principle applies equally well to single layer crops. On both farms the Verticillium occurred on the bottom layer in the first flush, often before mushrooms had been harvested, with very little on the second layer less on the third and none on the top.
As the crop progressed the patterned remained, often with none on the top even by the end of the third flush. Some houses were consistently worse with every crop badly affected although some remained more or less Verticillium free. On one farm the bed ends had most disease particularly at the ends nearest to the door, although not exclusively so.
Many theories were put forward to explain the distribution. Could flies be entering from the central corridor and landing initially on the bottom layer. This explanation could have accounted for the patches near to the door but not so likely those at the far end and would they always land on the bottom layer?
Airborne distribution of debris was considered to be possible as on both farms the ducted air blew down at the sides and circulated across the beds. The air was moving across all the beds although the bottom two always seemed to have more air movement.
Could this had brought more spores to the bottom beds? Verticillium spores are sticky and are not readily airborne so it seemed unlikely that such a consistent disease occurrence could be accounted for in this way.
Sporgon distribution was examined. Could it account for poor application on the lowest layer? One farm used a hand held hose and rose system of fungicide application, the other a watering tree. It seemed unlikely that such a consistent pattern could be accounted for in this way especially as every effort was made to treat the bottom layer after the spray theory had been proposed. Could the spores be introduced on wheels of vehicles used at house fill? Spores on the floor would then be splashed onto the lower layer at watering.
This seemed possible particularly with the hose system but less likely with the horizontal application of water from a watering tree. A disinfectant mat in the doorway, sufficiently large to allow wheel rotation, did not reduce disease levels.
The most likely explanation of Verticillium distribution, and I believe the correct one, was discovered by a chance observation.
It demonstrates how important it is to look at every farm operation as critically as possible even though the operation may have been done in the same way for years. Standing in the new crop just after casing, the Manager and I watched a very industrious worker cleaning the excess casing from the floor. This involved operating a high pressure hose and swilling the excess casing to a point where it could ultimately be picked up.
The concrete floor of the house was not in bad condition but had been laid in such a way that the water drained to the centre where there were drain holes. To get the correct shape, the floor had been laid in sections and at the corners there were joins running diagonally from the corner of the house towards the centre. These joins had worn and opened slightly so more casing was trapped in them than on the smooth floor. These cracks in the concrete were industriously hose out clean but the diagonal direction resulted in the debris being splashed onto the lowest layer of crop at the ends of the beds and probably some onto the second layer. These were just the places that the first disease was occurring.
It seemed very likely that spores of Verticillium or other pathogens washed off the beds would accumulate in the same places as the casing. Cracks in concrete are very likely places and in these cracks the spores would probably survive cook-out and disinfection.
The message from this saga is, firstly observe all that happens on the farm and try to think of the possible implications of what is being done.
Secondly do not hose the floor to clean up fallen casing but use a scrapper and shovel.
Thirdly, maintain cracks in concrete because it is possible that spores washed from beds will end up in these and can easily be returned to the beds by the use of power washing of the floor.
On the shelf farm, a change away from power washing after casing and subsequently during cropping resulted in a massive reduction in Verticillium with none showing in the first flush.
Unfortunately the other farm had gone out of business before the observation was made. John Fletcher August 2006 Consultant Plant Pathologist, Godmersham, Canterbury, CT4 7DL