Verticillium

This disease is caused by Verticillium, a species which produces sticky spores.

The symptoms produced vary with the developmental stage of the mushroom at the time of infection. 'Early' infection at pin head formation results in the production of malformed pin heads, which turn a grey/brown colour and remain leathery. Infection at a later stage causes a thickening of the stipe especially at the base, and also a crooked mushroom with a tilted cap and backward peeling stalk. Cap infection can occur at a very late stage in the development of the mushroom, resulting in circular superficial spots, initially pale brown in colour which become grey with age

Verticillium sp. commonly occur in the soil which may be a primary source of the infection, however, it is more likely that they come with casing or by transfer of infection from house to house by pickers, flies or machinery. Spores can lie dormant until they come into contact with mushroom mycelium which stimulates them to grow.

Because the spores are sticky the disease is spread on dust particles from the movement of earth or spent compost. This dust can contaminate fresh casing, or can enter through fans or doorways or be carried in by flies, pickers or even mites.

The disease within the production house can be spread by water splash. The spores can also be spread on any other equipment used in an infected growing house. Appearance of the disease at pin head formation signifies infection at an early stage of development, probably at the time of casing. Development of the disease at later stages of the production cycle usually indicates infection has occurred from other cropping houses or from outside sources.

Spread by water, flies and pickers can result in 30% of the crop being infected at the third flush and by the last flush virtually all of the crop.

The highest standards of hygiene are essential for the control of Verticillium. Other methods of control are as follows:

1. Avoid soil movement near mushroom houses especially on windy days. Take measures to avoid the accumulation of dust in the vicinity of mushroom houses and also dust movement into the vicinity of mushroom houses.

2. It is essential to control flies and every effort should be made to prevent their entry into cropping houses.

3. Infected bags should be picked last, to minimise the risk of pickers spreading spores.

4. Picking trays should not be set on the ground in the production house as dust is attracted to the sides of the trays by static electricity.

5. As diseased mushrooms appear they should be removed from the beds using a cloth or sponge soaked in disinfectant and then placed in a bucket containing disinfectant.

6. 'Table' salt on top of tissue paper can be used to contain disease.

7. All ventilation filters should be replaced after each crop