A new disease of A. bisporus – Putative Virus X

Richard Gaze

Introduction

Acceptance of the fact that a new disease may be occurring is a slow and difficult process. Experience and healthy scepticism indicate that the likelihood of such a phenomenon is extremely low. In the absence of any identifiable pathogen all other possible causes have first to be explored. Initially, whilst the problem is restricted, commercial confidentiality tends to stifle debate.

The purpose of this short review is to summarise what is currenntly known about a new disease phenomenon that appears to be occurring on a number of farms in Britain and to open the debate both for added stimulation and perhaps to allay some concerns that might occur due to the lack of information.

Background

In the autumn of 1996 a single farm in Britain was reported to have cropping problems for which no satisfactory explanation could be found. Several other farms experienced cropping problems of a transitory nature during 1997. These may or may not have shared a common cause with the original farm but they were similar in two ways. The problems involved an inability to pin and there remains no convincing explanation of the cause.

During late 1997 a second farm, unrelated to the first, began to experience similar problems to the original case which continued unabated and unresolved. During 1998 the number of farms being affected showly rose to approximately fifteen.

Symptoms

With the benefit of some hindsight one can say that the symptoms of this new disorder are quite diverse. Early, noticeable, symptoms were patches of non-productive bed. For this reason there is some danger that the disease will retain the early epithet of Patch Problem or Patch Disease. As the range of symptom becomes more apparent it seems inappropriate for this to be retained, as such a descriptive name can be misleading.

Symptoms are now known to include bare areas of casing of regular or irregular shape, sometimes with an edge of apparently vigorous sporophore development. In some instances the ‘patch’ produces mushrooms which are slower in development than normal areas, thus giving an effect of hills and hollows within the mushroom stand. An extreme symptom is where the bare areas exceed the productive area resulting in a form of clumping. On-farm experiments have shown that it is possible to transfer the symnptom to healthy trays by transfer of compost from within an infected area.

In addition to this type of symptom there have been instances of distorted mushrooms reminiscent of virus together with discoloured mushrooms of similar association. There have been apparently conflicting reports of symptoms becoming worse during the life of a crop and others where this is not the case.

A completely different symptom quite commonly observed is premature cap opening, sometimes associated with slightly imperfectly-formed cap structure. The disorder can undoubtedly also be symptomless apart from yield loss. As with orthodox virus disease yield loss is the one common universal symptom of the problem. Some growers have consistently reported a slight delay in cropping.

There is some considerable evidence to suggest that symptoms, apart from yield effects, can be masked or, conversely, increased by changes in husbandry or crop management suggesting that in may instances the disease is at the boundaries of symptom expression.

Causes

As the number of farms affected increased it became apparent that there appeared to be no obvious common factor to link any of the cases. Early suspicion fell on compost type, compost source, casing origin, spawn and husbandry techniques. The increasing diversity of all these factors from the increasing number of farms affected eliminated them from serious consideration.

Causal organisms were exhaustively sought but none were found. The original farm briefly demonstrated the presence of traditional virus disease (La France in the USA) but subsequent tests were unable to confirm this. Several other farms briefly demonstrated higher than normal, background levels of traditional virus but again these indications were not sustained. A breakthrough occurred when Peter Romaine of Penn State University was supplied with samples via an extended commercial link with the British Farms. When examining material for virus (35nm particles) by electrophoresis he discovered what appeared to be novel dsRNA bands, thus providing a plausible hypothesis for the cause of this problem.

Progress

In collaboration with Peter Romaine and the British research levy board, the HDS, who have funded the work, HRI has undertaken a project to investigate this problem.

Work so far has repeated and validated Peter Romaine’s initial findings and has further established that up to eight novel dsRNA bands are detectable in ‘infected’ material. The supposition is now quite positive that, following this validation work and a study to establish symptom correlation with test results, the cause of this disease is a virus or viruses of unknown type or form. Until further information is available we have named it ‘Virus X’.

Work is currently in hand to establish the form or identity of this virus or viruses, its routes of transmission and, with the aid of PCR tests being currently developed, to study the epidemiology of the disease with a view to formulating control strategies. There is some cause for limited optimism.

The problem does not appear to be becoming more intense or widespread and there are initial indications that increased precaution against traditional virus disease has considerable beneficial effects on some afflicted farms.

There remain, however, many enigmas relating to this problem, not least its ability to exist in the absence of traditional virus problems. The HRI contact, should anyone wish to discuss this work is Richard Gaze. Telephone +44 (0) 1789 472075 Fax +44 (0) 1789 472076.