Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK

Verticillium spp. remain one of the major threats to Agaricus bisporus production throughout the world. Little is known about the pathogen diversity or the interaction between pathogen and host. Symptoms are varied and are thought to depend on a number of factors such as developmental stage, the time of infection and genetic variability of the host. However, none of the commercially available spawns are resistant to this pathogen. In the UK, Verticillium is currently controlled by prochloraz manganese, the only available fungicide, and resistance to this fungicide has recently been detected in UK strains. The mechanisms of infection of Agaricus by fungal pathogens are poorly understood. Little or no information is available on biochemical or molecular interactions between host and pathogen.

Materials and methods
A world-wide collection of Verticillium spp. associated with mushroom cultivation has been created containing more than 200 strains. One-hundred and seventy strains of Agaricus have been obtained from the American Agaricus Resource Programme (ARP). Forty strains have been fruited and inoculated with V. fungicola. Eight strains were selected on the basis of displaying a high or low level of susceptibility to V. fungicola var. fungicola. Similarly, six strains of Verticillium were selected that displayed different levels of virulence and a propensity of cause either cap spotting or'bubble' symptoms. Agaricus cell wall material has been extracted from mushroom stipes and Verticillium cultured in liquid media containing cell wall material and assayed for enzyme production.

Results and conclusions
Analysis of Verticillium strains using RFLP, RAPD and DNA sequence analysis has shown that most isolates can be divided into three distinct groups and populations in the USA can easily be distinguished from those in Europe. Molecular analysis of Verticillium strains has shown that symptom variation (cap spotting or dry bubble) can be caused by three different Verticillium taxa, not solely V. fungicola var. fungicola as was previously thought.

Following inoculation of Verticillium onto Agaricus strains, significant reproducible differences in symptom expression have been recorded. No ARP strain tested showed absolute resistance to Verticillium with respect to cap spotting, although some pathogen host permutations did not produce ‘bubble’ symptoms.

Seventeen Verticillium strains were grown in liquid media containing Agaricus cell wall material. Verticillium was shown to produce exo- and endo-chitinases, P,1-3-glucanases, proteases and esterases. Multivariant analysis has suggested a tentative correlation between symptom type and enzyme production.