The genus Verticillium is divided into four sections (two new) and a residual group. The new section Nigrescentia comprises the well-known plant-pathogenic and some other saprophytic species with dark resting structures. The new section A lbo-erecta is characterized by white (or yellowish) colonies and erect conidiophores and contains mainly fungicolous species. Seven (two new) species and two new varieties are described and keyed out. Three other species have known teleomorphs of Nectriopsis (one new and two new combinations). This genus had been merged with Nectria by Samuels, but its retention is justified, as the original and the here described fungicolous (and myxomyceticolous) species are more suitably accomodated in Nectriopsis than in Hypomyces.The common causal agent of dry bubble in Agaricus bisporus, Verticillium fungicola var. fungicola, is redescribed and defined more narrowly than by Gams (1971) by its maximum growth temperature below 27 ~ A similar fungus with a maximum growth temperature near 33 ~ causing brown spots in A. bitorquis, is described as V. fungicola var. aleophilum. Isolates from wild agarics with a strongly reduced growth at 24 ~ and a maximum below 27 ~ a yellowish mycelium and inconspicuous sclerotia, are described as V. fungicola var. flavidum. V. biguttatum W. Gams, sp. nov., with cylindrical biguttulate conidia, is a common soil fungus and hyperparasite of Rhizoctonia solani. Gliocladium microspermum (Sacc.) W. Gams, comb. nov., the anamorph of Nectriopsis broomeana (Tul.) W. Gams, comb. nov., which is intermediate between Gliocladium and Verticillium, is included because of its fungicolous habit.

Effectivertess of benzimidazole fungicides in controlling Vertieillium disease of the commercial mushroom, Agarietts bisporus, was closely correlated with the differential in vitro activity. Thiophanatemethyl was an exception as greater disease control was achieved than expected from its in vitro activity.About one year after clearance for use of benzimidazole fungicides in mushroom growing, resistance in V. fungieola (Preuss) Hassebr. (syn. V. malthousei Ware) appeared on Dutch farms. In vitro, tests were made to determine the degree of resistance of two isolates, R1 and R2, obtained from diseased sporocarps from benomyl-sprayed crops. Mycelial growth of R2 was considerably less inhibited by MBC-fungicides than that of R1. In vivo, R1 was more pathogenic to mushrooms than a freshly made benomyl-sensitive isolate. Both benomyl-resistant isolates were cross-resistant to other MBC-fungicides and to thiabendazole and cypendazole. Likewise, two new experimental fungicides, imazalil and vinchlozolin, did not provide promising alternatives. When appearance of resistance in V. fungicola is suspected on mushroom farms, additional emphasis should be laid on farm hygiene to achieve disease control.

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Return-polyacrylamide gel electrophoresis (R-PAGE) is one of the methods used for the detection of viroids, in particular Chrysanthemum stunt viroid (CSVd) and Potato spindle tuber viroid (PSTVd). The method is based on temperature-related conformational changes of the circular single-stranded RNA molecule. This results in a characteristic electrophoretic behaviour, providing a clear separation between the viroid and nucleic acids of the plant. Since the description of R-PAGE, several modifications of the procedure have been published. In addition, several molecular techniques have been applied for the detection of viroids. However, R-PAGE is still particularly suitable for phytosanitary purposes because detection is not influenced by variations in the nucleotide sequence. This means that any Pospiviroid will be detected, provided the amount is above the detection threshold. In addition, relatively large amounts of leaf material can be handled in one test. This paper presents a recently developed R-PAGE procedure, based on the use of a horizontal electrophoresis system and paper wicks as buffer reservoirs. In comparison with the existing procedures, the new procedure is easy to perform and the use of chemicals is decreased. Results of the new procedure are shown and discussed in relation to those obtained by the previous procedure.

In recent years in the Netherlands a second mushroom species, Agaricus bitorquis, which prefers higher temperatures than A. bisporus and is less susceptible to certain diseases, is often commercially grown. Verticilliumfungicola var. fungicola, the causal agent of dry bubble, is responsible for considerable damage in crops ofA. bisporus. In A. bitorquis, however, dry bubble has hardly been noticed, but brown spots due to V. fungicola var. aleophilum resulted in inferior mushroom quality. The latter variety also caused brown spots in A. bisporus, but to a minor degree. In variety Les Miz 60 of A. bisporus, however, it also induced fruit-body deformation in a way different from dry bubble.Verticillium psalliotae, isolated from A. bitorquis in England, induced more confluent brown spots in A. bitorquis. In the Netherlands, where more A. bitorquis is grown than in other countries, V. psalliotae has not yet been encountered in crops of A. bitorquis. V. psalliotae, which has a high temperature optimum for mycelial growth, like V. fungicola var. aleophilum and A. bitorquis, did not infect A. bisporus in our trials.Artificial infection of A. bisporus or A. bitorquis could not be accomplished with the following related and/or fungicolous fungi: Verticillium lamellicola, V. fungicola var. flavidum, V. biguttatum, Nectriopsis tubariicola, Acremonium crotocinigenum and Aphanocladium album.