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Abstract: The present paper gives the first definite example of virus diseases in a fungus, they may cause severe losses. Symptom variation at first caused confusion, as it was profoundly affected by environmental conditions, the only consistent feature was the degeneration of the mycelium, hence the name 'Mushroom die-back'. Purified concentrated extracts from infected sporophores contained one, two or three kinds of virus particles (25 and 29 mµ diameter polyhedra, and 19 × 50 mµ bacilliform particles). Inoculation to healthy crops reproduced the disease. Preparations containing the 25 or 29 mµ. particles reacted specifically in serological tests, gave specific light-scattering zones in density-gradient centnfugation, and had ultraviolet absorption spectra very similar to two RNA plant viruses. Though characteristic symptoms occur in agar cultures, until recently no virus could be extracted. Now, by disrupting mycelium with ultrasound, virus can be detected with the electron microscope in as little as 1.0 mg of tissue. The three viruses move at different rates in mushroom mycelium, this partly explains the decrease in virulence shown by samples taken farther from the point of infection. But tests using only one virus have shown similar losses of virulence. Cultures of low virulence continued unaltered in sub-culture, but when inoculated to healthy mushroom trays full virulence was restored at fruiting. These changes were paralleled by similar changes in concentration of virus obtained in purified extracts. No serological relationship with viruses of flowering plants was found, and we have been unable to infect any standard test plants with the mushroom viruses. The 19 × 50 my particles occurred only in crops also containing one or both the other viruses The 25 mµ particles have been isolated, and their transmission through spores has been confirmed.
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