Studies on taxonomy and ecology of genus Typhula, causing winter rot diseases of overwintering dicots

A newfound disease on overwinterd carrots, which causes root rot under snow, was investigated. The pathogens are Typhula variabilis and T. japonica, and the disease was named Typhula winter rot. They also attack canola during the winter season, hence I marshaled diseases on carrots and canola under snow, and taxonomic muddle of three Typhula spices including T. variabilis, T. laschii, and T. intermedia. Ecology of T. variabilis, a main pathogen of carrots, was investigated. 1. Taxonomic study of four spices of Typhula Four Typhula species were obsereved with a light microscope, SEM, or TEM. The results of T. variabilis showed that the fungus has unique feature on the rind cell, i.e., the rind rugged, thick, often cracked, and the rind cells coalesced at the base but separate on the surface, and each rind cell of T. variabilis had a plateau. On the other hand, on T. laschii, the rind cell has ridge in the center. The rind cell of T. intermedia is rugged like that of T. variabilis, but they are different species because both fungi do not mate. T. variabilis, T. laschii, and T. intermedia had been treated the same species, but this study confirms that they are different morphologically and phylogenetically. This study designated the lectotype of T. variabilis, the neotype of T. laschii, and the lectotype and the epitype of T. japonica. 2. Pathogenicity of three spices of Typhula against overwinterd carrot and canola T. variabilis is pathogenic to carrot and canola, and T. japonica is also pathogenic to both crops, however, T. japonica causes root rot on carrot in only heavy snow areas. T. variabilis attacks carrot under snow regardlessof amounts of snow, and has stronger virulence than T. japonica against carrot. The main pathogen to overwintered carrot is T. variabilis. In addition, monokaryons of T. variabilis attack to carrot. On canola, the both fungi cause winter rot, and this is the first report of T. incarnata being pathogenic to canola. The root rot disease on carrots by genus Typhula is given a name as 'Typhula winter rot'. 3. Ecology of Typhula variabilis T. variabilis releases its basidiospores from September. The spore rain is controlled by air temperature change; it is active between 0 degC and 20 degC. Carrot leaves are infected with T. variabilis in Autumn and the disease develops under snow. Monokaryons of T. variabilis germinate from basidiospores and attack a host as soon as spores attach to a host. T. variabilis formed its sclerotia on carrot leaves at late of January, however root rot was recognized at eary March. Its portal entry to carrots is considered as leaves first, roots later through crown. The symptom after snowmelt supported it. When carrot crowns were covered with soil before snowing, Typhula winter rot was suppressed. Any fungisides are not registered to the disease currently, thus, cultural disease control is important.