Aphanomyces cochlioides . [Descriptions of Fungi and Bacteria

Abstract

Abstract A description is provided for Aphanomyces cochlioides . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Amaranthus blitoides, A. retroflexus, Beta lomatogona, B. patellaris, B. patula, B. trigyna, B. vulgaris, B. vulgaris var. cicla, Celosia argentea, Chenopodium album, Dianthus chinensis, Echinocloa crus-gallii, Escholtzia californica, Gomphrena globosa, Kochia scoparia, K. scoparia var. culta, Lychnis alba, Mollugo verticillata, Papaver rhoeas, Portulaca oleracea, Salsola kali, Saponaria ocymoides, Spinacia oleracea, Tetragonia tetragonioides . DISEASE: Blackroot of sugar beet; the fungus is a facultatively necrotrophic plant pathogen. There is an early acute phase of short duration (causing pre-emergence and post-emergence damping off) and a later chronic phase which may persist throughout the life of the plant. Infection during seed germination is indicated by poor stands with killed seeds remaining in the soil to infect young seedlings emerging elsewhere. Seedling hypocotyls are infected at ground level, a water-soaked area extending up and down the hypocotyl or the upper part of the young taproot from the point of entry. The invaded root or hypocotyl rapidly becomes brownish and then assumes the characteristic jet black discoloration from which the disease derives its name. Soon after, the cortex of the hypocotyl dries, and the stem and hypocotyl shrink, leaving a thin strand of tissue. Oospores are easily seen in the collapsed root and hypocotyl tissue on microscopic examination. The chronic phase first appears on plants in late June to August. A greenish-yellow discoloration of the swollen hypocotyl develops, affected root tissues becoming dark brown, soft, water-soaked, splitting apart and eventually shrivelling. Plants are stunted and lower leaves turn yellow. GEOGRAPHICAL DISTRIBUTION: Asia: Japan. Australasia & Oceania: Australia (Qld). Europe: Austria, Denmark, England, France, Germany (GDR & GFR), Hungary, Ireland, Poland, Sweden, USSR (Russia). North America: Canada (Alberta, NS, Ontario, Quebec), USA (California, Connecticut, Indiana, Michigan, Maine, MT, North Dakota, Ohio, South Dakota, Texas, Washington State, Wisconsin). South America: Chile. See CMI Distribution Maps of Plant Diseases 596. TRANSMISSION: Presumably in soil by oospores originating from sloughed-off root tissues and germinating to produce zoospores. The conditions favouring oospore germination are however largely unknown. Survival may occur on alternative hosts present in the crop, so the disease may be difficult to eliminate. The disease is particularly severe in warm, wet conditions, less so in cool, wet weather.