DEVELOPMENT OF BLACK ROOT-ROT CAUSED BY CHALARA-ELEGANS ON FRESH-MARKET CARROTS

Commercial carrot production fields in the Fraser Valley of British Columbia were surveyed for the presence of Chalara elegans (syn. Thielaviopsis basicola) during 1989 and 1990. About 70% of the soil samples originating from 27 fields were found to be infested with the pathogen using a carrot root disc baiting procedure. The development of disease (black root rot) on fresh market carrots, however, was initiated only after harvest and resulted predominantly from fungal colonization of tissues that were wounded during harvesting and grading. An inoculum density as low as four phialospores per disc resulted in disease development on carrot root discs under laboratory conditions. The proportion of colonized tissues was greatest at 25-degrees-C and was significantly lower at 15 or 30-degrees-C. Germination of phialospores of C. elegans and colonization of wound sites occurred within 24 h after inoculation. By 72 h, mycelial growth and sporulation were visible on the surface of inoculated tissues. Scanning electron microscopic studies revealed the development of both phialospores and chlamydospores on the tissue surface. Growth of the pathogen within the tissues was examined using light microscopy and was found to occur both intercellularly and intracellularly. Colonization of tissues was restricted to areas that had been wounded. Susceptibility of wounds to infection was reduced rapidly within 20 h after wounding. However, about 10% of the wounds were colonized for up to 40 h after wounding. Incubation of wounded carrots or root discs at 4-degrees-C for 24 h before inoculation delayed the rate of wound healing and enhanced disease; exposure to 4-degrees-C for 24 h after inoculation delayed disease development by limiting pathogen growth. A high temperature stress treatment (30-degrees-C for 24 h) of roots or discs either before or after inoculation enhanced disease development compared with constant 20-degrees-C. Roots with incipient infections that were stored at 4-degrees-C for up to 12 weeks showed resumed fungal colonization upon removal from storage. The development of C. elegans on fresh market carrots is enhanced by wounding of the periderm, and is influenced by inoculum level and by postharvest incubation temperatures.