Development of black root rot caused by Chalara elegans on fresh market carrots.
Punja, Z. K.; Chittaranjan, S.; Gaye, M. M.;
Canadian Journal of Plant Pathology Year: 1992 Vol: 14 Issue: 4 Pages: 299-309 Ref: 39 ref.
1992
บทคัดย่อ
Commercial carrot production fields in the Fraser Valley of British Columbia, Canada, were surveyed for the presence of C. elegans (Thielaviopsis basicola) during 1989 and 1990. Of the soil samples originating from 27 fields, c. 70% were infested with the pathogen using a carrot root disc baiting procedure. The development of black root rot on fresh market carrots, however, was only initiated after harvest and resulted predominantly from fungal colonization of tissues that were wounded during harvesting and grading. An inoculum density as low as 4 phialospores/disc resulted in disease development on carrot root discs under lab. conditions. The proportion of colonized tissues was greatest at 25 deg C and was significantly lower at 15 or 30 deg . Germination of phialospores of T. basicola 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. SEM 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 occurred both intercellularly and intracellularly. Colonization of tissues was restricted to areas that had been wounded. Susceptibility of wounds to infection was rapidly reduced within 20 h after wounding. However, c. 10% of the wounds were colonized for up to 40 h after wounding. Incubation of wounded carrots or root discs at 4 deg for 24 h before inoculation delayed the rate of wound healing and enhanced disease; exposure to 4 deg for 24 h after inoculation delayed disease development by limiting pathogen growth. A high temp. stress treatment (30 deg for 24 h) of roots or discs either before or after inoculation enhanced disease development compared with constant 20 deg C. Roots with incipient infections that were stored at 4 deg for up to 12 weeks showed resumed fungal colonization upon removal from storage. The development of T. basicola on fresh market carrots is enhanced by wounding of the periderm, and is influenced by inoculum level and by postharvest incubation temp.