The epidemiology and control of Pythium root dieback of muck-grown carrots

Olpidium was correlated with the frequency of precipitation greater than one half inch but was not correlated with root temperature, CO₂ or 0₂ concentration, saturated hydraulic conductivity, the height of the carrot beds, marketable yield or cull rate. Olpidium isolates with and without TNV did not produce lesions on carrot roots under greenhouse conditions. TNV was detected in both brown and white roots but only from problem fields. Carrot rootlets rub-inoculated with TNV failed to produce necrotic symptoms. Olpidium and TNV were found in onion, lettuce, celery and some weed species common to PRD problem fields. However, no root tip browning was observed in any of these hosts. Fast growing Pythium species were recovered equally frequently in brown and symptomless rootlets and from problem and non-problem soils. Most weeds, celery, onion and lettuce also had a high incidence of fast growing Pythium. The highly pathogenic, slow growing Pythium sulcatum was recovered only from problem soil. The recovery rate from symptomless roots was very low compared to brown roots. P. sulcatum was not isolated from celery or any of the weed species common in problem soil. Lettuce and onion were found to support low levels of infection. Evidence suggests that P. sulcatum is a primary incitant. PRD losses can be kept to a minimum and marketable yields increased by using tolerant varieties, such as HiPak; raised beds, if there is a readily available supply of irrigation water; precision seeding at 1 1/4 inches; and a crop rotation of onions preceding carrots. Matric potential was controlled in small containers separated from osmotic solutions of polyethylene glycol (PEG) 6000 by Pellicon ultrafiltration membranes (nominal molecular weight cutoff:500, Millipore Corp.). Matric potentials could be maintained for periods of 3-5 weeks before microbial breakdown of membranes occurred. Flow rate for the membranes was 1.0 cm³ cm⁻² day⁻¹ for a water potential difference across the membrane of 0.2 bar. Water potential measured with tensiometers or thermocouple psychrometers in a cylindrical container (4.3 cm diam. x 10 cm) with a membrane acrosss the bottom, remained relatively constant under conditions of soil surface evaporation but decreased rapidly when young plants were grown in the system. Soil cells (5.5 x 2.0 x 10 cm with one 43 mm diameter membrane in each side), containing two young carrots, and emersed in a -0.2 and -2.0 bar PEG solution had an average matric potential of -0.4 and -2.5 bars respectively over a three week period. The carrots transpired 7.8 and 3.9 ml/day at osmotic potentials of -0.2 and -2.0 bars respectively which suggests that sufficient water was passing through the membrane to meet the needs of a growing carrot.Land and Food Systems, Faculty ofGraduat