No description available.

The effect of interrupted leaf wetness (IWP) and %RH on infection by Cercospora carotae (Pass.) Solh. was studied by inoculating carrot leaves (Daucus carota var sativa DC. L.) and subjecting the plants to different IWP treatment, continuous leaf wetness (CWP) and to different combinations of %RH and temperature with and without an initial wet period of 6 hr. IWP significantly reduced infection as compared to CWP. Infection was optimal under leaf wetness and decreased with decrease in percent RH. The effect of temperature and duration of moist period on sporulation of C. carotae was studied on carrot plants under leaf wetness, 96%RH, and 96%RH with an initial 12 hr of leaf wetness. For all types of moisture conditions, sporulation increased with the increase in temperature up to the optimum (28$ sp circ$C) and then declined. Logistic and polynomial models were used to describe the effect of temperature and time on sporulation under these moisture conditions. The incubation period of Cercospora carotae was studied in the field. First lesions were observed 6 to 8 days after inoculation and new lesions appeared until the 10th to 14th day. The beginning, mean, and end of incubation period was modelled as a function of mean daily temperature and mean daily RH $ ge$ 90%. A model describing lesion appearance as a function of time was developed using a logistic function (R$ sp2$ = 0.84). A prediction model containing series of equations that described mathematically the interaction among predicted inoculum, infection and sporulation equivalents for the environment was developed and validated. In general, the model predicted adequately Cercospora blight progress. A weather-based forecasting system was developed to time the first fungicide spray to manage Cercospora blight of carrot based on the accumulation of critical number of disease severity units.

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 of / Graduate

No description available.

No description available.

No description available.

Forecasts for the first fungicide application to manage cercospora blight of carrot (Daucus carota L. subsp. sativus (Hoffim) Arcang) caused by Cercospora carotae (Passerini) Solheim were evaluated during the summers of 1991 and 1992. In the forecast based on infection, the blight severity values (BSV) were calculated for each day using duration of leaf wetness, relative humidity and temperature. The first fungicide was applied when the cumulative blight severity value reached 14 (CBSV 14) and 18 (CBSV 18). The forecasts were compared with the 50% disease incidence threshold method where the first fungicide was applied when the middle leaf of 50% of plants was diseased, and the conventional method where the first fungicide application was made when the plant height reached 15 cm. For the threshold of CBSV 14 and CBSV 18, no loss was observed in yield and fewer fungicides were applied as compared to conventional and 50% disease incidence threshold methods. Three years of weather data from experimental plots and 155 commercial field data were used to study the behaviour of CBSV at 50% disease incidence threshold. Only in 3% of the fields, the disease incidence of 50% was observed before the CBSV reached 11. The increase from CBSV 11 to 20 was rapid. Depending on environmental conditions the first fungicide at a CBSV range of 11 to 20 is proposed rather than a critical threshold since no loss in yield was observed at 18 in the experimental plots. No fungicide is applied until the CBSV reaches 11 and the application must not be delayed after CBSV reaches 20.

No description available.

No description available.

During the summer 1988 and 1989, surveys were conducted on the muck soils, South of Montreal to estimate the prevalence and incidence of the carrot diseases. Cercospora blight was the most prevalent disease; 91% and 96% of the fields and 99% and 92% of the plants sampled were diseased in 1988 and 1989, respectively. In decreasing order of occurrence the diseases present were: Crown gall, Alternaria blight, Root Knot, Sclerotinia rot and Aster yellows. / Greenhouse and field studies were carried out to evaluate partial resistance to Cercospora blight in 111 carrot cultivars based on the mean incubation period (MIP), the proportion of leaf area diseased (PLAD), and the sporulation / mm$ sp2$ lesion area (SPO). Significant differences among varieties were observed for all the parameters studied under greenhouse conditions and a significant negative correlation was found between PLAD and MIP (r = $-$0.29). Resistance equivalents were calculated for the PLAD, as proportions of the cultivar Dagger, so that they could be incorporated in a fundamental forecast model.