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New Zealand produces over 40% of the world’s carrot seed. This production is based in Mid-Canterbury, but recently the fungal pathogen Alternaria radicina has caused problems for carrot seed growers. The overall aim of this research project was to improve understanding of the disease cycle and to develop strategies for control of A. radicina in carrot seed production. In addition, to achieve these aims, two methods were developed to quantify the levels of A. radicina in soil samples. The use of a semi-selective agar medium was proved to be selective, sensitive and reliable, and suitable for commercial use. A quantitative PCR assay was also sensitive and very rapid, detecting a lower limit of 100 fg DNA and 10 conidia in soil per reaction (60 conidia/g soil), but had some technical problems which must be resolved before this method could be used commercially. A survey of 15 carrot fields found A. radicina soil populations of 33-233 CFUs/g soil, which were positively correlated with black root rot disease incidence and severity. A glasshouse trial confirmed that soil-borne A. radicina significantly reduced seedling emergence. A trial of pathogen propagule survival in soil demonstrated that conidia were the major survival propagules, and that the presence of plant debris increased survival of the pathogen. The prolonged cropping period for carrots allowed for overlapping periods between crops from consecutive seasons, providing a green bridge which allowed spread of the disease by wind-borne conidia that were detected up to 135 m from the crop. Conidium dispersal occurred over a broad timeframe, from 4 months after sowing to 7 weeks after harvest, with the maximum dispersal at harvest. Flaming and steaming treatments applied to carrots in September at three tractor speeds, viz. 1.7, 2.3 and 2.7 km/ha, in 2006 caused small but significant reductions in subsequent foliar disease, which lasted for 2 months after treatment. In 2007, another steaming trial using the same tractor speeds showed that treatment in June gave better control than a July treatment but the effects again lasted until only September and, at harvest, seed infection was not reduced significantly. A glasshouse study of crop rotation effects on survival of A. radicina propagules in soil found that growing wheat, barley and faba bean significantly reduced the amount of soil-borne inoculum. A field survey showed that A. radicina was not detectable in soils where carrot had last been grown ≥6 years previously, and was at lower levels (50-75 CFUs/g soil) in fields where wheat was grown in the crop rotation. In an irrigation trial plants irrigated by drip irrigation had significantly less foliar and root infection, and produced higher quality seed, but not greater yield, than those overhead irrigated. A disease susceptibility screening trial showed that while nursery parent lines differed in their susceptibility to A. radicina, the potential to reduce disease by this method is constrained by the overseas customers who specify the parent lines requirement. In vitro fungicide screening trials showed a range of efficacies against A. radicina, with iprodione being the best and azoxystrobin the least effective. In vivo studies showed that a single application of pyraclostrobin, or pyraclostrobin+boscalid, or difenoconazole in autumn was able to delay the onset of foliar disease but the initial control lasted only until spring. In a glasshouse trial, the pre- sowing incorporation of fungicides, a biological control agent and a fumigant reduced soil-borne inoculum and increased carrot seedling emergence. In a field trial, post-sowing incorporation of fungicides and biological control products into the soil significantly reduced black root rot disease, but had no effect on harvested seed infection levels. When fungicides or biological agents were applied to the umbels in field trials, only difenoconazole at 0.125 L a.i./ha applied a week before swathing, or two applications of Ulocladium oudemansii at 2 × 10¹¹ CFUs/ha, applied one and two weeks before swathing, significantly reduced A. radicina in the harvested seed and thereby improved seed germination. The fungicidal ability of diquat against A. radicina was demonstrated in an in vitro study and in field trials where 0.2, 0.4 or 0.6 L a.i./ha applied 4 days before swathing significantly reduced seed-borne infection and the percentage of abnormal seedlings in harvested seeds. The two lower rates allowed a small increase in germination but the highest rate reduced germination because it killed some seeds. A fungus new to New Zealand, A. carotiincultae, was detected on carrot seed produced in New Zealand and on seed imported from France. Its pathogenicity to carrot was demonstrated in a greenhouse trial. It is likely to have been orginally introduced to New Zealand on imported seed, as it was detected during seed health tests. While several of the methods investigated showed promise for control of A. radicina, none of them completely controlled the pathogen. Further research on a combination of methods is required before Canterbury’s carrot seed growers can be presented with effective and sustainable control strategies.