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There is interest among vegetable seed companies in the production of certified organic seed, primarily because of USA and EU legislation. Companies and farmers growing organic carrot seed in Canterbury, New Zealand have encountered a number of production issues, including appropriate crop densities, fungal pathogens, particularly Alternaria radicina, and weed management of overwintered intrarow weeds.
To determine suitable densities for organic carrot seed crops, the effect of crop densities of 5, 11, 20, 55 and 82 plants m‑2 on two F1 parent seed parent cultivars with contrasting morphologies was studied. There were large differences between the cultivars with respect to height, yield, yield response to density, germination, germination response to density and thousand seed weight (TSW). Yield varied between cultivars and among umbels with yield increasing with increasing density for cv. B but not A. TSW varied between cultivars and umbel levels. Seed vigour varied between umbel levels and A. radicina seed infestation increased with increasing density. The hypothesis stated in the literature that higher crop densities increases seed quality in commercial crops is supported by these results. It is also suggested that in order to correctly test the hypothesis, the practice of separating the umbel levels is methodologically incorrect.
A direct-fired steam weeder was developed, to avoid the practice used in standard boiler designs of superheating pressurised water to produce steam. This approach makes it possible to safely generate sufficient amounts of steam using tractor mounted equipment to effectively kill weeds. Steam weeders have a range of advantages over flame weeders including improved heat transmission due to the latent heat of condensation of steam and by stopping evapotranspirational cooling of plants after treatment. The direct-fired steam weeder also poses no fire risk, is unaffected by wind and is powered by raw vegetable oils, biofuels or diesel. This is offset by greater complexity, weight, use of large volumes of water and higher cost compared with flame weeders.
Tests on a range of weeds at various growth stages revealed that springtime thermal weeding of overwintered intrarow weeds is unlikely to kill most weed species. The dose response curves commonly used for optimising herbicide usage are less suitable for determining the optimal thermal doses to kill weeds. Instead, the transfer of heat onto and though a plant’s tissues to the hypocotyl stem and/or aerial meristems should be modelled.
Although springtime thermal treatment of intrarow weeds proved of limited use for weed control, it may have potential to eliminate foliar fungal pathogens. In a glasshouse experiment, the foliage of pot-grown carrots was artificially infested with A. radicina, Alternaria dauci, and Cercospora carotae. Thermal treatment caused complete disinfestation for all fungal pathogens. This effect needs to be confirmed under field conditions, and the impact of defoliation on carrot plant growth, seed yield and seed quality has to be established.
Five biological control agents (BCA), Bacillus subtilis, Trichoderma viride, Trichoderma atroviride, Rhizobium rhizogenes and Effective Microorganisms (EM) were tested for their ability to control A. dauci, A. radicina and C. carotae in a sequence of trials including in vitro laboratory assessments as well as in vivo glasshouse and field trials. Initial laboratory work showed that some BCA inhibited pathogen growth. However, glasshouse and field trials found no evidence that the biocontrol agents could satisfactorily control the three pathogens or other carrot infesting fungi.
Overall, the study confirmed current crop densities and spacings as suitable for organic carrot seed production, and that higher plant densities may increase pathogen infestation of seed. The BCA failed to control the fungal pathogens in the field; however, thermal weeders show promise for managing foliar fungal pathogens.
Keywords: organic agriculture, F1 hybrid, seed production, seed quality, Daucus carota, crop density, weed management, thermal weeding, steam weeding, flame weeding, Alternaria dauci, Alternaria radicina, Cercospora carotae, Bacillus subtilis, Trichoderma viride, Trichoderma atroviride, Rhizobium rhizogenes, Effective Microorganisms.