The pathogenesis of Fusarium oxysporum f. sp. narcissi and the role of antagonistic bulb-borne fungi in the chemical control of basal rot

The pathogenesis of Fusarium oxysporum f. sp. narcissiBasal plates and roots of narcissus were infected by Fusarium oxysporum Schlecht f. sp. narcissi (Cooke & Massee) Snyder & Hansen during the period of emergence of the roots in autumn only. This observation contrasts with the generally accepted view that infections mainly take place when roots die at senescence of the plant in early summer. It is concluded from histological observations that before root emergence the basal plate tissue is externally protected by an outer periderm. Shortly after the infection period during which roots rupture the periderm, bulbs and roots appear to possess a natural resistance against penetration and infection by fungi. New infection of the roots is prevented by suberization of the root exodermis. A periderm formed in the parenchymatous tissue of the basal plate surrounding the roots also acts as a barrier. The central disc itself becomes protected in early summer by formation of lignified abscission layers across the cortex and stele of senescent roots.For infection of the bulb and roots a temperature of above 13 °C and a high humidity favouring fungal growth are prerequisites. Root and basal rot will develop within a few weeks from the time that roots emerge. Decay of roots in early summer follows natural abscission of the roots.Latent and active infections may be discerned depending upon their position with respect to the inner periderm. The relationship between the site of infection and the development of symptoms is discussed.The commonly applied hot-water treatment contributes significantly to the spread of the pathogen. Control of basal rot requires killing of bulb-borne inoculum before planting and protection against soil-borne inoculum during the first week of root emergence after planting, which both can be achieved by application of a fungicide.The relation between autumnal infection of narcissus bulbs by Fusarium oxysporum f. sp. narcissi and disease development in the following seasonBulbs of narcissus contaminated with conidia of Fusarium oxysporum f. sp. narcissi, in absence or presence of fungicides during a hot-water treatment (HWT, 2 h at 43.5 °C), were planted in September and October. When, after the HWT, bulbs were stored for 4 days at 17 °C under humid conditions (RH>90%) and planted in soil at 13-15 °C, root rot was observed in December and basal rot in July. New basal rot developed during subsequent storage in August. However, when similarly treated bulbs, prior to planting at 8-12 °C, were stored during an additional period of 14 days at a RH of 70%, various types of bulb rot were present in December instead of root rot. Then sickle-, dwarf- and non-emerging plants were noticed in April, and very advanced basal rot in relatively high percentages of bulbs in July. For bulbs planted at 13-15 °C, a high positive correlation was found between the incidence of root rot in December and the presence of basal rot at the end of the season. For bulbs planted at 8-12 °C after 14 days of storage at RH of 70%, a correlation was found between the incidence of bulb rot in December and the subsequent development of symptoms in the foliage in April and the presence of basal rot in July.The relationship between autumnal infections of roots and basal plates and the development of symptoms later in the season holds for most situations in practice. Early knowledge about the health condition of a bulb lot in the field will support the setting up of an effective programme of roguing in spring and will indicate the most appropriate cultural measures to be employed from the time of lifting.The role of antagonists in the chemical control of Fusarium oxysporum f. sp. narcissiTreatment of narcissus bulbs with methoxy ethyl mercury chloride, pimaricin or thiram provided control of basal rot and primary root rot, caused by Fusarium oxysporum f. sp. narcissi and secondary root rot and 'skin disease' caused by F. oxysporum ff. spp. A similar treatment with formalin was only effective against basal rot and primary root rot. Newly formed roots of bulbs treated with the mercurial, pimaricin or thiram became more heavily colonized by Penicillium and Trichoderma spp. than those of untreated or formalin-treated bulbs. Especially P.janthinellum and T. viride appreared to protect the roots against the pathogens. These fungi were found to be antagonistic to F. oxysporum f. sp. narcissi and F. oxysproum ff. spp., both in experiments in vitro and in vivo. A synergism between P.janthinellum and thiram was observed with respect to inhibition of mycelial growth of F.oxysporum f. sp. narcissi in vitro and to control of root rot and basal rot in vivo. In vitro, similar effects were also found for pimaricin and organic mercury compounds. Pimaricin was found to stimulate the production of antibacterial metabolites by Cylindrocarpon destructans in vitro. In the field, a synergistic effect was observed between pimaricin and C.destructans with respect to control of basal rot.The influence of incubation conditions after lifting on development of basal rot and skin disease and on effectiveness of chemical control measuresFusarium oxysporum f. sp. narcissi and other formae speciales of F. oxysporum were more affected by heating during a hot-water treatment (HWT,2 h at 43.5 °C), than bulb-borne Penicillium and Trichoderma spp. were. The latter fungi also survived a forced drying process after a W better. Such treatment of bulbs and bulb parts resulted in a changed distribution of micro-organisms on the bulbs in favour of these saprophytes. Addition of either a mercurial thiram or pimaricin accelerated these effects on the bulb microflora, whereas formalin did not. A period of cool (< 15 °C) storage enhanced both the growth rate of Penicillium spp. in vitro and their capacity to colonize bulb tissue in natural soil. Higher temperatures stimulated development of Trichoderma viride. Cylindrocarpon destructans and F. oxysporum ff. spp. Late planting at temperatures below 8 ° C reduced disease development substantially.Further it was found that Penicillium corymbiferum, Penicillium janthinellum and Trichoderma viride are able to contribute to control of root rot and skin disease caused by F.oxysporum f. sp. narcissi andlor F. oxysporum ff. spp.The data obtained contribute to a better understanding how incubation conditions from lifting onwards can influence disease development in the following season. It was possible to elucidate why a low dose of a fungicide like pimaricin, applied during a HWT could give better control of basal rot and skin disease than higher dosages did.