EFFECTS OF INOCULUM DEPTH AND DENSITY ON FUSARIUM-WILT IN CARNATIONS

The effects of depth (0-30, 15-30, 30-45, 45-60, or 60-75 cm) and dens ity (0, 6, 25, 120, 770, or 3,500 propagules per gram of soil) of Fusa rium oxysporum f. sp. dianthi inoculum originating from naturally infe sted soil on wilt and flower yield in a highly susceptible cultivar of carnation (Dianthus caryophyllus 'Fantasia') were studied in concrete containers (100 X 50 cm) during three growing seasons. Depth of inocu lum placement was related directly to time until symptom onset and inv ersely to the final percentage of plants that were infected. Rate of d isease progress varied with depth of inoculum placement. At 15-30, 30- 45, 45-60, or 60-75 cm, mean wilt incidence 185 days after planting wa s 50, 45, 33, or 22%, respectively. Flower yields were correlated with depth of inoculum. Mean yield losses were 15, 23, 31, and 39% when in oculum was placed at depths of 60, 45, 30, and 15 cm, respectively, re lative to yield in pathogen-free soil. In experiments on the effect of inoculum density, time until symptom onset and final disease incidenc e were related inversely and directly, respectively, to propagule dens ity. The rate of disease progress was the same at all propagule densit ies. At densities of 6, 25, 120, 770, and 3,500 propagules per gram of soil, mean percentages of infected plants 185 days after planting wer e 2, 5, 13, 34, and 57%, respectively. Flower yields were related inve rsely to inoculum density. The effects of inoculum depth and density o n final disease incidence were fit by linear logistic models. At the e nd of the growing season, the vertical distribution of F. o. dianthi p ropagules through a 60-cm depth showed a similar trend whether the pro pagules had been placed at 0-30 cm or at 60-75 cm prior to planting. T he number of propagules placed at various inoculum densities prior to planting had increased 10- to 100-fold by the end of the growing seaso n.