Effects of Inoculum Depth and Density on Fusarium Wilt in Carnations

Abstract

The effects of depth (0-30, 15-30, 30-45, 45-60, or 60-75 cm) and density (0, 6, 25, 120, 770, or 3,500 propagules per gram of soil) of Fusarium oxysporum f. sp. dianthi inoculum originating from naturally infested soil on wilt and flower yield in a highly susceptible cultivar of carnation (Dianthus caryophyllus «Fantasia») were studied in concrete containers (100 × 50 cm) during three growing seasons. Depth of inoculum placement was related directly to time until symptom onset and inversely to the final percentage of plants that were infected. Rate of disease 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 was 50, 45, 33, or 22%, respectively. Flower yields were correlated with depth of inoculum. Mean yield losses were 15, 23, 31, and 39% when inoculum was placed at depths of 60, 45, 30, and 15 cm, respectively, relative to yield in pathogen-free soil. In experiments on the effect of inoculum density, time until symptom onset and final disease incidence were related inversely and directly, respectively, to propagule density. The rate of disease progress was the same at all propagule densities. At densities of 6, 25, 120, 770, and 3,500 propagules per gram of soil, mean percentages of infected plants 185 days after planting were 2, 5, 13, 34, and 57%, respectively. Flower yields were related inversely to inoculum density. The effects of inoculum depth and density on final disease incidence were fit by linear logistic models. At the end of the growing season, the vertical distribution of F. o. dianthi propagules through a 60-cm depth showed a similar trend whether the propagules had been placed at 0-30 cm or at 60-75 cm prior to planting. The number of propagules placed at various inoculum densities prior to planting had increased 10- to 100-fold by the end of the growing season