A Simple Quantitative Procedure for Inoculation of Safflower with Teliospores of the Rust Fungus, Puccinia carthami

A procedure is needed to determine susceptibility of safflower seedlings to teliospore inoculations of foreign accessions of Puccinia jaceae, a candidate agent for biological control of Centaurea solstitialis (yellow starthistle). The indigenous safflower rust pathogen, P. carthami, served as both model and basis for a future comparative study of these two rust fungi on safflower. The procedure involves inoculation of 2-day-old safflower seedlings on the hypocotyl near the base of the cotyledons with a known number of teliospores suspended in a 1-μl drop of a 0.1% water agar and wetting agent carrier. Advantages of this procedure include (i) quantification, by spore type, of inoculum applied to each seedling, (ii) precise placement of inoculum on test plants, and (iii) conservation of inoculum. Results were verified in studies with two safflower cultivars inoculated with 0, 15, 60, or 240 teliospores of either of two isolates of P. carthami. Plant height and dry weight data collected 6 weeks after inoculation revealed significant interactions for cultivar × inoculum concentration and for isolate × inoculum concentration. Similar results were achieved when safflower seeds were rolled in dry teliospores of P. carthami but not when seedlings grew through teliospore-infested sand. The latter two approaches (dry teliospore and infested sand inoculation) require much more inoculum and are only semiquantitative. The microliter drop procedure has potential usefulness in host range and quantitative studies involving other pathogens, including bacteria, fungi, and nematodes.