The Nature of Horizontal Resistance of Oats to Crown Rust

Abstract

Development of Puccinia coronata in an oat cultivar with horizontal resistance (HR) was assessed using fluorescence microscopy. The first visible response was fluorescence in mesophyll cell walls. This reaction occurred about 18 h after inoculation, and seemed to depend upon development of substomatal vesicles. The growth of the parasite at individual infection sites was inversely related to the degree of host response, which was expressed as the number of fluorescing cells. A rapid and severe host reaction resulted in the arrest of growth of the parasite before haustorial mother cells formed, but slow host response retarded hyphal growth. The arrest of growth was associated with a reduction in the number of uredinia, while the retardation of hyphal growth reduced size of the uredinia and increased the latent period. Thus, fluorescence of the mesophyll cell walls was correlated with these three components of HR. In some sites, the host did not react to invasion by the parasite. When this happened, the hyphae grew at a normal rate. Seven days after inoculation, the average area of the uredinia of a susceptible type was about three times larger than that of the cultivar with HR. My results suggest that the substomatal vesicle produces a compound that is recognised by mesophyll cells, and that the time of host recognisation and the speed of host response condition three major components of HR to crown rust.