Studies on sunflower rust. XI. Effect of temperature and light on germination and infection of sunflowers by Puccinia helianthi

Abstract

Urediospores of four races of Puccinia helianthi incubated on water agar for 6 h or on sunflower leaf discs for 16 h germinated equally well from 10 to 25 °C, but significantly less at 30° and least at 5 °C. Appressorium formation was highest at 10 to 25 °C, lower at 30 °C, and lowest at 5 °C for races 1 and 3; none occurred at 30 and 5° in races 2 and 4. The optimum for penetration by races 1 and 3 was 20 °C, for race 2, 15° to 20 °C, and for race 4, 20° to 25 °C. Results were the same on susceptible and resistant hosts.Germination percentage on water agar was reduced with increasing light intensity from 2200 to 17 600 lx during incubation for 6 h. Germination on sunflower leaf discs was reduced with increasing light intensity during incubation for 16 h. Appressorium formation and penetration were adversely affected by increasing light intensity.Inhibition by light was overcome during a subsequent dark period when inoculated leaf discs were exposed to various cycles of light and dark during a 16-h incubation period.