Development of barley leaf rust,Puccinia hordei,infections in barley. II. Importance of early events at the site of penetration for partial resistance

Summary The investigation involved three barley genotypes that varied from extremely susceptible (Akka) to an extreme level of partial resistance (17-6-16). The barley leaf rust colony size was measured in primary leaves 3, 6, 12 and 18 days after inoculation and in flag leaves 6, 12, 18, 24 and 30 days after inoculation with race 1-2-1. Akka always had the largest colonies, 17-5-16 the smallest, with Vada at an intermediate position. The genotypic differences were proportionally largest at the second sampling day and smallest at the last sampling day. The rate of colony growth decreased rapidly over time for all genotypes and in both plant stages. Measured at the same time (colony age the same) the rate of colony growth was largest for 17-5-16 and smallest for Akka in most periods. The time needed to reach a given colony size showed already large differences in the very early states especially in the flag leaves. Akka took 3.9 days to reach a size of 17 × 10^-3 mm² (only 5 to 10% of the colony size at the start of sporulation), Vada needed 8.3 days and 17-5-16 even 12.0 days. To reach a size of 320 × 10^-3 mm² the three genotypes needed 12.7, 18.0 and 22.8 days respectively, differences that are only slightly larger than those at the very small colony size.It was concluded that the partial resistance of barley to barley leaf rust is not primarily due to a reduced fungal growth in the partially resistant host tissue but predominantly so to an initial and temporary stagnation at the site of penetration. The longer this stagnation lasts, the longer the latent period and the higher the partial resistance are. Once this stagnation has been overcome the fungal growth rates do not vary much between genotypes with different levels of partial resistance.