Histopathological Study of the Alternaria solani Infection Process in Potato Cultivars with Different Levels of Early Blight Resistance

Histopathological analyses of the infection process of Alternaria solani were accomplished in three potato cultivars with different levels of early blight resistance. Leaflets of Aracy (resistant), Delta (moderately resistant) and Bintje (susceptible) cultivars were inoculated at the beginning of the flowering stage. In order to study the effect of leaf age, leaf samples were collected separately, in the lower, middle and upper third of the plants. Conidia germination, appressoria formation, penetration and number of penetration sites exhibiting hypersensitive response (HR) were qualitatively and quantitatively assessed at 6, 12, 24, 48 and 72 h after inoculation. Penetration occurred most frequently through the junctions of the epidermal cells. Penetrations through stomata were rarely observed. There was no association between the events and the resistance levels, except for the number of penetration sites showing HR that was highest in Aracy, intermediate in Delta and lowest in Bintje. Similarly, the number of sites with HR was the only event associated with the leaf age. Regardless of cultivar resistance levels, the number of the penetration sites with HR was higher in leaves in the upper part of the plant. These results suggest that HR may be one of the mechanisms associated with age-related and genetic resistance to early blight in potato.