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Leaf Position Prevails Over Plant Age and Leaf Age in Reflecting Resistance to Late Blight in Potato

First, second, third, and fifth authors: Plant Research International, P.O. Box 16, 6700 AA Wageningen, the Netherlands; first and sixth authors: Crop and Weed Ecology, Department of Plant Sciences, Wageningen University, P.O. Box 430, 6700 AK Wageningen, the Netherlands; and fourth author: Section of Phytopathology, Faculty of Biology, Utrecht University, P.O. Box 800.84, 3508 TB Utrecht, the Netherlands

The effects of plant age, leaf age, and leaf position on race-nonspecific resistance against Phytophthora infestans were investigated in a series of field and controlled environment experiments with five different potato (Solanum tuberosum) cultivars. Leaf position proved to be the most significant factor; apical leaves were far more resistant to late blight than basal leaves. Plant age and leaf age had only minor effects; therefore, the resistance of a specific leaf remained about the same during its entire lifetime. The gradual increase in late blight resistance from basal leaves to apical leaves appeared to be a general effect, irrespective of cultivar, growing conditions, or resistance test. Therefore, it is important to consider leaf position in tests for late blight resistance, because contrasts in resistance may be ascribed erroneously to differences between genotypes or treatments, whereas they are actually caused by differences in leaf position.