Potato Foliage Sugar Concentration in Relation to Infestation by the Potato Leaf hopper, Empoasca fabae (Homoptera: Cicadellidae)1

Total sugar in potato foliage was measured periodically from terminal, mid-range, and basal-fourth leaves of a potato leaf hopper tolerant variety, Sequoia, and a suscept, Irish Cobbler, both grown under conditions of (a) no nymphal infestation (i.e., DDT-sprayed), and of (b) natural infestation by Empoasca fabae (Harris). Sugar concentration was normally higher in the tolerant variety. In both varieties, concentrations were progressively less from terminal, to mid-range, to basal foliage. But following leafhopper infestation elevation of sugar concentrations was progressively greater from terminal, to midrange, to basal foliage. Greatest elevation of sugar concentration occurred in physiologically aged basal leaves of the suscept where necrosis and destruction of primary foliage is severe. Occurrence of elevated sugar concentration in leaves of infested plants suggests a feedback mechanism enhancing the nutritional suitability of the host spurred by potato leafhopper infestation, Under natural processes of evolution the suscept would not withstand the selective pressure of heavy E. fabae infestation.