Tissue localisation of lettuce resistance to the aphid Nasonovia ribisnigri using electrical penetration graphs

Electrical penetration graphs (EPG's) of Nasonovia ribisnigri (Mosley) (Homoptera, Aphididae) on resistant and susceptible lettuce ( Lactuca sativa , Compositae) showed a large reduction in the duration of the food uptake pattern (E2) on the resistant line. No differences in EPG's were observed before the phloem was reached. Therefore, resistance is believed to be located in the phloem vessel. Both mechanical blocking of the sieve element after puncturing and a difference in composition of the phloem sap are possible resistance factors. However, a chemical factor seems more likely because of the specificity of the resistance against N. ribisnigri.