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Phytoalexin Production in Locally Cross-Protected Harosoy and Harosoy-63 Soybeans. W. E. Svoboda, Student Assistant, Department of Plant Pathology, University of Illinois, Urbana-Champaign Campus, Urbana 61801; J. D. Paxton, Associate Professor, Department of Plant Pathology, University of Illinois, Urbana-Champaign Campus, Urbana 61801. Phytopathology 62:1457-1460. Accepted for publication 4 May 1972. DOI: 10.1094/Phyto-62-1457.

Hypocotyl tissues of 5-day-old Harosoy soybeans were cross-protected against local infection by Phytophthora megasperma var. sojae race 1 (Pms₁) by prior inoculation of the hypocotyl with a nonpathogen, Phytophthora cactorum. P. cactorum remained alive in the hypocotyl lesion during plant development, continuously triggering the production of phytoalexin (PA_k) inhibitory to the growth of the Pms₁ pathogen. The maintenance of the PA_k protective mechanism against infection by Pms₁ extended to a stage of plant development in which a form of adult plant resistance may become the predominating protective mechanism. Quantitatively, the PA_k reached peak concentration at 6 days after inoculation with P. cactorum; after 6 days, the concentration of PA_k remained relatively constant. No spectrophotometrically detectable breakdown product of the PA_k was found during the period studied. In the same manner, Harosoy-63 (H-63) soybean hypocotyls were cross-protected against local infection by an isolate of P. megasperma var. sojae (Pms) which can attack them. H-63 was cross-protected by prior inoculation of the hypocotyl with P. megasperma var. sojae race 1 (Pms₁), a nonpathogen of H-63. Pms₁ failed to persist in the hypocotyl of H-63 after 3 days. During this time, the initially induced PA_k broke down, allowing Pms infection to develop. With the cross-protective mechanism lost, the plants died.