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Symptom Development and Resistance in Safflower Hypocotyls to Phytophthora drechsleri. Lowell B. Johnson, Assistant Research Plant Pathologist, Department of Plant Pathology, University of California, Davis, Present address: Department of Plant Pathology, Kansas State University, Manhattan 66502. Phytopathology 60:534-537. Accepted for publication 22 October 1969. DOI: 10.1094/Phyto-60-534.

The hypersensitive response and number of dead plants of resistant Biggs safflower (Carthamus tinctorius) inoculated with Phytophthora drechsleri was greater under low than under high light intensities at 27 C. Hypocotyl cross sections, 24 hr after inoculation, showed no morphological basis for this difference under the two light intensities, although slightly more fungal mycelium was evident in the low-light plants. The preinoculation light effect on plant response and death was not modified by shifting plants from high to low light or vice versa or to darkness for up to 48 hr during and after inoculation. Longer exposures (3-5 days) to darkness or the covering of plant tops with aluminum foil resulted in extensive killing of inoculated Biggs. Excised Biggs leaves were capable of a resistant reaction. Terminal grafts of Biggs tops on Nebraska 10 (N10, susceptible) hypocotyls and roots and N10 tops on Biggs roots and hypocotyls showed that hypocotyl resistance or susceptibility was unchanged regardless of the reaction of the top. Hypocotyls of uprooted Biggs and N10 plants showed typical symptoms following a 24-hr inoculation and flooding period when compared to plants inoculated directly in crocks, as did plants with all roots removed. However, only a portion of those plants that were uprooted, and which had their tops excised at the cotyledonary node, showed any hypocotyl symptoms when flooded during and after inoculation. Hypocotyls of Biggs and N10, excised after a 2-hr inoculation period and immersed in 15 ml water in test tubes, showed no symptom development, although hypocotyls of uprooted plants in the same test tubes did. Symptoms developed if hypocotyls were instead plated on moist filter paper in petri plates. Material(s) showing a pink coloration and blue ultraviolet fluorescence leached through the hypocotyl epidermis when excised inoculated hypocotyls were wrapped in moist chromatography paper. This leaching was more intense in inoculated Biggs than in inoculated N10. Its relationship to resistance and symptom development was not determined.