VIEW ARTICLE

Relationship of Seedling Age to Development of Pythium ultimum on Roots of Antirrhinum majus. H. M. Mellano, Former Research Assistant, Department of Plant Pathology, University of California, Riverside 92502; D. E. Munnecke(2), and R. M. Endo(3). (2)(3)Professor, and Associate Professor, respectively, Department of Plant Pathology, University of California, Riverside 92502. Phytopathology 60:935-942. Accepted for publication 14 January 1970. DOI: 10.1094/Phyto-60-935.

Snapdragon (Antirrhinum majus) seedlings 15 days old or younger (susceptible) infected with Pythium ultimum were killed within 6 days because colonization of host tissue was rapid and unrestricted. However, 25-day-old (tolerant) plants developed tolerance to infection and host colonization that persisted for the rest of their life cycle. Infection of tolerant plants resulted in wilting and stunting intensified by high temp. Increased fertilization intensified root rot in seedling stages. Penetration and infection of roots of susceptible and tolerant plants were similar, but colonization of host tissues was different. On susceptible plants, appressoria arising from mycelia were formed commonly in the regions of elongation and maturation of roots, occasionally on the root cap or in the older regions of maturation, but never in the meristematic region. Several cells were invaded within 4-8 hr after appressoria formed; subsequent intracellular and intercellular colonization by mycelia was rapid and complete. Growth into meristematic areas appeared to be dependent upon death of the tissues. On tolerant plants, appressoria were formed abundantly on young tertiary and quaternary roots but sparingly on thickened primary and secondary roots. Colonization of the cortex of tertiary and quaternary roots was rapid but mycelial growth in the cortex of primary and secondary roots was greatly restricted. Oospores formed in infected tissue of both young and old roots 4-6 days after inoculation. The difference in response of tolerant and susceptible plants to infection could not be related to lignification of host cells. These observations indicated that tolerance to infection of snapdragon by P. ultimum was due to physiological rather than anatomical characteristics of the host tissues.