First, second, and fourth authors: Department of Plant Pathology; and third author; Department of Botany and Plant Sciences, University of California, Riverside 92521
Initial infection processes and the subsequent colonization of leaves and young stems of peach by Tranzschelia discolor were studied. On leaves where multiple disease cycles of peach rust occur during the growing season, urediniospores germinated after 4 h of wetness. Germ tubes became septate and formed appressoria only over leaf stomata beginning 18 h after inoculation. No appressoria, however, formed over stomata of positive replicas of leaf surfaces indicating nonthigmotropic responses of germ tubes. On young, primary-growth stems (ca. 8 weeks old), stomata were mostly closed, less frequent than on leaves, and recessed from the surface of the cuticle of the epidermis. Although appressoria formation was not observed on inoculated stems, germ tube growth of urediniospores was directional toward stomata. Penetration of stem tissue is apparently a less common event that was reflected by a lower occurrence of stem lesions compared with that of leaf lesions in our potted plant inoculation studies and previous field observations. Still, stem lesions are important as sources of primary inoculum each spring and were reproduced in this study for the first time. Fungal colonization of leaves and stems was subepidermal-intercellular and haustoria were commonly found within mesophyll or cortical cells, respectively. No fungal colonization was observed in cambial stem tissue. Vascular tissue was also not colonized and delimited lesions in leaves and stems. Morphological host responses were not observed in infections on either leaves or young stems. In older stems (>32 weeks old), however, the infection was delimited by a wound periderm after uredinial formation. Furthermore, with continued secondary growth, stems recovered and fungal lesions became part of the bark tissue of woody branches. Thus, the fungus must infect primary-growth branches each year to establish stem lesions.