Temporal Changes in Susceptibility of Citrus Phloem Tissue to Colonization by Phytophthora citrophthora and P. parasitica. M. E. Matheron, Extension Plant Pathologist, Department of Plant Pathology, University of Arizona, Yuma Agricultural Center, Yuma 85364. J. C. Matejka, Research Assistant, Department of Plant Pathology, University of Arizona, Yuma Agricultural Center, Yuma 85364. Plant Dis. 73:408-411. Accepted for publication 10 December 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0408.

Four rootstocks and two commercial cultivars of citrus were evaluated for temporal variation in susceptibility to colonization by Phytophthora citrophthora and P. parasitica. For 24 consecutive months, terminal shoots were collected from field-grown trees of Citrus macrophylla, C. jambhiri, C. aurantium, and C. volkameriana. They were then wounded, inoculated with P. citrophthora or P. parasitica, and incubated in moist chambers. Shoot pieces and bark strips of lemon (C. limon ‘Lisbon’) and tangelo (C. reticulata × C. paradisi ‘Orlando’) were inoculated and incubated in moist chambers at monthly intervals for 24 consecutive months. Shoot pieces of C. jambhiri, C. aurantium, and C. volkameriana inoculated with P. citrophthora or P. parasitica developed smaller lesions during December–February than during June–August. Colonization of excised shoot tissue of C. macrophylla by P. citrophthora was less during December–February than June–August, whereas colonization of the same plant tissue by P. parasitica was less during December–February than March–May. Minimum development of lesions occurred during November–December on bark strips from Lisbon lemon and Orlando tangelo. These findings demonstrate temporal fluctuations in the colonization of citrus rootstock and scion tissues by P. citrophthora and P. parasitica.