Disease Symptomatology and Variation in Susceptibility of Seed-Propagated Hybrid Geranium Varieties to Xanthomonas campestris pv. pelargonii. Christine T. Stephens, Associate Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. James Tuinier, Former Graduate Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Plant Dis. 73:559-562. Accepted for publication 27 January 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0559.

Ten of the 63 seed-propagated F₁ hybrid geranium varieties were found to possess some resistance to Xanthomonas campestris pv. pelargonii based on either a foliar disease rating index or plant survival. In two of the more resistant varieties, rapid leaf abscission and absence of a systemic population of the bacteria in the vascular system was observed. The optimal plant age for screening plants for resistance was 80–95 days after seeding. At this plant age, applications of chlormequat chloride to reduce the growth of plants had no effect on disease incidence or severity. Supplemental high-pressure sodium illumination also had no effect. The development of bacterial wilt on diploid seed-propagated geranium was similar to that on tetraploid cutting geranium except that in the former, lesions often coalesced into large, blighted, necrotic areas, a symptom not normally observed on the latter. The moderately resistant cultivars identified in this study appear to be useful for breeding programs.