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Ecology and Epidemiology

Relationship of Calcium to Potato Scab. D. H. Lambert, Assistant professor, Department of Botany and Plant Pathology, University of Maine, Orono 04469; F. E. Manzer, Professor, Department of Botany and Plant Pathology, University of Maine, Orono 04469. Phytopathology 81:632-636. Accepted for publication 10 January 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-632.

Field plots were treated with 4.5 MT/ha of dolomitic lime, an equivalent amount of gypsum (960 kg of Ca per hectare), or no Ca. At harvest, soil pHs in 1989 and 1990 were 4.4 and 4.4, 4.4 and 4.5, and 5.1 and 5.5 for nontreated, gypsum, and lime, respectively. In rows inoculated with Streptomyces scabies, incidences of tubers with scab lesions in 1989 and 1990 were 20 and 22, 18 and 22, and 53 and 80%, respectively, for the same treatments. Without reinoculation, scab incidences in 1990 were 0, 0, and 32%, respectively. Scab incidence was correlated with soil pH (P ? 0.001), but was not correlated with Ca concentrations in the soil, healthy tuber periderm, or medulla tissue. Calcium concentrations were four to five times higher in scab tissue than in healthy periderm from the same tubers. Therefore, in low pH soils, higher tissue Ca concentration is an effect rather than a cause of increased scab. Lesion diameter was inversely correlated with tuber Mg and Mn concentrations. Tubers with high Ca concentrations were less susceptible to soft rot when inoculated with Erwinia carotovora subsp. carotovora, indicating that treatment differences were sufficient to alter host reaction in a disease system known to be calcium-affected.