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Use of an Apparent Infection Threshold Population of Pseudomonas syringae to Predict Incidence and Severity of Brown Spot of Bean. J. Lindemann, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address: Advanced Genetic Sciences, Inc., 6701 San Pablo Avenue, Oakland, CA 94608; D. C. Arny(2), and C. D. Upper(3). (2)Department of Plant Pathology, University of Wisconsin, Madison 53706; (3)Plant Disease Resistance Research Unit, Agricultural Research Service, USDA, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 74:1334-1339. Accepted for publication 12 June 1984. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-1334.

A single seed lot of snap bean (Phaseolus vulgaris) was planted at 11 sites along an east-west transect in central Wisconsin. Epiphytic population sizes of naturally occurring Pseudomonas syringae pv. syringae pathogenic to bean (Psb) on symptomless bean leaflets differed among these sites. Bacterial brown spot was not detected at any site at which log10 (epiphytic Psb population size) was <4.0 on every bean leaflet sampled. Thus, 104 colony-forming units per gram of leaflet tissue may represent an apparent infection threshold population of Psb. The frequency with which Psb populations exceeded the apparent infection threshold level was estimated graphically. A model based on this frequency estimate was highly predictive of brown spot incidence 1 wk after full flower. The presence of very high Psb populations was a more reliable predictor of disease severity than was disease incidence. This predictive model based on infection threshold is presented as preferable to models based on mean pathogen populations because infections occur on individual plant parts, rather than on some theoretical mean plant part.