Inoculum Source and Density Influence Assessment of Fusiform Rust Resistance in Slash Pine. Margene M. Griggs, Plant Geneticist, Southern Forest Experiment Station, USDA Forest Service, Gulfport, MS 39503. Ronald J. Dinus, Manager, Western Forest Research Center, International Paper Company, Lebanon, OR; and Glenn A. Snow, Principal Plant Pathologist, Southern Forest Experiment Station, USDA Forest Service, Gulfport, MS 39503. Plant Dis. 68:770-774. Accepted for publication 1 March 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/PD-68-770.

Open-pollinated seedlings from resistant, moderately resistant, and susceptible slash pines (Pinus elliottii var. elliottii Engelm.) were inoculated with basidiospores of Cronartium quercuum f. sp. fusiforme from two geographic sources at three densities. Variation among pine families, inoculum densities, and sources was assessed in terms of presence or absence of initial symptoms (purple spots on stems and needles), seedlings galled 6 and 9 mo after inoculation, gall lengths, and seedling recovery. Recovery was indicated either by lack of gall formation or small galls that ceased to grow. More seedlings of the resistant and moderately resistant parents than of the susceptible parent developed initial symptoms without subsequently forming galls (“early recoveries”). Frequency of early recovery varied with inoculum density. Fewer seedlings from the resistant parent were galled 9 mo after inoculation, but the distinction between families diminished with increasing density. Most seedlings with small galls (<2.5 cm long) were free of external symptoms and active rust mycelium 33 mo after inoculation (“late recoveries”). Family effects on seedlings with small galls were significant. Family differences were increased when late recoveries were deducted from those galled at 9 mo. Results confirm that uniform, appropriate inoculum densities are prerequisite to accurate resistance tests and indicate that tests with varied sources and densities will help discern resistance types, degrees, and stabilities.