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Components of Partial Resistance in the Slash Pine-Fusiform Rust Pathosystem

First and third authors: School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, PO Box 110410, 32611-0410. Second author: CEPLAC/CEPEC/SEFIT, Cx. PO 07, 45600-000, Itabuna, BA Brazil. Fourth author: U.S. Department of Agriculture, Forest Service, Resistance Screening Center, 1579 Brevard Road, Asheville, NC 28806-9561

Progeny of rust-resistant, open-pollinated slash pine families exhibited components of partial resistance in greenhouse tests. Nine-month-old seedlings of some resistant families had (i) a greater frequency of short galls (≤ 25 mm long), and (ii) fewer sporulating galls compared with other resistant and susceptible families when exposed to Cronartium quercuum f. sp. fusiforme in greenhouse tests. These components of partial resistance were evident with (i) mixed- or single-gall inocula, (ii) varying inoculum concentration, and (iii) inoculation of wounds or intact tissues. Inocula effects were nonsignificant, but family effects were highly significant (P = 0.001 - 0.0001) for total galls (TG) and short galls (SG). Multivariate family means (TG-;SG clusters) were highly significant (P = 0.0001). Short galls decreased on all families with an increase in inoculum concentration. In 36 field trials (12 locations × 3 years) a family which exhibited components of partial resistance in greenhouse tests exhibited lower disease incidence (% trees infected), lower disease severity (galls per tree), and less variability in disease incidence and severity among trials compared with other resistant families. Partially resistant families would be useful for tree improvement and for research into the genetics and mechanisms of resistance in the slash pine-fusiform rust pathosystem.

Additional keywords: Cronartium quercuum f. sp. fusiforme , incomplete resistance, pine stem rust, Pinus elliottii var. elliottii .