Components of partial resistance in the slash pine-fusiform rust pathosystem

Progeny of rust-resistant, open-pollinated slash pine families exhibited co mponents of partial resistance in greenhouse tests. Nine-month-old seedling s of some resistant families had (i) a greater frequency of short galls (le ss than or equal to 25 mm long), and (ii) fewer sporulating galls compared with other resistant and susceptible families when exposed to Cronartium qu ercuum f. sp. fusiforme in greenhouse tests. These components of partial re sistance 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 signifi cant (P = 0.001 - 0.0001) for total galls (TG) and short galls (SG). Multiv ariate family means (TG-SG clusters) were highly significant (P = 0.0001). Short galls decreased on all families with an increase in inoculum concentr ation. In 36 field trials (12 locations x 3 years) a family which exhibited components of partial resistance in greenhouse tests exhibited lower disea se incidence (% trees infected), lower disease severity (galls per tree), a nd less variability in disease incidence and severity among trials compared with other resistant families. Partially resistant families would be usefu l for tree improvement and for research into the genetics and mechanisms of resistance in the slash pine-fusiform rust pathosystem.