Compatible host/parasite pairs enhance propagation of paintbrush (Castilleja spp.)

Abstract

Indian paintbrush, a parasitic member of the family Scrophulariaceae, is difficult to propagate due to dependence on an appropriate host plant species for long-term survival and health. A series of experiments identified prospective host species and procedures to improve viability for horticultural propagation of paintbrush (Castilleja spp.). Thirty-four paintbrush species were evaluated for tolerance to horticultural propagation techniques. Castilleja integra A. Gray, C. scabrida Eastw., and C. chromosa A. Nelson (Scrophulariaceae) demonstrated superior tolerance to transplanting and survived as seedlings in flats and pots when grown without a host. In subsequent preliminary and culminating experiments, compatibility of paintbrush/host species combinations was evaluated using a range of production conditions. Castilleja integra plants survived better after pot-to-pot transplanting with a host species when larger seedlings were utilized. Direct-seeding paintbrush and host plants into a common pot was approximately half as effective for establishing healthy C. integra compared with transplanting the species together after germination in independent flats. Castilleja integra showed a strong host-specific survival and vigor response. Higher levels of paintbrush survival, plant vigor, and aesthetic compatibility occurred when C. integra was paired with Artemisia frigida Willd. (Asteraceae), Artemisia michauxiana Besser (Asteraceae), Ericameria nauseosa Willd. (Asteraceae), Penstemon pinifolius Greene (Scrophulariaceae), Calylophus serrulatus (Nutt.) P.H. Raven (Onagraceae), Eriogonum strictum Benth. (Polygonaceae), and Eriogonum jamesii Benth. (Polygonaceae).