Infection of radiata and bishop pine by Mycosphaerella pini in California

Infection of radiata pine (Pinus radiata D. Don) and bishop pine (Pinus muricata D. Don) by Mycosphaerella pini Rostr. in Munk was determined on needles of infected forest trees in central to northern coastal California. Conidia from infected trees were used to inoculate radiata pine seedlings maintained in constant and (or) fluctuating moisture and temperature regimes. On needles of radiata pine but not bishop pine plantation trees, most conidial germ tubes grew directly towards and entered the nearest epistomatal opening. On inoculated radiata pine seedlings in infection chambers, germ tubes grew haphazardly and few entered openings. Germ tubes entered epistomatal openings more often on abaxial than adaxial needle surfaces of both plantation trees and inoculated seedlings, and more lesions developed on the abaxial surface. Simple hyphae penetrated through epistomatal chambers, between guard cells, and into substomatal chambers. On radiata pine plantation trees, germ tubes that penetrated below guard cells occasionally formed substomatal vesicles. On needles of northern race bishop pine that had few disease lesions per needle, substomatal vesicles were common and frequently partially disintegrated. On seedlings, "water-soaked" spots formed 5 days after hyphae of the fungus penetrated slightly below guard cells. Development of typical lesions was delayed when seedlings were initially exposed to up to 16 days of continuous mist spray and then kept dry for 8 weeks. Penetration on inoculated seedlings was significantly greater in a variable than in a constant air temperature regime on abaxial needle surfaces, and greater in 24 h/day than in a 16 h/day exposure of seedlings to mist spray. In both temperature regimes more needles were infected on seedlings exposed to 24 h/day mist spray and fewest were infected in the variable temperature, 16 h/day moisture regime.