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Infection of Austrian, Scots, and Ponderosa Pines by Diplodia pinea. L. W. Brookhouser, Former Graduate Assistant, Plant Pathology Department, University of Nebraska, Lincoln 68506, in cooperation with the College of Agriculture, University of Nebraska, Present address of senior author: Department of Plant Pathology, University of California, Berkeley 94720; Glenn W. Peterson, Plant Pathologist, Rocky Mountain Forest and Range Experiment Station, Forest Service, USDA, Lincoln, Nebraska 68503, in cooperation with the College of Agriculture, University of Nebraska. Phytopathology 61:409-414. Accepted for publication 12 November 1970. DOI: 10.1094/Phyto-61-409.

Spores of Diplodia pinea were disseminated from March to November in eastern Nebraska. Large numbers of spores were disseminated only during periods of rainfall. Spores germinated on water agar at temperatures ranging from 12 to 36 C. Per cent germination was highest at 24 C, and germ tube growth was maximum at 28 C. Spores started to germinate within 1.5 hr at 26 C; over 75% germinated within 2 hr. Fluorescent-labeled spores, plastic prints of needle surfaces, and microtome sections of inoculated needles were used in establishing that the pathogen penetrates needles of Austrian pine (Pinus nigra), Scots pine (P. sylvestris), and ponderosa pine (P. ponderosa) through stomata. Some germ tubes on needle surfaces grew directly toward and entered stomatal pits, but most grew randomly. Needles inoculated with fluorescent-labeled spores contained masses of fluorescent hyphae in stomatal pits. Hyphae from these masses grew through stomata and into the mesophyll. Direct penetration of the epidermis was never observed. Incubation for 12 hr in growth chambers at 100% relative humidity and 24 C was sufficient for infection of uninjured Austrian, Scots, and ponderosa pine needles. Initial symptoms, consisting of small, tan to reddish-brown lesions on the lower portion of young needles, appeared within 3 days after inoculation on Austrian and Scots pines and within 4 days on ponderosa pine. Needle discoloration and necrosis developed rapidly. Inoculations of 10-year-old trees in eastern Nebraska revealed that young shoots of all three species were highly susceptible to infection from late April until mid-June; previous years’ needles were not susceptible.