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Resistance

Variation in Susceptibility to the Pitch Canker Fungus Among Half-Sib and Full-Sib Families of Virginia Pine. Jane Barrows- Broaddus, Plant pathologist, USDA Forest Service, Southeastern Forest Experiment Station, Forestry Sciences Laboratory, Carlton Street, Athens, GA 30602; L. D. Dwinell, principal plant pathologist, USDA Forest Service, Southeastern Forest Experiment Station, Forestry Sciences Laboratory, Carlton Street, Athens, GA 30602. Phytopathology 74:438-444. Accepted for publication 1 November 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-438.

A survey for pitch canker among ramets of eight clones in a seed orchard of Virginia pine (Pinus virginiana) in Alabama indicated that the incidence of disease (0- 52% ramets with cankers) was related to specific clones. When seedlings of half-sib familes from these clones were inoculated with Fusarium moniliforme var. subglutinans, the causal agent of pine pitch canker, the ranking correlation of the families for susceptibility to the pathogen (based on 2 yr of trials) to the seed orchard survey averaged 0.77. In an 11-yr-old progeny test in Alabama, new shoots on full-sib families from four of the seed orchard clones were inoculated with F. moniliforme var. subglutinans for two consecutive years. The most susceptible family (x* = 83%) was related genetically to one of the most susceptible clones from the survey (52%) and a moderately susceptible (x* = 42%) family from the greenhouse trials. Ranking of susceptibility among the other families, however, was not consistent between years and did not match the survey or the greenhouse trials. Inoculated shoots in greenhouse and field trials initially developed resinous lesions and purple discoloration of the shoot. Sporodochia (1- 3 mm in diameter) and microscopic sporodochia (0.06- 0.2 mm in diameter) were observed on dead shoots. In the field, dead shoots became defoliated. Prior to needle drop, hyphae were observed on the surfaces of the needles, and microscopic sporodochia were observed emerging from the epidermis adjacent to the needle fascicles. Histological examination of surviving shoots from inoculated seedlings and branches indicated that the formation of a periderm in the cortex and reaction parenchyma in the xylem was a factor in delaying invasion by the pathogen.