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Cytology and Histology

Colonization of Resistant and Susceptible Oaks by Ceratocystis fagacearum. W. R. Jacobi, Graduate research assistant, Division of Plant Sciences, West Virginia University, Morgantown 26506, Present address of senior author: Department of Plant Pathology, North Carolina State University, Raleigh 27650; W. L. MacDonald, associate professor of plant pathology, Division of Plant Sciences, West Virginia University, Morgantown 26506. Phytopathology 70:618-623. Accepted for publication 3 December 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-618.

Colonization of northern red oak, Quercus rubra, chestnut oak, Q. prinus, and white oak, Q. alba, by Ceratocystis fagacearum from the time of inoculation to symptom expression was investigated by light microscopy. Two- to 3-yr-old branches from stump sprouts of each species were inoculated and stem sections were collected above and below the inoculation point before and after symptoms appeared. Symptoms were expressed by day 28 in red oaks and chestnut oaks and by day 53 in some white oaks. The pathogen was restricted laterally and longitudinally in vessels of white oak. The quantity of hyphae and conidia observed in white oak tissue was relatively constant by the 14th day after inoculation. In red oak, observable hyphae and conidia, initially observed by day 7, increased with time, spreading laterally and longitudinally into many small vessels and tracheids. Tyloses, gums, bubble-like structures, and darkly stained parenchyma cells were observed in all species in response to invasion. The ratio of tyloses to observable fungus was greater in white oaks than in red or chestnut oaks. A zone of darkly stained paratracheal parenchyma cells formed in white and chestnut oaks; in red oak this zone was more diffuse and involved both paratracheal and ray parenchyma. These differences in host responses may be related to the differential rate of colonization in red and white oaks.