Histopathology of blueberry twig blight caused by Phomopsis vaccinii

Emerging flower buds or fully opened flower clusters of highbush blueberry plants were inoculated with a conidial suspension of Phomopsis vaccinii. Samples were taken 3 days and 1, 2, 3, 4, 6, 8, and 10 wk after inoculation for histopathological study. P. vaccinii penetrated succulent floral parts. Hypertrophy and hyperplasia occurred in response to infections on ovaries and floral axes, respectively, but did not prevent growth of the hyphae. Hyphae grew intercellularly through the cortical cells of the pedicel and into the intercellular spaces in the cortex of the floral axes by 1 wk after inoculation. Stems were colonized initially by hyphal growth through the intercellular spaces in the cortex. Numerous tyloses were formed in the vessel elements in advance of this growth. All tissues in the infected stems and floral axes subsequently were colonized by P. vaccinii leading to necrosis and collapse of parenchyma cells in the cortex and vascular tissue. Gums were deposited in the infected vessel elements. Stem lesions ceased expanding by 8 wk after inoculation, and a meristematic layer of suberized cells developed in the cortex between the infected and healthy portions of the stem. Pycnidia were present in necrotic stem tissue by 10 wk after inoculation