VIEW ARTICLE

Ecology and Epidemiology

Biology and Epidemiology of Mycosphaerella pomi, Cause of Brooks Fruit Spot of Apple. T. B. Sutton, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; E. M. Brown(2), and D. J. Hawthorne(3). (2)Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; (3)Department of Entomology, North Carolina State University, Raleigh 27695-7613. Phytopathology 77:431-437. Accepted for publication 8 August 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-431.

The disease cycle of Mycosphaerella pomi, cause of Brooks fruit spot of apple, was elucidated. The fungus overwinters in apple leaves, and in the spring and early summer ascospores are discharged from pseudothecia during rain and dew periods and infect fruit and leaves. Leaf infections remain quiescent until late summer, when they appear as small purple flecks on the leaves. Extensive colonization of the leaf does not occur until after leaf fall. There was no evidence of secondary spread. Ascospores of M. pomi germinated within 6 hr at 16–24 C; germ tube elongation was greatest at 24 C. Penetration of the leaf occurred only through the stomata; appressoria formed over the stomatal openings. Ninety-six hours of continuous wetting at 20 C was necessary for leaf infection to occur. A 12- or 24-hr dry period after germination either had no effect on the incidence of infection or resulted in an increase in infection. This suggests that long continuous wetting periods are not necessarily required for infection to occur and that infection may be enhanced under alternating wetting and drying conditions in the orchard. Leaf age did not affect susceptibility. After leaf infection, thick-walled vesicle- or chlamydospore-like structures developed in the mesophyll beneath the stomata. Mycelium was observed growing from some of these structures into the leaf mesophyll 12–16 wk after infection. Leaf tissues are colonized during the winter and pseudothecia can form on either leaf surface. The use of ergosterol biosynthesis inhibiting fungicides in the post bloom period for Brooks spot control is discussed.