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Disease Note Occurrence of Black Rot and White Rot Cankers on Apple in Pennsylvania. J. W. Travis, J. L. Rytter, and K. D. Hickey, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 75:968. Accepted for publication 8 April 1991. DOI: 10.1094/PD-75-0968D. In 1989, the black rot fungus Botryosphaeria obtusa (Schwein.)
Shoemaker and the white rot fungus B. dothidea (Moug.:Fr.) Ces.
& De Not. were both isolated from limb cankers on apple (Malus
domestica Borkh.). Cankered wood with typical symptoms of black
rot was collected from various counties throughout Pennsylvania in
response to grower concern that the disease had become more prevalent,
was spreading throughout their orchards, and was killing young,
vigorously growing trees. Isolations were made from cankers onto
potato-dextrose agar plates that were placed under continuous light.
Within I mo, pycnidia containing characteristic spores of both fungi
were produced. Conidia of B. obtusa were approximately 25 X 10
µm, nonseptate, dark brown, and ovoid. Conidia of B. dothidea were
approximately 19 X 6 µm, hyaline, and nonseptate. Inoculations were
made in the laboratory by placing mycelium and/ or spores of each
pathogen in superficial wounds on excised limbs and on apple fruit.
Symptoms developed within 1-2 wk on all inoculated samples, and
fungi were readily reisolated from the samples. B. dothidea was isolated
from natural infections primarily in the spring and summer months,
and B. obtusa was isolated primarily in the fall and winter. This is
the first report of both fungi being isolated from apple wood in
Pennsylvania (1). |