VIEW ARTICLE

The Etiology and Histopathology of Helminthosporium Blister Canker of Pear. Gregoria N. Acedo, Formerly Graduate Assistant, Department of Plant Pathology, University of Massachusetts, Amherst 01002; G. N. Agrios, Associate Professor of Plant Pathology, Department of Plant Pathology, University of Massachusetts, Amherst 01002. Phytopathology 61:312-316. Accepted for publication 1 October 1970. DOI: 10.1094/Phyto-61-312.

A species of Helminthosporium was isolated from the bark of pear trees showing blister canker symptoms. Inoculations of pear seedlings with mycelial suspensions of the fungus proved it to be the cause of this disease on pear. The fungus from pear is similar to Helminthosporium papulosum from apples in cultural and morphological characteristics, but the isolate from pear does not infect apple and vice versa. The fungus on pear is therefore considered to be a distinct pathological form of H. papulosum, and is therefore designated Helminthosporium papulosum f. sp. pyri. Helminthosporium papulosum f. sp. pyri is a slow-growing fungus attaining a colony diameter of about 2.5 cm after 3 weeks of growth on nutrient media. Growth is favored by media rich in carbohydrates. The fungus rarely sporulates on nutrient media, but exposure of cultures to continuous fluorescent light enhances sporulation. Sporulation of the fungus in the field occurs in July and August, and coincides with the period of new infections in the field. Helminthosporium papulosum f. sp. pyri penetrates pear bark directly and invades the cortex both inter- and intracellularly. The mycelium also invades the phloem fiber cells, but its presence in the sieve tubes and xylem vessels was not demonstrated. In older lesions, browning and discoloration extend to the cambium and underlying wood. Hyperplasia and slight hypertrophy were observed in parenchymatous cells away from the invading hypha.