Abstract
Abstract A description is provided for Dibotryon morbosum . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: On Prunus americana, P. armeniaca, P. avium, P. besseyi, P. cerasus, P. domestica, P. maritima, P. melanocarpa, P. pennsylvanica, P. pumila, P. serotina, P. virginiana and other species of Prunus . DISEASE: Black knot of plum, damson, cherry, peach, apricot and other species of Prunus . The pathogen causes the formation of elongated black hypertrophied host tissue on the current year's branches or on trunks. These are generally confined to one side and are several times the normal diameter of the stem. The knots are olive-green and corky at first, later turning black and becoming hard and brittle. In late summer some may appear pink or white due to parasitism by Trichothecium roseum . On black cherry, P. serotina , large cankerous swellings 0.6 m or more long may be formed which render the tree worthless for timber. Attacked trees become stunted and dwarfed. GEOGRAPHICAL DISTRIBUTION: Confined to North America (Canada and U.S.A.) (CMI Map 48). TRANSMISSION: By ascospores which have a peak production in May (45, 3263h). Storage of knots from Prunus serotina at -20°C for 6 months had little effect on ascospore viability (44, 2699j). Conidia also tolerate low temperatures, surviving 192 days at -20°C but only 25 days at 21°C (47, 573). In addition to some conidia overwintering in a viable condition on plum and cherry, others develop from overwintering chlamydospores on plum buds and bark (14: 43). Prunings containing excised knots which are left lying in orchards can continue to produce viable spores for several weeks after being removed (9: 116; 24: 375). The pathogen may also be introduced to new areas on infected nursery stock (McClintock, 1945).
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