I. Introduction: Susceptible pear cultivars affected by Fabraea leaf spot often are defoliated by
midsummer, resulting in dwarfing of fruit and reduction of fruit buds. Infected fruit is
worthless since it is usually cracked, disfigured, or misshapen. In the nursery, early
defoliation and twig infection result in stunted growth of trees.
II. Symptoms: Leaf spot can be found on petioles,
leaves, shoots and fruits. Initial lesions on leaves are tiny, round, purplish-black
spots, which quickly enlarge to 1/8 to 1/4 inch in diameter and usually have a
blackish-brown center (photo 2-46). Spots coalesce and severely infected leaves fall to
the ground prematurely. A small black acervulus may develop in the center of each lesion,
from which conidia ooze in a creamy, white mass in wet weather. Fruit lesions (photo
2-47) are larger than those on leaves and cause the fruit to crack and drop. Lesions on
current season's shoots may be observed as small inconspicuous, purplish-black spots. Some
lesions develop into superficial cankers, but most are walled-off during the next growing
season, so that cankers rarely persist in two-year-old wood.
III. Disease Cycle: The
four-celled conidia (Entomosporium maculatum), with a distinctive insect-like
appearance, are spread mainly from overwintering leaf litter, and some from twig cankers,
by splashing water from rains or overhead irrigation. Wetting periods for infection may
vary from 8 to 12 hours at temperatures of 50 to 77 F (10-25 C). Lesions begin to
appear about 7 days after the beginning of an infection period. The disease may advance
rapidly in late summer as wind and rain distribute the conidia throughout the tree.
Susceptibility of leaves and fruit to infection does not decrease with maturity. Nearly
all pears of European descent are susceptible to this leaf spot.
IV. Monitoring: At midseason, examine 20 of the
lowest leaves on each sample tree for earliest symptoms (photo 2-46). One to ten
infections and greater than ten infections per 20 leaves represents moderate and high
V. Management: This disease is
controlled with applications of protectant fungicides. The timing and number of
applications varies depending upon the source and availability of primary inoculum, which
varies among regions of the eastern U.S. Early-season spray programs for pear scab
should also control early-season leaf spot infections. Where ascospores and conidia
of the fungus occur after petal fall, summer fungicide treatments are needed. In the
northeastern U.S., fungicide applications in June and July will generally control Fabraea
leaf spot, but mid-August and September applications are needed in wet seasons on
late-maturing cultivars such as Bosc.
Chemical control -
Chemical control - home
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Text prepared by T. van der Zwet and A. R.
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