I. Introduction: Pear scab is an
economically important disease throughout the world and can cause serious losses on
susceptible cultivars. The disease is more of a problem in European countries than in
North America, and is especially of major concern in Japan. Sometimes called black spot,
pear scab resembles apple scab (Venturia inaequalis) in
nearly all respects, and is caused by the closely related fungus, V. pirina. Pear
cultivars differ in susceptibility to scab; however, cultivars resistant in one region of
the country may not be resistant in another region.
II. Symptoms: Symptoms of pear scab are very similar
to apple scab. Lesions on leaves and petioles begin as round, brownish spots that
eventually become velvety in appearance. Within these lesions conidia are produced. Later
in the season, small spots can be observed on the lower surface of the leaves. These are
usually the result of late spring or early summer infections. Leaf infection of pear is
not as common as apple scab on apple leaves.
III. Disease Cycle: Scab lesions on fruit occur on
the calyx end and eventually on the sides of the fruit (photo 2-45). As these lesions
enlarge, they become dark brown and form large black areas as they coalesce. Lesions on
immature fruit are small, circular, velvety spots. Darker, pinpoint spots develop as the
fruit matures. Infected fruit often become irregular in shape.
Unlike apple scab, twig infections are common with pear
scab. Early in the growing season, lesions on young shoots appear as brown, velvety spots.
Later, these lesions become corky, canker-like areas. The following spring, pustules will
develop within these overwintered lesions. These pustules produce spores (conidia) that
perpetuate the spread of the disease.
The fungus overwinters in leaves on the ground and also as
mycelium in infected twigs. Infection of pear foliage and fruit occurs under conditions
similar to those required for infection of apple by the apple scab fungus. Ascospores are
the major source of primary inoculum. Infection occurs in the spring around the green-tip
stage of flower bud development. Ascospores in the overwintered leaves are released as the
result of rain and are carried by air currents to young leaves and fruit. Ascospores
continue to mature over a six to eight week period.
Conidia are the source of secondary inoculum and are
produced in either the primary lesions initiated by ascospores or within pustules on
infected twigs. Many secondary cycles may occur over a growing season. The length of the
wetting period and temperature required for infection depend on the number of hours of
continuous wetness and the temperature during this wetting period. The Mills chart for
determining apple scab infection periods (Table 2-1) along
with a leaf wetness recorder or hygrothermograph can provide the information for
determining the infection periods for pear scab. Scab lesions may develop in as few as
eight days after infection on young leaves and in as many as two months on older leaves.
Fruit are also more susceptible when young; however, mature fruit can be infected if the
length of wetting period is sufficiently long.
IV. Monitoring: No monitoring required by growers
during the dormant period. Consult with regional Cooperative Extension Service personnel
to determine the onset of ascospore maturity. An awareness of the scab inoculum situation
in adjacent abandoned or commercial orchards may influence early-season scab control
During the prebloom period and continuing through fruit
set, for both fresh and processing fruit, determine pear scab infection periods by
observing duration of leaf wetness and average temperatures during the wet period (Table
2-1, chapter 2). Using this Mill's table as an indicator of time required for lesion
expression, begin monitoring sample trees of earlydeveloping cultivars for first leaf
symptoms. Examine the upper and lower leaf surfaces on a minimum of ten leaf clusters on
each sample tree. In monitoring, walk around the perimeter of the tree and examine at
least two leaf clusters at each of the four compass directions. Record the total number of
clusters with scab lesions. For fresh market production, more than one infected leaf
cluster per tree represents potentially damaging levels of pear scab. For processing
pears, one to ten infected clusters represents a moderate risk, and more than ten infected
clusters represents a high risk.
At midseason and preharvest, no monitoring is required for
processing pears. For fresh market production, continue monitoring for lesions on leaves
of vegetative terminal shoots and on fruit. Walk around the perimeter of each sample tree
and examine at least two terminals at each of the four compass directions and 25 fruit per
tree. Record the total number of terminals and fruit with scab lesions. More than one
infected fruit per tree is a potentially damaging level for the fresh market.
For both fresh and processing pears, determine the percent
of leaves infected and number of lesions per infected leaf on six terminal shoots from
each sample tree after harvest and before natural defoliation begins. Greater than 0.5
percent leaves infected with an average of one lesion per leaf represents significant risk
of early scab infection next season.
V. Management: Fungicides that control
apple scab will control pear scab, but fewer applications are needed since pear scab
seldon is as severe as apple scab. Generally, spray applications should begin when
green tissue emerges from buds and should continue until the supply of ascospores is
depleted. The Mills Table used to evaluate temperature and wetting periods for apple
scab infection is applicable to pear scab.
Chemical control -
Chemical control - home
orchardists (pdf file - Acrobat Reader required)
Text prepared by J.W. Travis, J.L. Rytter,
and K.S. Yoder
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