Pear Scab Venturia pirina
J.W. Travis, J.L. Rytter, and K.S. Yoder
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.
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.
Scab lesions on fruit occur on the calyx end and eventually on the sides of the fruit. 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 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.
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 decisions. During the pre-bloom 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.
The Venturia pirina Infection Model designed by Spotts, R. A. and Cervantes, L. A. 1991
Environmental Input variables: Temperature, wetness duration.
Model description: Spotts and Cervantes present data from a controlled environment experiment with pear seedlings as well as in-field limb bagging experiments on the effects of temperature and wetness duration on conidial infections of pear seedlings, leaves and fruit. They have not evaluated Ascospore infection conditions but suggest that they should be quite similar to conidial infection conditions and therefore their model can be used to predict primary infection by Ascospores.
Action threshold: Model developers observed that the minimum wetness duration required for foliage infection by conidia all fell between the values required for "light" and "moderate" infection of apple by V. inequalis. Ascospores according to the Mills table. Therefore when using the Mills table for pear scab Ascospore or conidia infection, the authors recommend the use of hours of wetness for "light" infection to be more conservative.
(c) Dr. Heinrich Denzer, Pessl Instruments GmbH, Weiz, 2009