“We have a miniature Pear tree. The tree bears fruit very well but the fruit always has lots of small black spots. Some one told me that I should use a dormant spray for this problem. Would you give me advice on when to spray. We live in Massachusetts. Thanks much, Jim”
Thank you for the question. It sounds like your miniature pear tree is suffering from Pear Scab (Venturia pirina). Pear scab is a pretty serious disease and can cause serious losses on susceptible pear cultivars. The disease is more of a problem in European countries than here in the America. Although it is sometimes generically called black spot, pear scab resembles its cousin, apple scab (Venturia inaequalis) in nearly all respects. Pear cultivars differ in susceptibility to scab; however, cultivars resistant in one region of the country may not be resistant in another region.
The symptoms of pear scab are similar to apple scab; lesions on leaves and petioles begin as round, brownish spots that eventually become velvety in appearance. Within these lesions conidia (areas of spores) are produced. As the season progresses, 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 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.
There are a number of cultural controls you can incorporate into your management strategy to combat Pear Scab:
–Sanitation: Remove mummified fruit from trees, and dropped fruit from the ground. These can harbor inoculums of fruit diseases, complicating later chemical control and increasing reliance on pesticides. Some insects are also fostered by allowing dropped fruit to remain, such as the apple maggot.
–Host vigor: Maintain proper levels of host vigor. Nutrient-deficient trees are more prone to some diseases and insects; conversely, overly vigorous trees are more vulnerable to other pests.
–Pruning: Improve spray coverage through good pruning practices. Trees should be “opened up” to allow spray and sunlight penetration. Prune out all dead and decaying branches because such wood may harbor insect and diseases. Remove all healthy prunings from the tree because these can be colonized by rot fungi and increase inoculum levels of some rot diseases. Keep the height of the trees low to enable good coverage.
–Thinning: It is important to thin fruit properly to provide good disease and insect control. Thin all tree fruits so that the mature fruits will not touch each other. Protectant pesticides cannot effectively cover fruits that touch each other; hence, thisprovides a place for insects and diseases to become established.
–Tree size: It is almost impossible to produce high-quality fruit in the home orchard on old, large trees because the spray pressure commonly used is inadequate to force the pesticides to the tops of such trees. Therefore, old trees should be replaced with dwarf or semi-dwarf trees that are allowed to reach a height of no more than 12-15 feet.
–Ground cover management: If a weed-free strip is maintained in the tree row, most young larvae die of desiccation as they penetrate the soil surface to reach the roots (unless the orchard is irrigated).
I hope this infromation helps you out. If you have any other questions, please feel free to ask!
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