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Fire blight, also written fireblight, is a contagious disease affecting apples, pears, and some other members of the family Rosaceae. It is a serious concern to apple and pear producers. Under optimal conditions, it can destroy an entire orchard in a single growing season.

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Gymnosporangium libocedri, the Pacific Coast pear rust, is a plant pathogen and rust fungus. It produces orange gelatinous growths (telia) on incense cedar in the spring. Its secondary hosts include apple, crabapple, hawthorn, mountain ash, pear, quince, and serviceberry.

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Gymnosporangium sabinae is a fungal heteroecious plant pathogen with Juniperus as the primary (telial) host and only Pyrus as secondary (aecial) hosts. It is one of many types of rust fungi affecting plants.

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Monilinia fructigena is a plant pathogen in the fungus kingdom causing a fruit rot of apples, pears, plums, peaches and cherries.

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Neofabraea malicorticis is a plant pathogen that causes bull's-eye rot on apple and pear.

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Phyllachora pomigena is a plant pathogen responsible for Sooty blotch and flyspeck disease, a disease affecting apples and pears. It appears as a brown or black blotch on the fruit. Spots may coalesce to cover the entire fruit. During the summer these diseases develop during cool rainy weather, particularly in dense, unpruned trees with poor air circulation. Although unsightly, the fruit is still edible. The sooty blotch will wipe off of the fruit.

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Podosphaera leucotricha is a plant pathogen that can cause powdery mildew of apples and pears.

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Rosellinia necatrix is a fungal plant pathogen infecting several hosts including apples, apricots, avocados, cassava, strawberries, pears, hop. citruses and Narcissus, causing white root rot.

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Schizothyrium pomi is a plant pathogen of the sooty blotch and flyspeck complex, infecting apple, pear and citrus trees and carnations.

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Venturia pyrina is a species of fungus in the family Venturiaceae. A plant pathogen, it causes scab or black spot of pear. It has a widespread distribution in temperate and subtropical regions wherever pears are grown.