Pear Scab

Beginner Advanced

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Beginner

This pest affects:

Scientific Name
Venturia pirina

Identification
Leaves:

Lesions develop on both sides of leaves, usually developing first on the lower side
Young lesions are pinpoint spots, velvety brown to olive green with indistinct margins.
With time, olive green lesions turn dark brown to black.
Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf.
Leaves that are heavily infected with scab will curl, shrivel and fall from the tree.

Fruit:

Small, olive-black, velvety spots develop on fruit
As spots grow and become older, the centres lose the velvety appearance and become brown, corky and scabby
The entire fruit is susceptible to scab, but lesions often clustered around calyx end of fruit.
Heavily infected fruit becomes deformed and cracked and may drop prematurely
Fruit infections occurring late in the summer may not be visible at harvest to the naked eye, but enlarge in storage to pinhead size (pinpoint scab) and tend to cluster around the calyx end of the fruit
Pinpoint scab lesions may develop on maturing fruit late in the season or after they are placed in cold storage.

Other:

Lesions can appear on shoots (lead to canker-like areas)
Pear scab affect the terminals
Secondary infection occurs 8 to 17 days after the primary infection period

Often Confused With
Fabraea leaf spot –Leaf spots are purplish-black lesions with a blackish-brown center, which oozes a creamy, gelatinous mass of spores when the leaf is wet.

Period of Activity
From green tip until leaf drop in the fall.

Scouting Notes
Leaves and fruit should be monitored from green tip to harvest for signs of pear scab and results should be record. Temperature and hours of leaf wetness should be recorded in order to determine ascospore maturity, primary and secondary infection periods.

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. Examine the upper and lower leaf surfaces for scab lesions 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.

Thresholds
There is no threshold for initiating pear scab control. Sprays should be applied to prevent infection during period when weather facilitates the initiation or spread of pear scab. If no scab is observed following the primary infection period, the spray regime may be diminished accordingly.

Advanced

This pest affects:

Scientific Name
Venturia pirina

Identification
Leaves:

Lesions develop on both sides of leaves, usually developing first on the lower side
Young lesions are pinpoint spots, velvety brown to olive green with indistinct margins
With time, olive green lesions turn dark brown to black
Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf
Leaves that are heavily infected with scab will curl, shrivel and fall from the tree

Fruit:

Small, olive-black, velvety spots develop on fruit
As spots grow and become older, the centres lose the velvety appearance and become brown, corky and scabby
The entire fruit is susceptible to scab, but lesions often clustered around calyx end of fruit
Heavily infected fruit becomes deformed and cracked and may drop prematurely
Fruit infections occurring late in the summer may not be visible at harvest to the naked eye, but enlarge in storage to pinhead size (pinpoint scab) and tend to cluster around the calyx end of the fruit
Pinpoint scab lesions may develop on maturing fruit late in the season or after they are placed in cold storage

Other:

Lesions can appear on shoots (lead to canker-like areas)
Pear scab affect the terminals
Secondary infection occurs 8 to 17 days after the primary infection period

Often Confused With
Fabraea leaf spot –Leaf spots are purplish-black lesions with a blackish-brown center, which oozes a creamy, gelatinous mass of spores when the leaf is wet.

Biology
Pear scab affects only pears: it does not infect apples. The causal fungus overwinters in infected leaves on the ground. During the winter and early spring, small black pseudothecia develop in the infected leaves on the orchard floor. By early spring, ascospores, which serve as the primary inoculum for early season infections, are formed inside the pseudothecia.

Maturation of the ascospores in the dead leaves on the orchard floor usually occurs at the same time the pear tree is emerging from dormancy. Mature ascospores are present and ready to infect the first green tissue in spring. The percentage of mature ascospores in the orchard generally peaks when pears are at the white bud to early bloom stages of flower development.

Mature ascospores are discharged from the pseudothecia by rain or dew, primarily during the day, and carried up to emerging green tissue in the trees by wind currents. For infection to occur, plant tissue must remain wet for 10 to 25 hours. The specific length of time depends on temperature during the wetness period. In general, cooler temperatures require longer wetness periods, and warmer temperatures require shorter wetness periods for an infection to occur Olive green, velvety lesions appear 10-28 days after infection by an ascospore. The ascospores continue to mature over a six to eight week period. Lesions initiated by ascospores result in primary infections, and in turn, produce spores called conidia.

Conidia are spread by splashing rain drops and wind, and initiate further infections when the combination of temperature and leaf wetness enables them to germinate and become established. These secondary infections generally occur within a tree or between adjacent trees rather than at a long distance. The secondary cycle can be repeated many times during the growing season. With frequent rainfall, the control of pear scab becomes extremely difficult, particularly if the disease becomes established from primary infections in the spring.

Leaves are most susceptible to infection until they are fully expanded. Old leaves may again become susceptible to the fungus in late season, and previously inhibited mycelia inside the leaf tissues may resume growth, resulting in new visible lesions. This phase of epidemics in autumn has significant implications for disease management because it provides additional primary (ascospores) inoculum next spring.

Fruit are most susceptible to infection when young, but even mature fruit may become infected if wet periods last 24 hours or longer. Fruit that are infected a few weeks before harvest show symptoms after 2 to 6 months in storage. Scab-infected leaves can survive in fruit storage bins kept in cold storage with controlled atmosphere and air.

Period of Activity
From green tip until leaf drop in the fall.

Management Notes
Mow fallen leaves and apply urea to leaves just before leaf fall or before bud break in the spring.

Plant less susceptible cultivars. Seckel is very susceptible to scab; Bosc andAnjou, somewhat less so; Bartlett is relatively resistant.

Management with fungicides – Fungicides are used to control pear scab in commercial orchards. See OMAFRA Publication 360, Fruit Crop Protection Guide :

Chapter 5 - Tender Fruit or Pear Calendar only: Recommendations for pear scab at green tip, white bud, bloom, petal fall, first cover, second cover.

Some information included above excerpted from;