Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Pear -- Scab
See Also: Apple -- Scab
 
Cause: Venturia pirina, a fungus. The disease overwinters in infected fallen leaves and, in some areas, on pear twigs. Twig infection occurs sometimes in the Mosier and Medford, OR areas and commonly west of the Cascades and coastal British Columbia. The cultivars Forelle and Bartlett Red Sensation are very susceptible. The disease does not cause apple scab, nor can the apple scab fungus cause pear scab.

Scab on Asian pear is also caused by a different species, V. nashicola, that has not been reported in the PNW. There are some reports of scab on Asian pear in the Willamette Valley of Oregon but these have not been confirmed yet.

Symptoms: In spring, sooty spots with a soft velvet look appear on young fruit, stems, calyx lobes, or flower petals. Young infected fruit frequently drops or is misshapen. Scab spots expand with growth until halted by dry weather or sprays. Old fruit infections often crack open. Cracks are surrounded by russeted, corky tissue and then an olive-color ring of active fungus growth. If fruit is infected late in the season, about 2 weeks before harvest, pinpoint scab spots often show up in storage a month or more later.

On leaves, olive-black spots expand with leaf growth but often cause the leaf to twist abnormally. Infected twigs show small blisterlike infections the size of a pinhead and develop a corky layer. Many twig infections are sloughed off during the summer season.

Note the sunken, dark lesions on this fruit.

Scab can be clearly seen on the leaves and fruit.

Stem lesions are very troublesome as an overwintering site and need to be removed if observed.

 
Cultural control:

  1. Carefully discing to cover old leaves with soil, where practical, may help reduce spring infections.
  2. Pruning out infected twigs also offers some benefit.
  3. Applying dolomitic lime after leaf drop in fall to increase soil pH also helps reduce inoculum the next spring.
Chemical control: Spray delayed dormant compounds before bud scales drop. Apply in season sprays at preblossom (prepink), pink, calyx, and first cover. Forecasting systems are available to time sprays to control pear scab.

  1. A delayed dormant application is effective against twig infections in orchards that had a lot of disease the previous year(s).
    1. Lime sulfur (29%) at 6 to 11 gal/100 gal water. Polysul, Lily Miller Dormant Spray for Disease and Bonide Lime Sulfur Spray are registered for home use. 48-hr reentry.
  2. Foliar applications during the growing season.
    1. Flint 50 WG at 2 to 2.5 oz/A. Use as a protectant and not a curative. Rotate with other fungicides that have different modes of action. Do not apply more than twice consecutively, exceed 4 applications total per season, use more than 11 oz/A/season, or use within 14 days of harvest. ‘Concord’ grapes may be injured if accidentally sprayed. 12-hr reentry.
    2. Mancozeb products such as Mancozeb 80 WP, Penncozeb 75 DF, or Manzate 75 DF at 3 or 6 lb/A. Do not combine 6 lb/A prebloom or 3 lb/A all-season schedule. See label for details. Do not apply within 77 days of harvest. 24-hr reentry.
    3. Microthiol Disperss (80% sulfur) at 10 to 20 lb/A. Do not use a spreader sticker or apply to 'Anjou'. 24-hr reentry.
    4. Pristine at 14.5 to 18.5 oz/A. Do not use more than two (2) consecutive applications or more than four (4) times/year. Can be used day of harvest. 12-hr reentry.
    5. Procure 480 SC at 8 to 16 fl oz/A plus a standard protectant fungicide. Do not apply within 14 days of harvest or more than 64 oz/A/year. 12-hr reentry.
    6. Rubigan EC at 8 to 12 oz/A plus a surfactant. Do not apply until after petal fall. Do not use less than 6 oz/A. Do not apply within 30 days of harvest. Mix with a protectant fungicide. 12-hr reentry.
    7. Scala SC at 7 to 10 fl oz/A alone or at 5 fl oz/A when tank-mixed with another fungicide active against scab. Do not use at any time in the orchard if your packing house uses Penbotec post harvest. Do not apply more than three (3) applications alone, more than 40 fl oz/A/season, or within 72 days of harvest. 12-hr reentry.
    8. Sovran at 3.2 to 6.4 oz/A. Rotate with other fungicides that have different modes of action. Do not apply more than twice consecutively or within 30 days of harvest. Some sweet cherries, such as 'Van', may be injured if accidentally sprayed. 12-hr reentry.
    9. Syllit FL at 3 to 4.5 pt/A. Alternate with other products. Do not apply within 7 days of harvest. 48-hr reentry.
    10. Topsin M WSB at 1 lb/A plus another fungicide. Do not use more than 4 lb/A/season or within 1 day of harvest. Do not use at any time in the orchard if your packing house uses a benzimidazole (such as Mertect or Decco Salt 19) post harvest. This material will kill earthworms which help decompose scab infected leaves. 3-day reentry.
    11. Vangard 75 WG at 3 to 5 oz/A tank-mixed with another fungicide. Do not apply alone, or within 72 days of harvest, or more than 22 oz/A/season. Use in spring when temperatures are cooler. 12-hr reentry.
    12. Ziram 76 DF at 6 to 8 lb/A. Do not use within 5 days of harvest or more than 24 lb/A/season. 48-hr reentry.

    Chemical control only for flowering pear

    1. Eagle 20 EW at 6 to 12 fl oz/100 gal water. 24-hr reentry.
    2. Mancozeb-based products. 24-hr reentry.
      1. Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
      2. Pentathlon DF at 1 to 2 lb/A or per 100 gal water.
      3. Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
    3. Pageant at 6 to 12 oz/100 gal water. Do not use more than two (2) consecutive applications before switching to a different fungicide group. A group 7 and 11 fungicide. 24-hr reentry.
    4. Spectracide Immunox at 1 fl oz/gal water.
Forecasting: Pear scab infection periods can be determined by measuring temperature and leaf wetness with weather monitoring equipment. The scab infection season starts when 320 degree days (base 32oF) have accumulated and ends after 1620 degree days (base 32? F, starting when bud scales separate) have accumulated followed by at least 0.01 inch of rain or dew. If the orchard is free of scab up to this date, no additional fungicide applications are necessary for the season, regardless of subsequent infection periods.
References:
Lennox, C.L. and Spotts, R.A. 2003. Sensitivity of populations of Botrytis cinerea from pear-related sources to benzimidazole and dicarboximide fungicides. Plant Disease 87:645-649.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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