Cause Venturia pyrina, a fungus that 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 Cascade Range and coastal British Columbia. Fallen leaves produce ascospores in the spring. Spores are generally released during rainstorms over a 3-to-4 month period but primarily during bloom. Infection occurs when leaves are wet for 10 to 25 hours and symptoms are seen in 2 to 3 weeks. Conidia are produced in these new scab spots and can infect healthy foliage or fruit.
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 Pacific Northwest. There are some reports of scab on Asian pear in the Willamette Valley of Oregon, but these have not been confirmed.
Symptoms In spring, sooty spots with a soft velvet look appear on young fruit, stems, calyx lobes, or flower petals. Young infected fruit frequently drop or are 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-size 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 blister-like infections the size of a pinhead and develop a corky layer. Many twig infections are sloughed off during the summer season.
- Apply nitrogen (urea) to leaves in fall to enhance decomposition of fallen leaves and make them more palatable to earthworms.
- Shred fallen leaves with a flail mower to help speed decomposition of infected leaves.
- Pruning out infected twigs also offers some benefit.
- Applying dolomitic lime after leaf drop in fall to increase soil pH also helps reduce inoculum the next spring.
- Reduce irrigation sets so leaves do not stay wet for extended periods of time. Use sprinkler heads that do not wet the foliage of the tree or use drip irrigation.
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. Forecasting is especially useful in arid areas with few infection periods. Resistance to Group 3 and 11 fungicides have been detected in the Medford, OR area. In Pennsylvania, not using dormant copper sprays, older trees, larger orchards, orchards with ≤10 cultivars, and application of >4 DMI sprays were positively correlated with the incidence of resistant apple scab isolates.
- A delayed dormant application is effective against twig infections in orchards that had a lot of disease the previous year(s).
- Lime Sulfur Ultra (27% lime sulfur) at 2 to 3 gal/100 gal water. 48-hr reentry. O
- Rex Lime Sulfur Solution (28%) at 8 to 10 gal/100 gal water depending on growth stage. 48-hr reentry. O
- Foliar applications during the growing season. Alternate or tank-mix products from different groups that have different modes of action. Also, limit applications from any particular group to two (2) or fewer per year. Selection of products for rotation and/or mixing must consider group 7 fungicides when used through the irrigation as a nematicide.
- Aprovia at 5.5 to 7 fl oz/A plus another fungicide and an adjuvant. Do not use within 30 days of harvest. Group 7 fungicide. 12-hr reentry.
- Bonide Fruit Tree and Plant Guard RTS is registered but also contains an insecticide in addition to a group 7 + 11 fungicide (see Pristine). Do not use within 21 days of harvest. H
- Cueva at 0.5 to 1 gal/100 gal water/A. May be used on day of harvest. Group M1 fungicide.4-hr reentry. O
- Ferbam Granuflo at 6 lb/A. Do not apply within 7 days of harvest. Not recommended for late season use due to black residue on fruit. Group M3 fungicide. 24-hr reentry.
- Flint Extra at 2.5 to 2.9 fl oz/A. Use as a protectant and not a curative. Do not apply within 14 days of harvest. 'Concord' grapes may be injured if accidentally sprayed. Group 11 fungicide. 12-hr reentry.
- Fontelis at 16 to 20 fl oz/A plus another fungicide. Do not use within 28 days of harvest or with thinning agents. Group 7 fungicide. 12-hr reentry.
- Inspire Super at 12 fl oz/A. Do not apply within 14 days of harvest. Group 3 + 9 fungicide. 12-hr reentry.
- Luna Sensation at 4 to 5.8 fl oz/A. Do not use within 14 day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Luna Tranquility at 11.2 to 16 fl oz/A. Do not use within 72 days of harvest. Group 7 + 9 fungicide. 12-hr reentry.
- Mancozeb products such as Dithane, Manzate Pro-Stick, or Penncozeb 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. Group M3 fungicides. 24-hr reentry.
- Merivon at 4 to 5.5 fl oz/A. Do not use with EC or oil-based products. May be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Microthiol Disperss (80% sulfur) at 10 to 20 lb/A. Do not use a spreader-sticker or apply to 'Anjou'. Group M2 fungicide. 24-hr reentry.
- Miravis at 3.4 fl oz/A. Do not use within 30 days of harvest. Group 7 fungicide. 4-hr reentry.
- OxiPhos at 1.3 to 5 quarts/A. Might be used to supplement a normal program. Group P7 fungicide. 4-hr reentry.
- Phospho-Jet is registered as a basal bark spray at 62.4 fl oz in 62.4 fl oz water plus 3 fl oz of Pentra-Bark. Spray first 5 feet of trunk including scaffold limbs until runoff. Group P7 fungicide. 4-hr reentry.
- Pristine at 14.5 to 18.5 oz/A. Do not use with an HMO. Can be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Procure 480 SC at 8 to 16 fl oz/A plus a standard protectant fungicide. Do not apply within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
- Rex Lime Sulfur Solution (28%) at 1.5 to 3 gal/100 gal water depending on growth stage. Different formulations of lime sulfur may have different rates. Do not use on 'd'Anjou', 'Comice' or 'Seckle' or with oil. 48-hr reentry.
- Scala SC at 7 to 10 fl oz/A alone or at 5 to 10 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 postharvest. Do not apply within 72 days of harvest. Group 9 fungicide. 12-hr reentry.
- Sovran at 3.2 to 6.4 oz/A. Do not apply within 30 days of harvest. Some sweet cherries, such as 'Van', may be injured if accidentally sprayed. Group 11 fungicide. 12-hr reentry.
- Syllit FL at 3 pints/A plus another fungicide. Do not apply within 7 days of harvest. Group U12 fungicide. 48-hr reentry.
- Tebuconazole-based fungicides are registered. It is unknown if there is a PGR effect on shape when using these products during bloom. Do not use within 75 days of harvest. Group 3 fungicides.
- Tebucon 45 DF at 4 to 8 oz/A. 5-day reentry.
- Unicorn DF at 2 to 3 lb/A. Includes sulfur in the formulation. 5-day reentry.
- Topguard at 13 fl oz/A plus another fungicide. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
- 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) postharvest. This material will kill earthworms, which help decompose scab infected leaves. Group 1 fungicide. 2-day reentry.
- Trionic 4 SC at 8 to 16 fl oz/A plus a standard protectant fungicide. Do not apply within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
- Vangard 75 WG at 3 to 5 oz/A tank-mixed with another fungicide. May be applied day of harvest. Use in spring when temperatures are cooler. Group 9 fungicide. 12-hr reentry.
- Ziram 76 DF at 6 lb/A. Do not use within 14 days of harvest. Group M3 fungicide. 48-hr reentry.
Notes Some registered products offer only suppression of this disease and thus are not recommended for use. These products include DoubleNickel 55, Oso, Ph-D, and Sonata.
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 32°F) 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. Additional degree days may be needed during extended periods of warm dry weather. In arid areas, if there are 5 or fewer leaves with scab in the fall after examination of all the leaves on 10 shoots from 10 trees in a 2-acre area of the orchard with a history of scab then the first scab spray of the season can be skipped.
- Serenade ASO (Bacillus subtilis strain QST 713) at 2 to 4 quarts/A. Active ingredient is a small protein. 4-hr reentry. O
Chemical control only for flowering pear
- Cleary's 3336 EG at 12 to 16 oz/100 gal water plus another fungicide. Group 1 fungicide. 12-hr reentry.
- CuPRO 5000 at 1.5 to 5 lb/A. Group M1 fungicide. 48-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Group 11 fungicide. 12-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Pageant at 6 to 12 oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Phospho-Jet is registered as a basal bark spray at 62.4 fl oz in 62.4 fl oz water plus 3 fl oz of Pentra-Bark. Spray first 5-feet of trunk including scaffold limbs until runoff. Group P7 fungicide. 4-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
References Eikemo, H., Gadoury, D.M., Spotts, R.A., Villalta, O., Creemers, P., Seem, R.C., and Stensvand, A. 2011. Evaluation of six models to estimate ascospore maturation in Venturia pyrina. Plant Disease 95:279-284.
Spotts, R.A., Cervantes, L.A., and Niederholzer, F.J.A. 2000. Pear scab: Components of potential ascospore dose and validation of an ascospore maturity model. Plant Disease 84:681-683.