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Sooty blotch & flyspeck

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Sooty blotch & flyspeck of pear, a disease complex caused by a number of fungi

Overview

  • Sooty blotch and flyspeck (SBFS) is for management purposes one disease that blemishes the surface of apple and pear fruit.
  • Infectious spores of the many fungi that cause SBFS come primarily from wild plants along orchard borders.
  • SBFS activity is most prevalent during the period one to two weeks after petal fall, through to harvest.
  • Cultural controls such as pruning, mowing and removing host plants on orchard borders decrease SBFS risk.
  • Fungicides are the primary control for SBFS. The initial fungicide application each season can be timed using accumulated leaf wetness hours from petal fall. Later fungicide applications should be timed according the amount of rain or the time that’s elapsed since the previous application.

Symptoms & disease cycle

Symptoms and signs

Sooty blotch shows as dark, irregularly shaped areas, like charcoal smudges on fruit. Flyspeck develops distinct black, pinhead-sized spots generally clustered in groups of 10 to 50. These symptoms often appear together on fruit, but one or the other may occur alone. Details of the symptoms, such as speck size or blotch margins may vary significantly, as many different fungi can cause SBFS, and disease pressure from weather varies. Symptoms are limited to the waxy apple or pear cuticle, and can generally be removed by rubbing with water or a bleach solution.

Note that on pear sooty mold appearing as a result of pear psylla infestation can be confused with SBFS (sooty blotch in particular), however, it's usually pretty easy to see the sooty mold on the twigs and foliage in addition to the fruit with sooty mold. (See sooty mold pict in Gallery.) SBFS only affects the fruit. Sooty mold is also generally easily washed off whereas SBFS is not. Sooty mold is generally NOT controlled with fungicides (except for maybe Topsin-M), pear psylla needs to be controlled early in the season with timely insecticide application(s).

Disease cycle

The different life cycles for many fungi that contribute to the SBFS complex are not well understood. Generally the fungi first produce spores that infect apple fruit in mid- to late summer. However, the fungi may infest very young fruit and remain invisible for several weeks, first appearing in late summer or early fall. SBFS apparently has secondary spore production and infection cycles related to rain, with higher rates of disease occurring in years with heavy or frequent rain. These fungi appear to overwinter on plants adjacent to apple & pear orchards.

Chemical control

Fungicides applied approximately every two to three weeks, starting with second cover, will generally control SBFS. The most effective fungicides against include the QoI’s (strobilurins: Flint, Sovran, Pristine) and thiophanate-methyl (Topsin, T-Methyl). Captan, Inspire Super and other pre-mixes that contain a QoI (Luna Sensation, Merivon) are not quite as effective but still provide good control.

Accumulated leaf wetness hours from fruit set (170 to 220 h) can be used to more accurately time the first SBFS fungicide application. After that, timing should be based on the amount of rain and the time from the previous fungicide application.

Fungicides should be re-applied when they are depleted, either by rain or breakdown over time. Use the following depletion rules, applying whichever comes first.

  • Pristine (14.5 oz/A) – 2.5 inches rain or 21 days. (Should generally be reserved for the last application before harvest)
  • Flint (2.5 oz/A), Sovran (6.4 oz/A), or thiophanate methyl (0.8 lb/A or 20 fl. oz/A) PLUS Captan (2.5 lb/A Captan 80 or equivalent rate for other formulations) – 2.0 inches of rain or 21 days
  • Captan (4 lb/A Captan 80 or equivalent rate for other formulations) – 1.5 inches of rain or 14 days.

Other Fungicides. Sulfur, liquid lime sulfur and phosphorous acid compounds (for example Prophyte, Phostrol) also suppress SBFS, though less is known about their depletion rates, and with liquid lime sulfur there is risk of russeting on fruit and foliar stress.

Post-harvest chlorine bleach. SBFS blemishes may be removed or significantly reduced using postharvest fruit dip treatments in low-concentration chlorine bleach solutions (500 to 800 ppm chlorine) followed by brushing on a commercial grading line.

Note: always check label to make sure the fungicide is registered for use on pear.

Resistance (Eastern U.S.)

None reported

Non-chemical control

Cultural Controls

Anything that slows drying in apple and pear tree canopies encourages SBFS development. So larger trees that are poorly pruned develop more disease. Similarly, trees in areas where air circulation is poor develop more disease. The source of many of the SBFS fungi is wild plant hosts in woods or hedgerows adjacent to orchards. Cutting back these border plants, particularly well-known hosts such as wild blackberries, reduces disease pressure. Keep grass in the orchard mowed to reduce humidity in tree canopies.

Varietal Susceptibility

Apple and pear cultivars vary in the amount of SBFS at harvest, but this is primarily related to harvest date rather than resistance pathogen colonization. Later harvested cultivars have the highest SBFS incidence. Lower SBFS incidence on the earlier maturing cultivars apparently results from disease avoidance, as these apples are exposed to fewer hours of wetting and high relative humidity, environmental factors favorable for growth of SBFS fungi.