Managing Black Rot in Ontario Apple Orchards

Black rot is an important and troublesome disease in Ontario apple orchards. This disease can go unrecognized. The disease is caused by the fungus Botryosphaeria obtusa (Schwein), which overwinters on dead limbs, twigs, cankers (Fig 2), and mummified fruit. During the season, the fungus releases ascospores during rainstorms that are rain splashed, washed or blown onto fruit and foliage. Often the first symptoms of black rot appears as small purple lesions on leaves shortly after petal fall. The lesions eventually expand to form tan coloured spots surrounded by purple margins (Fig. 3). Severely infected leaves become discoloured and may drop prematurely.

Figure 1 - Black rot canker on limb, OMAFRA, 1999.

Figure. 2 - Frog eye leaf spot on leaves, OMAFRA, 1999.

Fruit can become infected early in the season, however, infection may occur at any time throughout the season. Black rot infections commonly appear on fruit that are in close proximity to infected fruit mummies or cankers. Initially small reddish dots appear on infected fruit that develop into purple slightly raised pimple-like spots (Fig. 4). These spots expand over the season, rotting the fruit and eventually forming alternating black and brown concentric rings (Fig. 5). Late season infections often appear as black irregularly shaped dots, surrounded by a red halo. These infections are likely initiated early in the season, however, they remain quiescent throughout the summer as their fungal growth is suppressed by natural inhibitors in the green fruit (Rosenberger, 2000). Unfortunately as the fruit ripens, these natural inhibitors decrease in concentration and the symptoms of the disease become more apparent. Black rot infections that are initiated in the orchard can increase storage decays after harvest, frustrating growers and packers alike. In fact, black rot has been reported to cause 25% to 50% fruit losses, in some areas in southeastern US (Jones and Aldwinckle, 1990).

Figure. 3 - Early symptoms of black rot on fruit, OMAFRA, 2004.

Figure. 4 - Late symptoms of black rot on fruit, note concentric circles, OMAFRA, 1999.

A recent study in the U.S. indicates that apple varieties differ in their susceptibility to black rot (Biggs and Miller, 2004). Among the twenty-three varieties evaluated for black rot susceptibility, these researchers found that “Pristine” and “Sunrise” were most susceptible to black rot; whereas “GingerGold”, “Senshu”, “Honeycrisp” and “Pioneer™Mac” appeared moderately susceptible, and “Golden Delicious”, “Enterprise”, “Golden Supreme”, and “Fuji” were the least susceptible. In Ontario, we commonly find black rot infections on “Idared”, “Northern Spy”, “Golden Delicious”, “Honeycrisp” and “Gala”.

Removal and burning of infected wood and mummified fruit will help to reduce the spread of the disease throughout orchards. Early season fungicide sprays of Captan (3.75 kg/ha), can also help to manage black rot. Fungicide sprays should begin as early as petal fall, and 2–3 applications should be applied 7–10 days apart. Strobilurin fungicides, such as Sovran and Flint, also have activity against black rot. The EBDC and sterol inhibitor fungicides do not have effective activity against black rot.

Biggs, A. and S.M. Miller. 2004. Relative susceptibility of selected apple cultivars to fruit rot caused by Botryosphaeria obtusa. Hortscience 39 (2): 303-306.

Jones, A.L. and H.S. Aldwinckle. 1990. Compendium of apple and pear diseases. APS Press, St. Paul, Minn.

Rosenberger, D. 2000. Disease developments. Scaffolds Fruit Journal Vol. 9, No. 12, June 5, 2000.