Dealing with Pre-harvest and Storage Disease

With the frequent rains since petal fall, fungal pathogens, such as black rot and bitter rot infections have been observed in many apple orchards across the province this year. With harvest in full swing, take the time to consider management of these diseases and prepare for other storage issues that may arise. Since many storage issues are actually initiated in the orchard, careful thought should go into control at this level. Often pre-harvest symptoms are not obvious and infection can come as quite a surprise when fruit is later brought out of storage.

Black rot (Botryosphaeria obtuse) overwinters in cankers on twigs, branches and trunk, as well as on mummified fruit. This often makes apple cultivars which retain mummified fruit, such as Gala, Honeycrisp, Cortland and Northern Spy more susceptible to infection, as well as trees damaged by winter injury or fire blight.

Late season black rot infection often appears just before harvest as black spots on the fruit associated with mummified fruit left from chemical thinners (Figure 1). Eventually, the lesions enlarge into a brown rot with concentric dark and light rings (Figure 2), which late in the season or in storage, may develop tiny pimple-like black pycnidia (Figure 3). The flesh of fruit infected with black rot remains firm, in contrast to several other apple rots and may increase storage decays after harvest, frustrating growers and packers alike.

Figure 1. Late season black rot infection appears as black spots on fruit associated with mummified fruit left from chemical thinners.

Figure 1. Late season black rot infection appears as black spots on fruit associated with mummified fruit left from chemical thinners.

Figure 2. Black rot lesions enlarge with concentric dark and light rings.

Figure 2. Black rot lesions enlarge with concentric dark and light rings.

Figure 3. In storage, black rot infected fruit remain firm and may produce tiny pimple-like black pycnidia.

Figure 3. In storage, black rot infected fruit remain firm and may produce tiny pimple-like black pycnidia.

Bitter rot, caused by the fungus Colletrotrichum gloeosporiodes and C. acutatum, overwinters in mummified fruit, crevices in the bark, and cankers. Typically, symptoms appear late summer to harvest. This year, spots on maturing apples have been observed across the province, particularly on Golden Delicious, Gala, Honeycrisp, Gingergold and Spy. Unlike black rot, the bitter rot fungus does not require fruit wounding to establish an infection and can directly penetrate the fruit skin.
Early infections on fruit appear as tiny, grey or brown spots that enlarge to sunken, dark brown lesions, often surrounded by a red halo (Figure 4). Especially during wet or humid conditions, masses of salmon-coloured spores are produced on the surface of the expanding fruit lesions. A diagnostic V-shaped rot can also be observed progressing towards the core when infected fruit are cut open (Figure 5).

Figure 4. Bitter rot appears as sunken spots, surrounded by a red halo. Masses of salmon-coloured spores are produced on the surface of the enlarged lesion.

Figure 4. Bitter rot appears as sunken spots, surrounded by a red halo. Masses of salmon-coloured spores are produced on the surface of the enlarged lesion.

Figure 5. Bitter rot has a diagnostic V-shape rot progressing towards the core.

Figure 5. Bitter rot has a diagnostic V-shape rot progressing towards the core.

Blue and gray molds (Penicillium expansum and Botrytis cinerea, respectively) are two important diseases of stored apples. Both pathogens require a wound to cause disease, which most frequently occurs during picking and handling operations. Spores can survive long periods of unfavourable conditions, as well as from season to season on picking boxes, contaminated bins, storage walls and packing equipment. The pathogens may also invade fruit that are over mature at harvest or fruit held in storage too long.

The symptoms of blue mold begin as soft watery light brown areas around injuries or lenticels on the fruit surface. Fruit affected by gray mold is spongy, light to dark brown, and diseased tissue is not separable from the healthy tissue. Blue mold initially has tufts of snow-white spores that turn to blue-green (Figure 6) and has a musty odour, while gray mold has fluffy white or grey mycelium and dark grey spores masses (Figure 7) with a "cedar-like" smell only in advanced stages.

Figure 6. Blue mold initially has tufts of white spores that turn blue-green.

Figure 6. Blue mold initially has tufts of white spores that turn blue-green.

Figure 7. Gray mold has fluffy white or grey mycelium and dark grey spore masses.

Figure 7. Gray mold has fluffy white or grey mycelium and dark grey spore masses.

As you're getting your orchards ready for harvest, the following are some general management techniques to keep in mind to reduce postharvest fruit rots:

This season

  • Handle fruit carefully during harvest -bruised or wounded fruit are susceptible to storage diseases.
  • Harvest fruit at proper maturity -the more mature, the more susceptible a fruit is to storage diseases.
  • Minimize inoculum sources -use clean bins, remove damaged fruit before storing, limit the amount of soil and plant debris brought in and sanitize storage equipment.
  • Keep fruit cool after harvest -warm temperatures encourage pathogens to grow.

Next season

  • Practice orchard sanitation to remove inoculum sources -prune out cankers or dead branches, and remove rotten or mummified fruit.
  • Dormant sprays of copper and sulphur may help to reduce overwintering inoculum.
  • Apply calcium sprays to improve fruit quality.
  • Prune to increase air flow, promote faster drying time and ensure better spray penetration.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca