What are these spots on my apples?

While most of us were melting in the hot spell a few weeks ago, some disease pathogens were thriving. Spots on maturing apples have since been observed across the province, particularly on Golden Delicious, Gala, Honeycrisp, Gingergold and Spy. Although some samples are still awaiting confirmation from the Pest Diagnostic Lab, bitter rot has been identified as the causal pathogen in orchards from Norfolk and Northumberland Counties.

What is it?

Bitter rot, caused by the pathogenic fungi Colletrotrichum gloeosporiodes and C. acutatum, overwinters in mummified fruit, crevices in the bark, and cankers. Fruit maturity, warm temperatures (>26 ºC), high humidity (80-100%), and history of disease are factors that determine when bitter rot manifests, but typically symptoms appear late summer to harvest.

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 1). 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 2).

Figure 1. Bitter rot infections on fruit appear as sunken grey or brown spots, and may be surrounded by a red halo on lighter skinned varieties (right).

Figure 1. Bitter rot infections on fruit appear as sunken grey or brown spots, and may be surrounded by a red halo on lighter skinned varieties (right). During wet or humid conditions, masses of salmon-coloured spores are produced on the surface of bitter rot lesions (left).

Figure 2. Bitter rot has a diagnostic V-shape rot that can be observed progressing towards the core when infected fruit are cut open.

Figure 2. Bitter rot has a diagnostic V-shape rot that can be observed progressing towards the core when infected fruit are cut open.

Leaf symptoms of this fungus, known as Glomerella leaf blotch, appear as irregular brown blotches with concentric rings, giving the blotch a "wood grain" appearance (Figure 3). Often the remaining tissue of severely infected leaves turns yellow and fall prematurely.

Figure 3. Glomerella leaf blotch lesions form irregular brown blotches with concentric rings that give a "wood grain" appearance.

Figure 3. Glomerella leaf blotch lesions form irregular brown blotches with concentric rings that give a "wood grain" appearance.

Management

Unlike other rots, the bitter rot fungus does not require fruit wounding to establish an infection and can directly penetrate the fruit skin. Because of this, it's critical if the pathogen is in your orchard to protect the maturing fruit. Researchers from Penn State and Cornell recommend summer fungicide sprays on a 10-14 d interval through to harvest, more frequent under favorable conditions. Captan and Pristine (Group 7 & 11) seem to provide good protective control, but be sure to practice resistance management with Pristine and be mindful of the pre-harvest interval for any product used. Removing cankers, dead wood, and fruit mummies, as well as mulching leaves in the fall will also significantly reduce the overwintering inoculum.

Wanted: Bitter rot and Glomerella leaf blotch survey

Do you suspect bitter rot or Glomerella leaf blotch? OMAF and MRA specialists are conducting a provincial survey this year to learn about the biology of this new disease, cultivar susceptibility and control options. Please contact OMAF and MRA at 519-426-4322 or kristy.grigg-mcguffin@ontario.ca, so we can do a site visit, collect samples and submit them to the lab for disease diagnosis.


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