As we enter early summer in Wisconsin, apple fruit diseases become a critical concern, especially with the recent wet weather conditions. This season’s rain increases the risk of disease development, making it essential to understand when pathogens infect, the conducive conditions for their spread, and effective management strategies. A review of several important apple fruit diseases is provided below to help minimize their impact on apple production.

Black Rot is one of the most common fruit rot diseases that apple growers encounter. Symptoms appear as black and brown concentric rings on fruits, usually containing small, black fungal fruiting structures. These symptoms are typically found at the calyx end (bottom) of the apple fruit. Infections often occur 4 to 6 weeks prior to harvest, so well-timed preventative fungicides during these periods are crucial. The black rot pathogen can also infect leaves (frogeye leaf spot) and branches (canker infections).

Bitter Rot is becoming increasingly common in Wisconsin apple production. Symptoms appear as sunken, circular lesions that are light brown to dark brown in color. Fungal fruiting structures are formed in concentric circles in the lesions, producing a significant number of spores. During wet conditions, salmon to orange-colored spores exude in a circular pattern. Cross-sections of infected apples may reveal a V-shaped lesion. Fruits are susceptible to infection at all developmental stages, and lesion expansion occurs rapidly at 86°F. Epidemics are most severe under warm and wet conditions, with fruit infections common from mid-to-late summer but potentially developing before, during, or after bloom.

White rot symptoms can be similar to black rot and may appear around the same time. However, the white rot pathogen does not cause leaf spots like the black rot pathogen. Lesions on the fruit are sunken, circular brown to tan spots, sometimes surrounded by a red margin (or dark purple margin on red fruits). The rot extends towards the core of the fruit in a cylindrical shape, unlike bitter rot, which extends in a V-shape. In warm conditions (77-86°F), the fruits turn a light brown to tan color and become soft and watery. In cool conditions, the symptoms can resemble those seen with black rot – firm rotted areas that are darker in color. The fungus forms in the lesions on the fruit.

Sooty Blotch and Flyspeck (SBFS) is a common and cosmetic issue of apple fruits that can lower fruit quality. Numerous fungi have been associated with SBFS; these fungi grow superficially on the fruit, unlike the rot diseases mentioned above, which can affect both the skin and flesh of the apple. Symptoms of SBFS typically appear as dark blotches on the fruit (SB) and small specks clustered together (FS). The fungi overwinter on fruits, twigs, and leaves of other host plants, including apple and pear, and infections of apple begin in late May and early June. NEWA has a risk prediction model for the SBFS disease complex, which can be found in a previous article on SBFS.

Effective disease control in apple orchards involves a combination of good sanitation practices and the use of protectant fungicides.

• Throughout the growing season, promptly remove any diseased fruit to reduce the spread of pathogens.
• Remove shoots affected by fire blight, including both current seasons blighted shoots and older shoots, to prevent the buildup of the pathogen. This is particularly important for managing bitter rot.
• Avoid tightly clustered fruit, as they can create a conducive microclimate for disease development and limit spray coverage. Proper thinning is recommended to mitigate these risks.
• Always be mindful of pre-harvest intervals on the fungicides you select.
  • Captan: This fungicide is effective against summer fruit rots and SBFS when applied at higher rates and at intervals of no more than two weeks.
  • Strobilurins (e.g., Flint Extra, Sovran, Pristine): These fungicides are effective against SBFS and fruit rots. Tank mixing with captan provides better results.
  • SDHI + Strobilurin (e.g., Luna Sensation, Merivon): These combinations are effective against both SBFS and fruit rot diseases.
  • DMI Fungicides (e.g., Indar, Inspire Super): These are effective against SBFS but not fruit rots. If using DMI or SDHI fungicides for apple scab control in the spring, avoid using them for summer fruit diseases if apple scab lesions are visible, as this can enhance the risk of fungicide resistance.
  • Liquid Lime Sulfur or Elemental Sulfur: These are effective against SBFS, though there is a risk of phytotoxicity to the fruit.
  • Products like Oxidate, Serenade, potassium bicarbonate, and Regalia may be effective under low disease pressure but have shown inconsistent results in field trials. These treatments will require frequent re-application in high disease pressure conditions.

This article was posted in Apples, Disease and tagged apple disease, Apple Sooty Blotch, Apples, Bitter Rot, Black Rot, disease, flyspeck, Leslie Holland, sooty blotch, white rot.