Apple (Malus spp.)-Diplodia Canker

Cause Several Diplodia spp. can cause branch cankers on apple but Diplodia seriata (sexual Botryosphaeria obtusa), the cause of black rot of apple, has been found most often, especially from eastern Washington. Both D. seriata and D. mutila have been found in the Okanagan Valley of British Columbia. Cankers usually develop around cold-injured tissue, mechanical wounds, or fire blight infections. This fungus also causes leaf spots and fruit rots. Black rot of apple has been found in eastern Washington apple orchards. The fungus survives from season to season in cankers or mummified fruit. Warm and wet weather favors spore dispersal, infection, and disease development. Cultivars 'Delicious' and 'Pristine' are susceptible while 'Golden Delicious', 'Braeburn', and 'Fuji' are relatively resistant. Black rot, however, was observed on 'Fuji', 'Aurora', and 'Golden Gala', apples in Washington but less so on 'Pink Lady'.

Symptom The bark of infected limbs becomes slightly sunken with reddish-brown areas. The canker may just be a superficial roughening of the bark or develop into necrotic cracks with discolored vascular cambium. Cankers can become a few feet long.

Leaf symptoms include the development of a leaf spot with a purple margin and tan center commonly referred to as frogeye leafspot. Infected leaves fall prematurely from the tree. A blossom end fruit rot may also develop starting as a purple spot with a red ring. Lesions may also occur at insect injuries. The fruit lesions in Washington did not develop concentric rings but were light-to-dark-brown with defined margins. Pycnidia may develop in the lesions. In advanced stage the fruit was completely rotted, spongy to firm, and light brown. Fruit mummify and remain attached to the tree.

Cultural control

  • Grow resistant cultivars.
  • Remove and destroy dead wood, wood prunings, and mummified fruit.
  • Flail chopping prunings as fine as possible and leaving them in the orchard is acceptable.

Chemical control

  • Captan 80 WDG at 2.5 to 5 lb/A. May be applied up to the day of harvest. Do not use with oils, lime, or alkaline materials. Group M4 fungicide. 24-hr reentry.
  • Flint Extra at 2.5 to 2.9 fl oz/A. Use on a protectant schedule and not curatively. Do not use within 14 days of harvest. Injury may occur to Concord grapes if accidentally sprayed. Group 11 fungicide. 12-hr reentry.
  • Luna Sensation at 4 to 5.8 fl oz/A. Do not use within 14 days of harvest. Group 7 + 11 fungicide. 12-hr reentry.
  • Merivon at 4 to 5.5 fl oz/A. Do not use with EC or oil-based products. May be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
  • Pristine at 14.5 to 18.5 oz/A. Do not use more than two (2) consecutive applications or with thinning agents. The addition of a silicone-based surfactant has improved control. Can be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
  • Sovran at 3.2 to 6.4 oz/A. Do not make more than two (2) consecutive applications, or within 30 days of harvest. Injury may occur to some sweet cherries, such as 'Van', if accidentally sprayed. Group 11 fungicide. 12-hr reentry.
  • Topsin 4.5 FL at 15 to 20 fl oz/A plus another fungicide. Do not use within 1 day of harvest. Do not use at any time in the orchard if your packing house uses a benzimidazole (such as Mertect or Decco Salt 19) postharvest. This material will kill earthworms, which help decompose scab infected leaves. Group 1 fungicide. 2-day reentry.

References Kim, Y.K., Kwak, J.H., Aguilar, C.G., and Xiao, C.L. 2016. First report of black rot on apple fruit caused by Diplodia seriata in Washington state. Plant Disease 100:1499

Úrbez-Torres, J.R., Boulé, J., and O'Gorman, D.T. 2016. First Report of Diplodia seriata and D. mutila causing apple dieback in British Columbia. Plant Disease 100:1243-1244