July 18, 2001

Apple producers who do not adequately protect their orchards may experience a bitter harvest of rotted fruit. One of the most devastating diseases for apples in the summer is bitter rot.

Periods of hot and humid weather are perfect conditions for bitter rot, which is caused by the fungus Glomerella cingulata. Usually it shows up as a tan or brown, sunken, decayed lesion on the fruit's surface. The decay extends in a cone shape toward the apple core and the rotted area is firm and brown.

"In humid or rainy weather, the lesions on the fruit surface may develop orange, slimy spores that ooze from fruiting structures," said John Hartman, Extension plant pathologist for the University of Kentucky College of Agriculture. "Spores are splashed by rain to other fruit."

Hartman said bitter rot develops rapidly in warm weather and slowly in cool weather. Fruits can develop bitter rot during or just after harvest. "Bitter rot decay may be commonplace on what was thought to be healthy fruit coming out of storage," he added.

In orchards, bitter rot is more likely to be found near wooded areas and fence rows, and it appears to spread from trees in these border areas. The fungus may cause leaf spots on shade tress and the spores can be blown into apple orchards during windy rainstorms in August and September. If fall hurricanes bring extended windy and rainy weather, late-maturing fruits on unprotected trees near wooded areas could be at great risk for bitter rot. The good news is that bitter rot is manageable with fungicides and cultural practices.

Cultural practices include sanitation as removal of mummified fruit, dead wood, and fire blighted twigs in winter. Producers should also avoid planting apples near wooded areas. Even with good cultural practices, summer fungicide sprays may also be needed.

"To reduce disease spread now, growers should pick and destroy diseased apples as they appear in the orchard," Hartman said.

It's better to be prepared and prevent bitter rot before it has a chance to cause problems in the orchard and ruin a profitable harvest.

"Captan fungicide can be applied during late summer and it has good activity against bitter rot," Hartman said. "However, since Captan is relatively ineffective against other summer diseases such as sooty blotch and flyspeck, tank mixes with other fungicides are needed for best control."

Hartman recommends using a Benlate/Captan or Topsin-M/Captan combination at biweekly intervals through the latter weeks of each season.

"As an alternative, growers will want to try the new strobilurin fungicides such as Flint or Sovran," he said. "These new fungicides are reduced risk fungicides and have good activity against several summer diseases."

Growers with high-risk orchards for bitter rot, namely orchards with poor air drainage near woodlots, need to reevaluate their fungicide and orchard management programs if they have a bitter rot problem.

For more information about controlling fruit diseases, apple growers can contact their county Extension office for advice and copies of the Kentucky Commercial Tree Fruit Spray Schedule.


John Hartman 859-257-5779