Fruit growing can be a magical experience — after all those years of hard work, training, pruning and caring for your young fruit tree, it finally bears those perfect fruits you’ve been dreaming of for many seasons. Sadly, not all fruit fantasies have a happy ending; sometimes they end with the souring of fruit, an unpleasant condition that will leave a bad taste in any gardener’s mouth.
What is Souring of Fruit?
Fruit souring in plants is a very common problem and appears frequently in citrus, figs and grapes. It’s caused by a variety of soil-borne yeasts that gain entry through the skins of ripening fruits, where they feed, resulting in the fruit’s fermentation. Wounds may be so small that they’re difficult to see with the naked eye, but soon water-soaked spots appear and spread across the infected fruit’s surface.
As the yeasts work through affected fruit, they break down the tissues, which become slimy or almost completely liquid and ooze from the skin. Gas bubbles may erupt from broken areas in the fruit’s surface and a white to cream colored layer of mycelium often appears. Affected fruits may change colors, but this color change is heavily dependent on species and variety.
How to Fix Sour Fruit
You can’t save fruits already affected with sour rot, but you can work to prevent it in others. Remove any fruit showing signs of sour rot and those fruits nearby, being careful not to squeeze or split them. This may be difficult on grapes, so you may need to remove the entire bunch. Souring of fruit tends to be more severe on plants with tightly clustered fruits.
Damage from pest insects like wasps, vinegar flies and beetles, as well as birds and hail, open the skin of fruits, allowing easy access for yeast colonies. Insects may carry spores on their bodies that they inadvertently rub into wounds as they move around on fruit. Controlling this damage is vital to preventing fruit souring in plants.
Trapping wasps and flies, or installing a screen house around your troubled plant can reduce the chances of future infection. Opening the canopy more to allow greater air penetration and thinning fruits may improve your chances as well, since yeast have a difficult time surviving in dry environments.
There is no chemical control designed with sour rot in mind, but kaolin clay applied to fruits early in their development and reapplied frequently is a known deterrent to vinegar flies.