As a follow-up on Dr. Sial’s post from last week, I have also received several calls from Extension Agents about the prevalence of soft fruit and fruit splits. Furthermore, in addition to reports of berry splits, we’ve received reports that yeast rot has been found affecting some of this fruit. Since yeast rot is an uncommon issue on blueberry, I have received several questions about it in recent days.
Yeast rot is caused by the fungus Aureobasidium pullulans. This rot is a sporadic post-harvest rot of blueberries that is rarely reported. It can cause fruit to rapidly collapse and take on a wet, slimy appearance (images can be found in the Michigan State Fruit Rot ID Guide here). Fruit heavily affected by yeast rot may have a distinct fermented odor. There is very little information about the environmental conditions that can lead to yeast rot, but it is likely that warm, wet, humid conditions would favor the growth of this fungus. This fungus is known for its ability to colonize fruit surfaces, especially within wounds. Overripe, split, or hail-damaged fruit can be a contributing factor to issues with yeast rot. Unfortunately, management options for this post-harvest issue are not known.
It is likely that the recently reported issues with split fruit and fly infestations are contributing to the yeast rot issues that we are seeing. The splits on the fruit (and the holes/punctures caused by flies like SWD) are wounds that can promote the growth of the fungus that causes yeast rot. It is likely also that the fermented odor of fruit with yeast rot attracts even more flies which further spreads and exacerbates the problem.
We have experienced unusual weather patterns this year including a warm winter, low night-time temperatures lingering late into the season, and excessive rain events over the past few weeks. These may have caused the issues with fruit splits that we are seeing right now. The most common causes of blueberry splits are rainfall or excessive/irregular irrigation and freeze scars. Rain splitting occurs when water is imbibed through the skin of ripe berries (usually after the surface of the berry is wet for 24-48 hrs). Scar tissue left over from freezes can also reduce the elasticity of berry skins and result in splits when berries expand. Regardless of whether scar tissue or rain splitting is to blame for the berry splits, the yeast rot that we are seeing now is more likely to be a result of the splits rather than a cause.