What is Sour Rot

I came across this paper"Influence of sour rotten grapes on the chemical composition and quality of grape must and wine" which was of interest to me because we have had sour rot problems in our vineyard, most noticeably in 2018 when we chose to leave our grapes hanging so that we could increase the sugar content but in the meantime due to the rains that came, we lost a significant portion of our crop to sour rot.
So, what is sour rot?
Sour rot occurs due to the action of a complex group of organisms involved in changing fruit composition as a result of the production of high levels of a wide range of undesirable metabolites including acetic acid, glycerol, ethyl acetate, ethanol, acetaldehyde, galacturonic acid, and gluconic acid.
Sour rot is troublesome with tightly packed, thin-skinned cultivars, and dense bunches close to harvesting, even when produced with low canopy densities.
Sour rot may cause heavy crop losses and reduce vintage quality, mainly when heavy rainfalls during fruit maturation and often determines harvest date in warm and humid regions. Sour rot, is becoming more frequent, probably due either to better gray rot, Botrytis cinerea prevention or to climate changes that favor the growth of other microorganisms.
Especially nearing harvest time, sour rot shows distinct and clearly recognizable symptoms:

• grape pulp browning
• disaggregation of the internal tissues and detachment of the rotten berry from the pedicel
• juice leaking from higher to lower berries in the cluster
• release of a pungent odor of acetic acid (vinegar) and ethyl acetate from the rotten berries
• the presence of Drosophila fruit flies, all around and on rotten bunches
• in the final stages, the berries are completely empty and the skins are dried out as a result of the complete fermentative breakdown of the pulp

The sudden appearance of sour rot is dependent on a combination of factors:

• (a) biotic and abiotic agents that damage the skin, produce microscopic epidermal fissures, or stimulate berry splitting (insects, birds, diseases such as mildews and rots, pests such as grape moths, excessive swelling of the berries and the pressure of surrounding grapes, and mechanical and physiological injuries)
• (b) fruit flies (Drosophila spp.), quickly attracted by the injured grapes, carrying yeasts and acetic acid bacteria that enhance the process and speed up its spread throughout the entire cluster
• (c) climate conditions, mainly high temperature, that promote the growth of microorganisms responsible for the sour rot
• (d) opportunistic microorganisms (mainly yeasts, acetic acid bacteria, and other fungi) that colonize the injured skins and inner tissues of grape berry

References:
1. Andre ́ Barata, Alda Pais, Manuel Malfeito-Ferreira, Virgılio Loureiro, "Influence of sour rotten grapes on the chemical composition and quality of grape must and wine", Eur Food Res Technol, (2011), 233:183–194.
2. Illustration from Purdue University, Bruce Bordelon, Grapes: The Sour Rot Situation, August 25. 2016.