Grapes rotting on vine? 5 causes and how to save your crop
What you need to know when grapes turn dark and rot on the vine (photo: Styler)
Ripe grape clusters can start to spoil before you even have time to harvest. The berries may darken, crack, soften, or develop a coating, and sometimes the cluster develops a distinctive sour smell.
Styler explains why grapes rot right on the vine and what you can do to stop the problem from spreading.
Moisture, gray mold
One of the most common causes is gray mold, caused by the fungus Botrytis cinerea. The problem develops especially actively in wet weather, when berries and clusters stay damp for a long time.
According to Penn State Extension, cool temperatures and wet conditions promote gray mold. Individual berries begin to soften and rot, and the infection can eventually spread throughout the entire cluster.
What to do: Remove damaged berries and clusters so they don't become a source of further spread. Also check whether the vine is too dense and whether the clusters dry quickly enough after rain.
Berries are cracking
Cracks in the skin provide an entry point for various fungi and microorganisms. A berry can become damaged after a sudden influx of moisture, by insects or birds, or through mechanical injury.
Experts at UC Agriculture and Natural Resources IPM note that damaged berries are particularly vulnerable to various rot pathogens. Insects, birds, cracks that develop as berries grow, or damage associated with other grape diseases can cause these injuries.
So if cracked berries have already appeared on the clusters, it is best not to leave them among healthy berries for too long.
Sour rot, fruit flies
If the berries turn brown, break down, release juice, and the cluster smells vinegar-like, the cause may be so-called sour rot.
Penn State Extension explains that damaged berries, yeasts, acetic acid bacteria, and fruit flies can all contribute to this problem. Insects are attracted to juice from damaged berries, which can help the problem spread further through the clusters.
In this case, don't leave rotten berries on the vine, and inspect nearby clusters regularly.
Too much foliage
When clusters are constantly surrounded by dense foliage, air circulation around them is reduced. After rain or dew, moisture can remain on the berries longer, creating favorable conditions for rot.
Penn State Extension experts note that better air circulation and less shade can help lower the risk of certain types of rot.
However, clusters should not be exposed too much late in the season either: berries that have spent a long time in the shade can be damaged by sudden exposure to sunlight.
You waited too long to harvest
Overripe grapes gradually soften and become more vulnerable to damage. If berries have already started cracking or the first signs of rot have appeared on some clusters, waiting too long can be risky.
This is especially important in warm and humid weather. Under such conditions, rot can spread quite quickly from individual berries to the entire cluster.
If most of the grapes are already ripe, check the clusters regularly and keep healthy grapes away from those that have already started to spoil.
How to reduce the risk of rot
To prevent the problem from recurring next year, you should:
- Regularly inspect clusters and remove damaged berries
- Avoid excessive vine growth and overcrowding
- Watch for pests that can damage the berries
- Not leave overripe or rotten clusters on the vine
- Take weather conditions into account before harvesting
- Maintain good air circulation around the clusters.
If the grapes have already started to rot extensively, first determine the nature of the problem. Gray, fuzzy growth, cracks, darkening berries, or a pronounced vinegar-like smell can point to different causes, so your approach to future prevention will vary.
This material is for informational purposes only and should not be used for medical diagnosis or self-treatment. Our goal is to provide readers with accurate information about symptoms, causes, and methods of detecting diseases. RBС-Ukraine is not responsible for any diagnoses that readers may make based on materials from the resource. We do not recommend self-treatment and advise consulting a doctor in case of any health concerns.
