Things worth knowing about "Rain rot"
Rain rot is a moisture-induced plant disease affecting fruits and vegetables. Fungi and bacteria decompose plant tissue during prolonged wet conditions.
What is Rain Rot?
Rain rot (German: Regenfäule) refers to a group of rot diseases in plants — particularly affecting fruits, vegetables, and berries — that are promoted by prolonged rainfall, high humidity, and poor air circulation. The term does not describe a single pathogen, but rather a weather-induced damage pattern in which various fungi and bacteria decompose plant tissue.
Causes
Rain rot develops through the interaction of several factors:
- Fungal pathogens: Common culprits include moulds such as Botrytis cinerea (grey mould), Phytophthora species, and Monilinia species.
- Bacterial pathogens: Bacteria such as Erwinia species can also cause rotting.
- Climatic conditions: Persistent rain, high relative humidity, and low sunlight create ideal conditions for rot-causing organisms.
- Mechanical damage: Cracks in fruit skin caused by rain or hail allow pathogens to enter the tissue.
- Dense planting: Poor air circulation between plants promotes the spread of disease.
Affected Crops
Rain rot can affect many cultivated plants, including:
- Strawberries and raspberries
- Tomatoes and peppers
- Apples, pears, and cherries
- Grapes
- Lettuce and cabbage
- Potatoes (tuber rot)
Symptoms
Typical signs of rain rot include:
- Brown, mushy, or discoloured patches on fruits or leaves
- Soft, disintegrating plant tissue
- Grey or whitish mould growth (especially in the case of Botrytis)
- Unpleasant rotting odour
- Rapid spread to neighbouring plant parts or plants
Diagnosis
Diagnosis is usually made by visual inspection of the affected plant parts. For accurate pathogen identification, microbiological laboratory tests or phytopathological analyses can be performed. In agricultural practice, diagnosis is often based on experience and the typical damage pattern observed.
Treatment and Prevention
Cultural Measures
- Maintain adequate plant spacing to improve air circulation
- Water plants in the morning to allow them to dry out during the day
- Remove affected plant parts immediately and dispose of them properly
- Use mulch to reduce soil splashing and spore dispersal
Biological and Chemical Control
- Biological agents: Products based on Bacillus subtilis or Trichoderma species can be used preventively.
- Copper-based products: Copper preparations are approved in organic farming and are effective against various fungal pathogens.
- Synthetic fungicides: In conventional farming, approved fungicides are available that target specific pathogens.
Variety and Site Selection
Choosing resistant or robust plant varieties and a suitable, well-drained site can significantly reduce the occurrence of rain rot.
Relevance to Food Safety
Food affected by rain rot should not be consumed. Rot-causing fungi can produce mycotoxins (fungal toxins) that may still be present in the food even after the visibly affected area has been removed. For fruits and berries in particular, it is recommended to discard heavily affected produce entirely.
References
- Agrios, G. N. - Plant Pathology, 5th Edition. Academic Press, 2005.
- European and Mediterranean Plant Protection Organization (EPPO) - Botrytis cinerea and related pathogens. www.gd.eppo.int (accessed 2024).
- Food and Agriculture Organization of the United Nations (FAO) - Manual on the Prevention of Post-Harvest Fruit and Vegetable Losses. FAO, Rome.