Introduction
The genus Botrytis displays significant diversity, characterised by numerous species exhibiting variations in their biology, ecology, morphological features, and host range. While the genus encompasses approximately 28 well-described species, new species are continually being discovered and described. While most Botrytis species exhibit a limited host range, targeting only a few species of either monocotyledonous or dicotyledonous plants, some, such as the snowdrop fungus Botrytis galanthina, seem to be host-specific1,2.
However, the most common and significant species of Botrytis is B. cinerea that causes grey mould rot and contributes to substantial losses in crops, particularly strawberries, and leads to post-harvest spoilage of numerous fruits. B. cinerea infects over 200 species of plants, including monocots, dicots, gymnosperms, pteridophytes, bryophytes, and macroalgae. It is a prevalent plant pathogen causing significant damage to a wide range of bulbous, vegetable, fruit, flower, fiber, and oilseed crops, both in the field and greenhouse environments, as well as during storage and shipment3,4.
Understanding Botrytis
The life cycle of Botrytis fungi begins with the production of asexual spores (conidia) from conidiophores. These spores germinate upon landing on a host plant's surface, influenced by factors like temperature and moisture. During winter, sclerotia are produced, providing a hardened resting body for survival in extreme conditions. Upon favourable conditions, hyphal threads within sclerotia germinate. Some species also produce chlamydospores for temporary survival. Spores germinate rapidly upon landing on a host plant, forming germ tubes and structures like appressoria for penetration. Enzymes aid in penetration, followed by growth into host cells and release of damaging chemicals. After the host's death, Botrytis may live as saprophytes, releasing conidiophores for spore dispersal. Sexual reproduction, involving the formation of apothecia and ascospores, is rare in nature for Botrytis cinerea5,6.
Favourable Environments
Temperature: Botrytis cinerea thrives within a moderate temperature range, typically between 15°C and 30°C.
Relative Humidity (RH): High humidity levels (between 85-100%) are conducive to infection by B. cinerea. Disease occurrence and severity tend to escalate with increasing RH. While no disease manifests at 65% RH, its incidence rises significantly as RH levels increase.
pH: The pH level plays a crucial role in determining the interaction between B. cinerea and its host plants. B. cinerea utilise different extracellular enzymes to infect hosts according to the ambient pH conditions. B. cinerea shows a preference for acidic pH conditions, as indicated by its modulation of the secretome at pH 4 and 6 to mimic the pH values of fruits and other tissues. This ability to modulate pH conditions enhances its pathogenicity and facilitates successful infection7.
Impact on Plants
Grey mould, caused by the fungus B. cinerea, is a prevalent disease affecting various plants. It typically enters through wounds or infects plants under stress but can also target healthy plants, especially in humid conditions. The disease manifests as fuzzy grey-brown mould on decaying buds, leaves, flowers, or fruit, particularly in above-ground plant parts like buds and flowers, resulting in shrivelling and death8. Botrytis infection can reduce the yield and marketability of strawberries, raspberries, tomatoes, lettuce, and many other crops. It has a significant economic impact on agriculture and horticulture, as it can cause substantial losses in crop production and quality. Annual losses caused by Botrytis can range from 10% to 44% (pre- and post harvest), with most loss occurring post harvest9.
Botrytis Control Strategies
Chemical control: This involves applying synthetic fungicides to prevent or reduce infection by B. cinerea. However, this method has some drawbacks, such as environmental pollution, human health risks, and development of fungicide resistance.
Resistance inducer: This involves using plant signalling molecules, such as salicylic acid (SA) or jasmonic acid (JA), to activate the plant’s own defence system against B. cinerea. This method can enhance the plant’s resistance and reduce the need for fungicides.
Biological control: This involves using beneficial microorganisms, such as bacteria or fungi, to antagonise or inhibit B. cinerea. These microorganisms can act directly by parasitism, antibiosis, or competition, or indirectly by inducing systemic plant resistance. Some examples of biological control agents are Bacillus, Pseudomonas, and Trichoderma.
Cultural management practices: such as controlling humidity and reducing plant surface wetness can significantly aid in disease suppression4,10,11.
Conclusion
Botrytis cinerea is a fungal pathogen that causes grey mould or botrytis bunch rot on many plant species, especially those grown in humid conditions. It can infect various plant organs, such as fruits, flowers, leaves, and shoots, and cause symptoms such as water-soaked spots, soft decay, and fuzzy grey-brown mould. The disease can result in significant economic losses for both field and greenhouse crops, as well as affect the quality of strawberries. To prevent and control botrytis cinerea, good hygiene, ventilation, and reduced humidity are important, as well as organic or chemical treatments if necessary.
