Botrytis cinerea Pers. |
Phylum: Ascomycota (anamorphic fungi - hyphomycetes)
British distribution: Terrestrial, ubiquitous
World distribution: Cosmopolitan.
As everyone knows, strawberries are easily damaged, and the bruised areas quickly begin to develop the first signs of mould. Ever wonder what would happen if the mould is left to grow?
The mould in this case is Botrytis cinerea, much the most common spoilage mould on strawberries, though other hyphomycetes such as Penicillium, Cladosporium and Alternaria, and zygomycetes such as Rhizopus can also occur.
Under the high power of the microscope (original photograph at × 500), the fungus looks like bunches of grapes. Large numbers of rounded conidia are budded off at the branched ends of the long (to 2 mm), stiffly upright conidiophores.
|The conidia are colourless (or nearly so), as are the tips of the conidiophores. The vegetative mycelium of the fungus is also colourless, appearing white to the naked eye in its "fluffy" stage. But mature fungal colonies are a dingy grey.
The colour is in the lower parts of the conidiophores, which are distinctly brown and thick-walled when seen under the microscope, as shown here in this rather poor, tangled mount, originally photographed at × 125. (This is a normal bright-field photomicrograph, the one above uses Nomarski Differential Interference, which enhances transparent objects but loses colour).
Botrytis cinerea is not merely a spoilage mould of soft fruit, but is a very common saprotroph and pathogen on all kinds of damp plant material, rapidly causing a rot that causes collapse of the tissues. It can be seen in the wild on, for example, dead flower petals, particularly in damp weather. When conditions are right it can spread rapidly (note the one apparently fresh and healthy strawberry in the top photograph; it is developing mould within a day of contact). In humid conditions (as in glasshouses) it can be highly destructive, particularly on lettuces.
On the other hand, it can sometimes (rarely!) be regarded as a benefit. On over-ripe grapes it causes the 'Noble Rot', which aids in the production of certain sweet wines.
Botrytis commonly forms resting structures, sclerotia, in the remains of the substrate. In the wild these can sometimes be seen on dead, standing stems of tougher herbaceous plants. Normally these germinate to produce new mycelium and a continuation of the asexual, conidial stage. Only rarely is the sexual fruitbody (teleomorph) seen; this is a small cup-fungus, Sclerotinia fuckeliana (named after the distinguished mycologist, Leopold Fuckel, if anyone wants to know).
Since it reproduces almost entirely asexually, any chance mutations, if successful, will be perpetuated as individual genotypes. This means that over the course of time, B. cinerea has become represented by many variants. Some of these have become more specialised and in many cases can be regarded as distinct species. Amongst these related but more host-specific species are:
|• ||B. allii, which causes serious damage to onions and shallots in storage (as do some other Botrytis species);|
|• ||B. narcissicola, the cause of "smoulder mould" on bulbs and leaves of daffodils;|
|• ||B. tulipae, cause of "fire" on living tulip leaves (another species, B. polyblastis causes "fire" of daffodils);|
|• ||B. fabae, cause of "chocolate spot" on broad beans.|
Accounts of this fungus will be found in most mycology textbooks. A useful summary of Botrytis as a pathogen was given by Agrios (1988).
Photographed material: yet more contents of the author's refrigerator, 1998.
|• ||Agrios, G.N., (1988). Plant pathology, 3rd ed., Academic Press, San Diego.|
|• ||Ellis, M.B., (1971). Dematiaceous hyphomycetes, Commonwealth Mycological Institute, Kew.|