Taxonomic position: Domain: Eukaryota; Kingdom: Fungi; Phylum: Ascomycota; Class: Ascomycetes; Subclass: Leotiomycetidae; Order: Heliotales; Family: Sclerotiniaceae
Description: B. cinerea is the causal agent of grey mould-rot; it infects more than 200 different plant species and produces serious economic losses, both before and after harvest. The pathogen can affect crops at any of their developmental stages and can also affect any section of the part.
During the infection cycle, the spores of B. cinerea can be produced on any plant matter and carried to long distances by draughts of wind. B. cinerea is a cosmopolitan, highly polyphagous fungus that survives as saprophyte on plant debris and, passively, as sclerotia. The pathogen produces a new generation of spores, which can start a new infection cycle.
In grapevine crops, several plants are needed for the disease to occur, as well as good spore dispersal and favorable humidity and temperature conditions. The fungus depends to a great extent on the environmental conditions, mainly humidity. It overwinters as sclerotia on the vine shoots bark or as mycelium under the bark epidermis and among the dormant bud scales. Asexual spores are generated in the spring; they spread through water or wind. Young racemes and leaves become thus infected. For asexual spores to germinate an optimal temperature of 18 ºC is required; temperatures may sometimes range between 15 and 20 ºC.
The disease may occur with or without water; in this latter case, it is necessary that the relative humidity be more than 90%. Germinated asexual spores cannot infect vulnerable tissues if the temperature is below 8 ºC. Previous damage and injuries provide excellent chances for infection. The fungus develops on grapevine plants until the end of the summer and produces an increasing number of spores, even before the winter begins. By this time, the fungus generates resistance structures in order to endure unfavorable conditions.
Characteristics of vulnerable cultivars:
- Tightly-clustered bunches: Chenin, Pinot Gris, Chardonnay, Tempranilla
- Lax bunches: cherry, La Rioja muscatel
During the spring, the fungus attacks buds, as well as young shoots and leaves. The former turn brown, necrose and dry off, whereas leaves show irregular spots on the blade margins. This damage is not as significant as that occurring in racemes. Inflorescences may be damaged during the blooming stage; necrosis or rot may occur, thus reducing the amount of harvested produce.
Moreover, the attacked inflorescences that do not come off may serve, after blooming, as inoculum. Grape petioles and raceme peduncles may thus be infected; these may fall from the plant. During the veraison stage, the fungus directly attacks the epidermis of the fruits, or it may attack fruits through injuries. Tightly clustered racemes will, consequently, suffer greater rot. White grapes turn brown, and red ones acquire different reddish tonalities. Berries dry off on account of dry weather, but they show cracks if there is humidity, and their surface is covered with grayish, dark chestnut or bluish green efflorescence.
In tomato crops, the symptoms may appear at any section of the plant and at any of its developmental stages. However, the most significant damage occurs in adult plants. It is worth mentioning, though, that the fungus may attack seedlings and kill them after transplant, specially if they are buried deep and if the predisposing conditions are adequate. In this case, the infection comes from dry cotyledons and moves down to the neck of the plant. This is finally surrounded by a brownish rot that strangles the plant and kills it.
In greenhouses, symptoms like necrosis in flowers, stems, leaves and petioles of adult plants are very frequent. The invasion in leaves occurs where infected petals fall. Stamen residues and pollen, which serve as exogenous sources of energy, also contribute to fungal penetration. Needless to say, injuries (specially those resulting from inadequate pruning) also allow the infection in. The lesions expand progressively from the inlet point onwards; a brownish necrosis with concentric rings invades the entire leaf, petiole and stem. In the case of the stem, the lesions can be as long as ten centimeters, and they may surround it and produce plant wilting in the sections that lie above.
Fruit rot starts by the calyx or pistil, more often than not, through debris of petals that become stuck to them. Injuries by insects are also helpful. A soft, grayish green rot occurs, which progresses quickly. The fungus fruits under high humidity conditions and develops brownish-gray mycelium.
A typical, though not very frequent, symptom occurring in tomato crops is the “ghost spot” that appears on fruits. This usually happens in high humidity conditions and in cold weather followed by dry weather. The infecting mycelium does not usually produce rot; however, white, concentric rings appear. These rings, which have a diameter of about eight millimeters, affect the market quality of the fruit.
In apple crops, earlier symptoms consist of a reddish stain at the base of one or more sepals in the fruit calyx. Such reddish area gradually increases and turns light brownish. It can surround the entire calyx or comprise only one side. The affected area sinks.
The epidermis of the infected area turns dark brownish to black and eventually stretches; this affected part usually comes off the neighboring healthy area. Affected fruits usually fall off the tree before time.
In strawberry crops, the disease affects flowers and fruits, mainly during harvest and post-harvest.
The fungus is evident in flowers and fruits in the shape of light to dark brown stains. Abundant gray mycelium is produced, as well as long, branching conidiophores and bunches of ovoid, unicellular, either colorless or gray conidia; these resemble a bunch of grapes. The conidia are released in high relative humidity and cool weather. They are dispersed by wind and splashing. These conidia are the main source of inoculum; mycelium on dead leaves is the main source of production.
In asparagus, the disease starts during pre-harvest: the apexes wilt and eventually rot after the harvest. Cold, humid weather favors the development of B. cinerea; white mycelium may develop on the turions. This mycelium turns gray because it becomes covered with abundant fruiting made up of conidiophores and conidia.
Sunflower seedlings may be killed. However, attacks to the capitulum are more frequent. Flowers become covered with gray mold; soft, brownish spots appear in the receptacle which progress rapidly. Black sclerotia similar to those produced by S. sclerotium may develop.
In blueberry, the fungus usually occurs in fruits during post-harvest. Flower abortion may also occur. In Argentina, B. cinerea is considered a primary pest due to its wide distribution.
The fungus is dispersed by rain, wind and draughts of air below plastic coverings.
In grapevine crops, predisposing conditions are: temperatures ranging from 22 to 30 ºC, relative humidity over 90%, free water. Sensitivity will largely depend on the racemes, as well as the thickness, skin and chemical composition of berries and the degree of resistance of the cultivars.
In tomato crops, dim light, high relative humidity and temperatures ranging from 15 to 20 ºC foster the development of the disease. Sources of exogenous energy, such as wilted petals falling on leaves or stems, are necessary for the fungus to be able to infect somewhat healthy tissues.
In strawberry crops, the optimal temperature for sporulation ranges between 15 and 22 ºC. Weeds also provide an excellent source of reinfection. The pathogen becomes actives in low temperatures and produces considerable loss in fruits that have been stored for long periods, in temperatures ranging from 0 to 10 ºC. Germinated spores rarely penetrate actively growing tissues in a direct way; however, they do so through injuries.
The information contained in the system is subject to constant changes and revisions. The Bureau of Surveillance and Monitoring reserves its right to make all necessary amendments in its listings and in the contents of the data sheets whenever appropriate.
Importing countries may, if required, contact the National Directorate of Plant Protection for an official technical report on the phytosanitary status of the crops. The Directorate shall prepare such report in accordance with the information contained in this database.