The inoculum of the fungus is highly abundant and ubiquitous and usually comes from infected plant tissues . B. cinerea mainly enters the host via wounds or natural openings . Infections of non-senescing or unripe plant organs usually lead to limited damage and quiescent infections . Different types of quiescence have been described: delay of conidia germination or growth arrest after germination , endophytic symptomless growth in the apoplast , colonization of abscising flower organs followed by growth into ovaries or receptacles where growth arrests . Independent of the type of infection, the pathogen generally enters a short asymptomatic, biotrophic phase at the beginning of the disease cycle . An aggressive necrotrophic phase commonly succeeds the quiescent or asymptomatic phase once plant organs start to senesce or ripen, during which B. cinerea causes rapid decay of the infected tissues . B. cinerea’s infection mechanisms have been studied in model organisms and further characterized thanks to the availability of high-quality reference genome sequences . The fungus is known to actively promote plant susceptibility by employing a variety of virulence factors . In early stages, B. cinerea deploys sRNAs and effector proteins to suppress premature host cell death and immune responses, which enables the fungus to establish inside the host and accumulate biomass prior to the necrotrophic phase . It was demonstrated that B. cinerea Dicer-like proteins DCL1 and DCL2 produce sRNAs that are secreted from fungal hyphae and translocated to the plant cell where they interfere with the host RNAi mechanisms to silence host immune response genes in Arabidopsis and tomato leaves .

Some secreted virulence factors can lead to host cell death,blueberry pot like effector proteins, toxins and enzymes involved in reactive oxygen species production . B. cinerea can also secrete oxalic acid that lowers the pH of the host tissues and stimulates the production and activity of fungal enzymes like pectinases, laccases and proteases . Furthermore, oxalic acid accumulation leads to Ca2+ chelation, which in turn weakens the pectin structures of plant cell walls and inhibits the deposition of callose . Other virulence factors are cell wall degrading enzymes that enable B. cinerea to cause plant cell lysis and loosen walls to facilitate tissue penetration . The fungus is known to produce plant hormones or hormone analogues that may disturb the host’s cellular metabolism and immune responses. The relevance of these mechanisms for the capacity of B. cinerea to infect strawberry remains unknown.Grey mould in strawberries can result from B. cinerea infections of open flowers or by penetration of fruit receptacle tissues . In primary infections, B. cinerea infects flower organs during or right after flowering, allowing hyphae to grow into the receptacle . The sources of primary inoculum range from overwintering sclerotia to conidia or mycelium from infected neighbouring plants . Infected senescent petals, stamens and calyxes can facilitate primary infections in fruit . Histological studies have shown that even though styles are frequently infected, fungal growth appears to be strongly inhibited and never reaches the receptacle. In contrast, fungal growth in colonized stamens can reach the receptacle in some cultivars . Following infection of the unripe receptacle by B. cinerea, fungal growth is usually arrested and a symptomless quiescent phase occurs. The mechanisms that lead to quiescent infections are not yet fully understood. Proanthocyanins appear to induce B. cinerea quiescence in unripe fruit by restricting the activity of fungal enzymes like polygalacturonases that are necessary for aggressive infection of hosts .

Even though PA content in fruit remains constant during ripening, increasing polymerization of PAs leads to lower inhibitory activity in ripe fruit . Similarly, anthocyanins might delay B. cinerea infections or cause quiescence . For instance, strawberries illuminated with white fluorescent light showed increased anthocyanin content and delayed development of grey mould . Reduced fruit decay has also been observed in raspberries with high pigmentation and in transgenic tomatoes that accumulate anthocyanins . Other small phenolics, especially catechins, may have a role in quiescence. High levels of catechins inhibit fungal growth, and a decrease in catechins is correlated with a reduction of other anti-fungal compounds such as lipoxygenases . Interestingly, young and ripe fruit have low catechin concentration, suggesting that initial infections of young receptacles are possible because they do not yet accumulate enough catechins to stop colonization . B. cinerea quiescence is complex and involves additional factors besides the accumulation of phenolic compounds. It has been proposed that quiescence in unripe fruit is initiated by: lack of nutrients such as sugars from the host, presence of preformed anti-fungal compounds, unsuitable environment for fungal virulence factors . In unripe strawberries, factors from all three categories are present, including lack of available sugars , preformed anti-fungal compounds , and high activity of PG-inhibiting proteins . Induction of the necrotrophic phase in ripe strawberries could be triggered by changes in biochemical composition of the host tissues associated with the ripening process, such as increased sugar content, volatile production and alteration of plant deffences . These modifications promote not only fungal growth but also host susceptibility, e.g. via the release of oxalic acid and efflux of toxins .

During secondary infections, the fungus initiates the necrotrophic phase without quiescence . The sources of conidia for secondary infections can also be diverse, from senescent leaves to infected fruit . Conidia from B. cinerea-infected flower parts are major sources of secondary inoculum . It has been estimated that more than 64% of the strawberry infections result from organic fragments that are in contact with the fruit, such as petals and stamens . Contrary to other fruit , senescent flower parts often adhere to strawberries long enough to retain water films for at least 8 h, which is the time needed for B. cinerea conidia germination . Secondary infections can also result from nesting, which corresponds to direct penetration of mycelia growing on neighbouring plant organs such as infected leaves and fruit . Generally, secondary infections proceed rapidly and B. cinerea can complete its germination and infection as fast as 16 h post-inoculation with a rapid increase in fungal biomass at 48 hpi . Early responses of strawberries to infection include higher expression of the defence genes FaPGIP and FaChi 2-1 , whereas lower expression of the reference gene DNA Binding Protein – FaDBP indicates extensive cell death induced by B. cinerea at late stages of infection .Fruit ripening influences the susceptibility of strawberry fruit to B. cinerea . Strawberries are mostly resistant to infection in their unripe stage, where they restrict fungal growth by causing quiescence. However, in the ripe stage, strawberries are highly susceptible and decay rapidly. Fruit susceptibility to fungal disease increases as ripening progresses; hence, B. cinerea appears to promote susceptibility in unripe fruit by activating specific ripening-related processes . In tomato fruit, master transcriptional regulators of ripening have been shown to have different roles in disease susceptibility. For example, the activity of the tomato transcription factor NON-RIPENING favours B. cinerea infection . Strawberries are non-climacteric fruit with a ripening programme different from that of climacteric tomatoes. Thus, a deeper understanding of strawberry ripening regulation and how B. cinerea may modulate particular ripening events are pivotal to characterize the dynamics of the strawberry-B. cinerea pathosystem. Recent transcriptomic studies of developing strawberries point out that ripening events start between the ‘large green’ and ‘white’ stages, and involve changes in cell wall composition, sugar metabolism, hormone biosynthesis and responses, pigmentation and antioxidant levels . Moreover, a general decrease of oxidative phosphorylation processes has been observed during strawberry ripening . Normal strawberry ripening involves a variety of biochemical and physiological processes,nursery pots some of which are discussed below in the context of B. cinerea interactions.Ripening is associated with the disassembly of the fruit cell walls, which leads to tissue softening.

Cell wall degradation benefits B. cinerea as it reduces mechanical barriers to infection and spread, increases the possibilities of bruising and provides the fungus with access to simple sugars as a carbon source . In strawberry, cell wall solubilization occurs early in fruit development when the walls start to swell . Cell wall solubilization correlates with an increase in fruit sugar content, resulting from polysaccharide breakdown. A decrease of acid-soluble pectins and the alcohol-insoluble fraction of cell walls occur during ripening, whereas the water-soluble content increases . The degree of pectin solubilization and depolymerization is highly related to strawberry fruit firmness . Silencing of an endogenous pectin lyase gene in strawberry resulted in fruit with higher external and internal firmness, mostly due to low pectin solubilization, stiffer cell walls, and increased cell to cell adhesion . Besides PL, other enzymes that may have affected strawberry firmness include PGs, β-galactosidases, endoglucanases, α-arabinofuranosidases and β-xylosidases . In addition to the fruit endogenous cell wall disassembly, B. cinerea secretes an extensive array of CWDEs that target most polysaccharides in the fruit cell walls, particularly pectins . These CWDEs include fungal PGs, such as Bcpg2, a gene that is mainly active in the early penetration stage . The expression of B. cinerea PGs is dependent on the host species, the plant tissue, temperature and the stage of infection .Another barrier for B. cinerea infection is the fruit cuticle. During fruit expansion and ripening the cuticle gets thinner, which makes strawberries more susceptible to initial penetration by germinating conidia. B. cinerea can penetrate the plant cuticle by secretion of cutinases . Additionally, cuticle properties can result in higher incidence of cracks and other damages through which B. cinerea can enter the fruit without the need of cutinases . Studies on strawberry cuticles are scarce and only exist for leaf tissues . In tomato fruit, thicker and stiffer cuticles lead to higher resistance to initial B. cinerea infections. Moreover, it is known that the chemical composition of the cuticle changes during tomato ripening, and this is likely to be the case in strawberry .During ripening, the content of sugar in strawberries increases and therefore can serve as nutrients for B. cinerea. In unripe strawberries, the main sugars are glucose and fructose with low concentrations of sucrose. Sucrose levels increase rapidly during de-greening and red colouring . In tomato, it has been shown that the Cnr mutant, which does not accumulate high levels of sugars is still highly susceptible to B. cinerea infection . This observation suggests that even though sugars may serve as a susceptibility factor, high sugar concentrations are not essential for B. cinerea infection. However, sugar content could still influence susceptibility to B. cinerea as specific sugars may serve as ripening initiation signals. For instance, sucrose regulates abscisic acid levels in strawberries, which are necessary for normal ripening and could influence fruit susceptibility as described below . Like other ripening-related events, B. cinerea can alter neutral sugar and sugar acid levels in the infected host tissues, mainly by degradation and depolymerization of cell walls. This was reported for infections in tobacco and Arabidopsis leaves, where the fungus degrades pectins to release the monosaccharide galacturonic acid .ABA biosynthesis during fruit ripening is triggered by a decrease in pH, turgor changes, sugar accumulation, and the switch of sugars from mainly glucose and fructose to sucrose . Effects of ABA on strawberry susceptibility to fungal disease have not been extensively studied, but down regulation of the ABA biosynthetic gene β-glucosidase FaBG3 has been reported to result in fruit with limited ripening and higher B. cinerea resistance . In tomato, ABA accumulation is related to higher pathogen susceptibility, probably via activation of senescence . During strawberry ripening, the increase of ABA is correlated with a decrease of auxin, which induces early fruit growth and expansion but is known to inhibit ripening processes . The role of auxin in fruit susceptibility seems to depend on the plant species, as indole acetic acid treatment in Arabidopsis leads to susceptibility, whereas IAA-treated tomato leaves and eggplant fruit show lower infection severity . Ethylene has a secondary organ-specific role in strawberry ripening, particularly in achenes and green and white receptacles . Ethylene increases the susceptibility of tomato to B. cinerea by inducing ripening; however, its functions during strawberry infections are yet to be fully characterized.