All these factors are likely subjects to variation throughout the infection process of B. cinerea and the transcriptional regulation of protease-encoding genes by ambient pH has been previously shown, suggesting that expression of these proteases may depend on pH dynamics during infection. Hence, acidic proteases might be expressed during the first steps of colonization that is accompanied by an ambient pH decrease, while the pH increases during the later stages may facilitate the expression of serine proteases whose activities are observed only at alkaline pH. The loss of a single protease likely will not affect virulence as shown for the deletion of bacp8. The down-regulation of a whole set of proteases, however, may very well explain the reduced ability of bcvel1 mutants to colonize plant tissues. The group of over-expressed genes in bcvel1 mutants mainly comprises those predicted to be involved in nutrient acquisition. Thus, several genes encoding MFS sugar transporters, ammonium Evofosfamide transporters and nitrate and amino acid transporters showed increased expression levels suggesting that the Dbcvel1 mutants sense nutrient starvation conditions possibly due to its inability to kill plant cells and to deconstruct plant tissue in a wild-type-like manner. Furthermore, the over-expression of several MFS multidrug transporter-encoding genes in the bcvel1 deletion mutant could reflect an elevated response to plant defense responses, as active efflux by ABC and MFS transporters may provide resistance to toxic compounds such as antibiotics, plant defense compounds and fungicides. However, BcAtrB known to export the phytoalexins camalexin and resveratrol and therefore being essential for full virulence on Arabidopsis and grape vine, does not belong to the set of differentially expressed transporters in this approach. To determine whether mutations in bcvel1 also occurred in other isolates, we screened several natural isolates of different origins for their capacity to produce OA and to form sclerotia during incubation in constant darkness. We found that eight out of 70 isolates tested were also affected in these traits. Sequencing of bcvel1 in these eight isolates revealed the presence of several SNPs, one of them resulting in an early stop codon and consequently in a truncated protein shorter than that of isolate T4. These data support our suggestion that mutations of bcvel1 are quite common thereby contributing to genetic variation in field populations. Even though the other seven OA- and sclerotiadeficient isolates did not contain mutations in BcVEL1, their phenotype might be associated with mutations of BcVEL2 as similar functions have been described for VeA/ VelB homologues in several fungi. In conclusion, our results provide evidence that a single point mutation in the natural B. cinerea isolate T4 is responsible for the deregulation of light-dependent development, loss of OA formation and reduced virulence compared to strain B05.10. The resultant truncated BcVEL1 led to reduced fitness of strain T4 and at least of one additional natural isolate. The persistence of these deleterious mutations is not yet understood and remains to be elucidated. One hypothesis is that the formation of conidia even in the absence of light may, in some particular ecological niches e.g. on some hosts, confer a fitness advantage in terms of higher survival rates and spatial distribution of the fungus over the ability to form sclerotia as survival structures and full virulence on all host plants.