Appressorium turgor pressure of Colletotrichum kahawae might have a role in coffee cuticle penetration

The method of penetration of fungi through the host cuticle by means of cutinase versus mechanical pressure exerted by melanized appresoria has been the subject of debate. Colletotrichum kahawae Bridge & Waller infects green coffee berries in Africa, inducing 70-80% losses. Turgor pressure (TP) of the appresoria was estimated in Nitro to be 2.6 MPa, about twice the osmotic pressure (OP) of the green berries. Appresoria exposed in Nitro to polyethylene glycol (PEG) solution with OPs of 7.0 MPa and above immediately collapsed. However, collapsed appresoria subjected to OP as high as 46.5 MPa could recover. Green berries inoculated with conidial suspensions, if subjected to OP of 28.5 MPa, showed 7% of them with necrotic lesions. Total inhibition of infection was achieved at 46.5 MPa. The OP of PEG solutions applied to inoculated green fruit decreased to the OP of the green berries in 48 h. The resistance of appresoria to osmotic stress, combined with the rapid dilution of PEG by solutes (water) from the fruit might explain the rate of infection even at very high OP. Unmelanized appresoria induced by tricyclazole showed TPs as low as one-quarter of melanized ones and, as a consequence, the percentage of infection on leaves and green berries was much lower. Cutinase was present in conidial mucilage and in extracellular fluids of germinated conidia in Nitro and in planta. Cutinase was induced by growing the fungus in Czapek-Dox medium if cutin was used as the sole carbon source. Diisopropyl fluorophosphate, a cutinase inhibitor, totally inhibited cutinase activity of culture filtrates and extracellular fluids but did not prevent infection. It is suggested that the TP of C. kahawae appresoria might play a major role in coffee cuticle penetration, according to our results.