Botrytis cinerea is one of number of different fungi that can cause bunch rot in grapevines.

Botrytis rot is a weather-driven disease that can cause significant loss of grape yield and quality, even after application of a full program of fungicides.

Photo: K Evans, TIA

Botrytis spores are almost always present in vineyards. Infection can be initiated from spores carried over from the previous season, in sources such as cane debris, bunch remnants, tendrils, leaf petioles and leaf blades. Important sources of infection in growing vines are infected damaged leaves, decaying floral parts (mainly caps) and aborted and rotting berries. The fungus can rest in a quiet (latent) state and then resume growth when the developing berry begins to soften. After latent botrytis resumes its invasion of a grape berry, it can then spread from berry to berry, which is especially rapid in compact bunches of thin-skinned varieties. Botrytis also spreads readily from bunch to bunch in crowded fruit zones.

Botrytis infects grapevine tissue via wounds and natural openings, including microfissures in the berry skin and wounds made by insects, powdery mildew, berry splitting, loose pedicels or other physical damage. Spore germination is stimulated by sugars and amino acids exuded from ripening berries. The fungus secretes enzymes to kill plant tissue in advance of its colonisation and then absorbs nutrients from dead tissue. Any decaying grape tissue, especially damaged leaves, dead floral parts and ripe berries, is a prime target for botrytis colonisation and subsequent spore production. Botrytis thrives in weather conditions which are consistently wet or humid.

Critical control points for managing botrytis include reducing spore load, reducing flower and fruit infection, limiting re-growth of latent infections and limiting disease spread. Each control point has a number of management measures, some of which may only apply to certain regions and/or grape varieties. Selection of a particular management practice will depend on objectives for grape yield and quality and whether the control measure is cost-effective.

Top image, photo courtesy of K Evans, TIA. Bottom image, photo courtesy of R Emmet, DEPI.