Botrytis cinerea is a fungal disease that occurs in various grape producing regions around the world, including SA. Infection often occurs in the vineyard and will go
unnoticed for a large part of the season, and under these circumstances only becomes apparent during the late season or during cold storage, when grey mould appears. This makes the disease particularly difficult to manage.
VINEYARDS: Botrytis is mainly dispersed as conidia in air currents and less commonly in raindrops. According to research, the presence of conidia is high in air currents from pre-bloom to the late pea-size stage of grape development. It then decreases drastically. Fewconidia are present in the air currents after bunch closure. This means that Botrytis infection occurs between bloom and the late pea-size stage of the season.
IN SHOOTS: Research has shown that the presence of the fungus on healthy shoots is highest early in the season. Colonisation generally occurs on the nodes and leaves. In instances where colonisation has taken place on the leaves, the fungus mainly infects the leaf blade, which shows no symptoms and therefore carries the fungus latently.
IN BUNCHES: Colonisation of the bunches occurs during bloom and the pea-size stage of grape development. Infection usually occurs at the berry base and the pedicel, but infection can also occur at the rachis and laterals. The cheek of the berry is seldom colonised, and the stylar end is virtually free of the fungus.
Grey mould in practice
Grey mould presents itself (becomes symptomatic) late in the season, normally after véraison. A stress trigger (e.g. damage to the berry) is required for the fungus to become symptomatic in the vineyard. Factors that encourage the occurrence of the stress trigger are bunch density, turgor, berry crack and insect damage. Climate plays a significant role in the pattern of development of grey mould – the fungus produces its conidia on infected tissue in cool, wet conditions. Subsequent cool, wet periods are required to encourage the conidia to germinate, grow and colonise on the bunch parts. Furthermore, the extent of infection is related to the presence of inoculum and the occurrence of subsequent infection cycles. Due to the spreading nature of the pathogen, extensive berry and – consequently – severe bunch rot can develop from a single berry that becomes symptomatic at the berry base/pedicel area.
The control of grey mould
It is appropriate at this stage to remind the reader that disease control is a practice that involves choosing suitable control measures and applying them at specific stages of the disease cycle on an integrated basis, in order to reduce the disease to an economically acceptable level and to maintain it there. Understanding the behaviour of the pathogen is of critical importance in applying disease control.
Building blocks of a control
• A critical review of the behaviour of the fungus and its life cycle on grapes shows that attempts to control the disease (in vineyard) must be focussed on the early stage of season and protection of the inner bunch.
• Good production practices (fertilisation, viticultural management, canopy management, soil moisture management, sanitation practices, fungicide and biocontrol) all play a role in disease management. Biocontrol and fungicide use are particularly important.
• Fungicide control is effective and must be applied at three critical stages. To ensure proper fungicide coverage, spray apparatus must work properly and be properly calibrated.
o The first application must take place before bloom, in order to prevent leaf infection, since canopy management practices early in the season can, due to leaf damage, supply secondary inocula that can drastically increase inoculum levels in air currents during bloom to pea- size stage. These inocula then land on the young vine bunches.
o The second application must be done between bloom and pea-size stage in order to reduce the number of fungal spores that land on young bunches, and to prevent floral debris carrying the fungus and sporulate on it.
o The third application must be done during bunch closure in order to reach and kill the inocula on the inner bunch parts. This recommendation is of particular importance for cultivars with tight bunches.