Managing late season wet weather

20 January 2016

Sustained rainfall in the period from veraison to harvest can put vines under significant threat of disease. This can leave growers and vineyard managers with some difficult decisions to make. In some cases the usual control options of spraying, slashing or trimming may not be viable or may be of limited value and some crop loss may be inevitable. This eBulletin outlines a selection of points to consider when making vineyard management decisions after late season rain.

Fungicide sprays
It’s important to be aware of the range of diseases and bunch rots that the vines might be experiencing and their relative importance. For example, downy mildew on leaves late in the season will not be the highest priority if there is a risk of bunch rot. Decisions on whether to use fungicide sprays need to take into account both the impact of withholding periods and whether the spray will be able to achieve the coverage needed to be effective.

The application of any fungicide spray restricts picking date because a withholding period has to be completed before the fruit can be harvested. This can be a disadvantage in terms of reducing flexibility in the timing of picking decisions. When bad weather is around, the ability to pick a bit early (or before the next storm) can sometimes make all the difference.

When canopies are big and leafy, spray coverage where it is needed (in and on bunches) can be almost impossible. To be effective, the spray has to get across the whole berry and into the rachis. Without this level of protection, the disease can start within the bunch and spread outwards. In some cases, the most economical action may be not to spray if achieving the required coverage is unlikely.

If a decision is made to spray, there is only a short list of spray options for Botrytis in the weeks before harvest that meet export requirements. The best choice of chemical will depend on a number of factors. The ‘desiccant’ options such as hydrogen peroxide + peroxyacetic acid may not be worthwhile if more rain is due but could be very useful a few days before harvest to dry up any infected material. The ‘protectant’ options such as iprodione should only be used when good coverage is possible and the fruit is in reasonable condition. Applying chemicals where infection is well established has been shown to lead to the establishment of chemical resistance in the population and must be avoided.

Other options For high value crops it may be economical to drop infected fruit ahead of the mechanical harvester or to conduct selective harvesting by hand. Detailed block assessment may identify parcels of fruit with manageable levels of disease that could be harvested separately. Some varieties (with thicker skins and looser clusters) will manage the conditions better than others and efforts should be focused on these varieties. Slashing and trimming to promote airflow is a good strategy if access to the block is possible. This can provide a viable alternative to spraying.

Safety of vineyard operations should always be considered. Wet and slippery conditions can be dangerous on a tractor towing a heavy spray cart, especially on sloping ground. If safe access is not possible, this can limit spray, slash or trimming opportunities.

Fruit quality
The consequences of harvesting poor quality fruit should also be considered. There are significant winemaking challenges involved in processing diseased fruit and thought should be given to the eventual market for a potentially inferior product. In some cases it may be more viable to leave severely disease-affected fruit in the field rather than incurring additional harvesting and winemaking costs only to produce wine of questionable quality.

Final points
When conditions are challenging, adaptability and flexibility is important. Management strategies should be reviewed every few days as conditions, the weather forecast and fruit ripeness changes.

Technical advice and assistance are available from the AWRI helpdesk – 08 8313 6600 or helpdesk@awri.com.au.

Additional resources on managing Botrytis can be found here:

The AWRI thanks Liz Riley from Vitibit Pty Ltd for her assistance with the preparation of this eBulletin.