Best practice Protection from effects of rain

Protecting grapes from the effects of rain

When it rains, grapes respond in a variety of ways. Not all growers have entire crops covered with plastic, so with a wet season forecast, it’s important to prepare for and manage the effects of rain. During veraison

Grapes are particularly susceptible to moisture during veraison – especially grapes with thinner skin. Generally, the crispier or crunchier the texture, the worse the impact.

Look out for soil saturation, which can be as bad as over-irrigating.

Rain and grape production

Botrytis is more prevalent during wet seasons. Fungal spore levels can easily be 25 times greater. Wet seasons typically produce weaker, softer grapes that are more sensitive to SO2 (sulphur dioxide), which can lead to a higher risk of bleaching.

Rain and harvest

During the day, water flows from the roots to the leaves and also from the fruit to the leaves. Photosynthesis results in the constant expulsion of moisture.

However, at night, the situation reverses. Excess moisture from the plant now flows toward the grape berries and they quickly swell, often resulting in splitting. After rain, it is prudent to suspend harvest for at least three days. If fungal infections do occur, this time period allows symptoms to develop and pickers to trim infected and split berries. Even so, fruit picked soon after rain should be sold quickly, or closely monitored in storage for signs of decay.

Check the Brix levels of your fruit before harvest, as these do change after rain.

Also, following heavy rain, it is best to harvest in the afternoon (contrary to popular logic of morning harvest) to exploit the ebb and flow of water in the vine.

Optimising post-harvest quality after rain

The most important factor in maximising grapes’ storage potential is to keep the grapes dry. Forced air coolers can help dry fruit before it condenses as free water inside the liner bag – better to dare a little dry stem than to have a split berry in the box.

If padding and lidding must occur immediately after harvest, it is critical to use liners and cartons with perforations, which will hasten the cooling period and allow heat to escape.

Consider a 5mm perforated liner and tissue/wadding on the bottom of the box to absorb any extra moisture, and be sure to select the right SO2 sheet for grape variety and box size.

Depending on seasonal growing conditions, the timing of harvest, grape variety, and vineyard management, utilising an extra half SO2 sheet can help if harvesting and storing after rain. It’s vital to monitor fruit for fungal development. v Compiled with the support of Tessara Pty Ltd.