What Does All This Rain Mean For Our Grapes?

As we head into another growing season, and with the recent confirmation that a third successive La Nina season is happening, a number of people have asked "what effect is all of this rain having?" 

In the short term, if it rains on and off until mid December, and keeps the soil moisture levels at a moderate level, then things will be good, as the vines will be healthy, have good canopies and not be stressed. 

In saying that, there are a number of factors that growers need to be aware of that may lead to increased disease pressure, such as the volume and timing of precipitation, along with the associated temperature. If there is sufficient rainfall to actively promote good canopies, although only enough to result in low disease pressure, then canopy management becomes more important than normal.

Excessive foliage will require a trim to maintain good airflow, or else the disease pressure will more than likely increase as the humid air becomes trapped in the canopy. Not only do dense canopies reduce airflow, they become a barrier to spray efficacy. Therefore dense untrimmed canopies would most likely lead to mildew and botrytis infections. Furthermore, timing of the rainfall is critical. If the rainfall occurs during important physiological stages, such as flowering, then the level of fruit set may be greatly reduced.
So, in short, small amounts of regular rainfall this side of Christmas can be beneficial to allow canopies to grow, whereas excessive rainfall and the bad timing of rainfall can lead to "big" canopies and escalate unwanted disease. pressure, while also impacting important physiological stages.