South Africa is prone to drought, which makes the cultivation of crops an ongoing challenge. The efficient cultivation of vineyards relies heavily on effective irrigation practices. Dr Markus Keller, a viticulture professor at Washington State University’s Irrigated Agriculture Research and Extension Centre, recently shed some light on this topic during the South African Society for Enology and Viticulture’s first-ever virtual conference that took place from 3 to 5 November 2020.

As one of the keynote speakers, he elaborated on vineyard irrigation for optimum grape yield and quality. Furthermore, he explained how producers can monitor heat stress to benefit harvests.

“The global climate is becoming hotter, coupled with the fact that many countries do not have sufficient water reserves to properly irrigate their crops. These drought conditions, along with heatwaves, can lead to heat stress. However, by carefully monitoring and managing irrigation practices, producers can save water and increase the quality and quantity of their grapes,” Dr Keller explained.

The water stress spectrum

When it comes to the water stress spectrum, most producers know that there is an optimum amount of water needed to ensure high-quality grapes, which is less than the amount needed to ensure a higher yield. This not only rings true for wine grapes but also for table grapes.

There is a consensus in the viticulture community that if water stress is applied at the right time of production, it can lead to higher grape quality.

On the one hand, overwatering a vineyard can lead to canopy density, high acidity levels, poorly coloured grapes and worst of all, large berries. On the other hand, extreme water stress or underwatering can lead to an inadequate leaf-to-fruit ratio. When this ratio is not optimal, it leads to reduced photosynthesis and a drop in sugar production. It can also cause excessive leaf shedding, vine death, and ultimately result in lower yields.

The water stress spectrum.

However, when proper water stress monitoring and management is applied, more shoot growth is evident. This allows producers to measure the water potential in either the stems or leaves, keep an eye on stomatal conductance, and measure soil moisture in order to keep their vineyards in an optimum range on the water stress spectrum.

Irrigating according to grape variety

Dr Keller found that the response to various levels of soil moisture is variety specific when it comes to wine grapes. In terms of red wine grapes, low to moderate yields with small, sun-exposed berries are desirable.

Therefore, producers must irrigate with a moderate to severe pre-veraison water deficit that equals 25 to 50% vineyard evapotranspiration (ETv). According to Dr Keller, you can induce more water stress for bigger, bolder wines and less water stress for lighter red wines.

The response to various levels of soil moisture is variety specific when it comes to wine grapes.

When producing white wine, moderate yields are desirable, with berry size and sun exposure being less important. A mild pre-veraison water deficit of 50 to 75% ETv can be applied, with less water stress needed for fruitier wines.

Higher yields and bigger berries are desirable when cultivating table grapes, with sun exposure being less important. In this case, only a mild pre-veraison water deficit of 75 and 80% ETv will be needed.

Mitigating heat stress

Heatwaves are a recurring threat in grape production and require producers to develop innovative ways to mitigate heat stress.

One of the options available to producers is shade netting. However, it is expensive and causes less light to reach the vineyard, leading to lower levels of photosynthesis. Reflective particle films can be useful but need to be sprayed on the crops, which increases labour costs. Overhead sprinklers can also be utilised, but they are not always as effective in terms of water distribution. In addition, they can cause bunch rot. Heat-tolerant grape varieties can be bought but will require field grafting or replanting.

Although the abovementioned solutions could aid in mitigating heat stress, they are not always geared towards a specific grape, nor are they cost effective.

Dr Keller says his research team have been developing a mist-type evaporative cooling system (MECS) over the past few years. This device has an infrared thermometer that monitors canopy heat and leaf moisture sensors that pick up moisture on leaf surfaces. If the canopy reaches 35°C or the leaves are dry, the device switches on.

Dr Keller and his research team’s mist-type evaporative cooling system (MECS).

The system switches off if the canopy’s temperature drops below 32°C or if the leaves are wet. Misting nozzles on a drip tube, which is attached to foliage wire, create a thin film of water on the west side of the canopy, without wetting the leaves.

Compared to overhead sprinklers, this system reduces water usage by up to 90% and keeps the canopy temperature around 32 to 35°C during heat waves. No effect on disease incidence, yield or berry weight has been recorded yet. – Claudi Nortjé, AgriOrbit