There have been reports of serious crop losses from vineyards in South Australia and Victoria as a result of the current heatwave. The most seriously-affected vineyards appear to be those that have had limited or no water available for irrigation, not only for the duration of the heatwave but also for the two to three months prior to the onset of the heatwave.
Vineyards that have had limited shoot growth this season due to lack of water since budburst have, as a consequence, very open canopies with few leaf layers and well-exposed bunches, particularly if a VSP trellis is employed. During heatwave conditions, even though they may have been irrigated, such vineyards were very prone to leaf loss and bunch damage during the heatwave because there was limited opportunity for protection of inner leaf layers and bunches by outer leaf layers. Also, such vineyards are likely to have high under-vine soil temperatures due to lack of soil surface shading leading to death of roots in the dripper wetting-zone.
Vineyards that had ‘reasonable’ canopy development before the heatwave, and which were irrigated frequently during the heatwave, were best placed to avoid severe defoliation and bunch loss. However, it is difficult to define ‘reasonable’ in this context because it is possible that vines with big canopies and high yields may be susceptible for other reasons.
It is important to note that when leaf stomata are closedas they will have been for most of the time during the heatwaveleaf temperature can be up to 10°C higher than ambient. Similarly, exposed berries can be up to 15°C above ambient. Therefore, exposed leaves and bunches will have experienced temperatures that greatly exceeded the threshold for tissue damage and death.
Given that even well-shaded bunches will have experienced lengthy periods when tissue temperature exceeded 45°C in some locations, it is likely that there will have been negative effects on fruit composition and consequent wine quality. The magnitude of such negative effects will be very much a consequence of variety, phenological stage, canopy architecture and location.
What to do now?
Heatwave conditions can result in significant leaf loss and bunch damage. Identify the extent and nature of crop losses now. Determine which areas of your vineyard may warrant preferential treatment because the fruit is fit for purpose and, with selective harvesting, will meet winery requirements. Consultation with your grower liaison officer is recommended.
Consider the removal of unwanted fruit that is capable of ripening. Anecdotal evidence indicates that bunches left un-picked can divert resources away from carbohydrate storage in the woody parts. This may lead to poor budburst and spring growth in the following season.
To prepare for a heatwave next season, irrigate early if rootzone moisture is lacking to ensure that spring growth is adequate and the canopy size is at least moderate. Use mulch undervine to reduce soil temperature during heatwaves. Throughout the season encourage active root activity at depth, e.g. by using longer, less frequent irrigations. This will place the vine in a better position to make the most of soil moisture during heatwaves.