Effect of Smoke on Grape Production in the San Joaquin Valley

George Zhuang, UCCE Viticulture Advisor Fresno

Matthew Fidelibus, Department of Viticulture and Enology, UC Davis

Karl Lund, UCCE Viticulture Advisor Madera, Merced, and Mariposa Counties

In 2020 a global pandemic and extreme heat waves in California have caused disruption and damage to agricultural operations. Moreover, some of the largest and most destructive wildfires in California history developed at the end of the summer, many in close proximity to vineyards. Wineries and other structures have been destroyed and the wine grape crop in some areas was exposed to heavy smoke, potentially tainting the grapes. Effects of smoke on grape and wine is a relatively new research topic, pioneered by Australians whose industry has also had to contend with devastating wildfires. Increasingly frequent and severe wildfires in California have prompted local industries and universities to undertake research towards the effects that smoke may have on grape and wine quality.

Dr. Anita Oberholster, a UC Davis extension specialist in Enology, spoke on the subject of smoke taint in grape and wine at the recent San Joaquin Valley Virtual Grape Symposium in December of 2020. She went into the mechanisms of how burning wood releases volatile phenols, such as free guaiacol and 4-methylguaiacol, along with o, p, m-cresol, syringol, and 4-methylsyringol. These chemicals can be absorbed through the skin of the grape and lead to wines that have an ashy, bonfire, or smoky flavor. The ability for these chemicals to absorb through the skin of the grape does increase after veraison; however, even very young berries can still be affected by smoke taint. As these chemicals do need to be absorbed by grape, their effect is only felt during the current season. Once these volatile phenolics are inside the grape they become glycosylated, chemically bound to sugar molecules inside the grape. This bond makes the volatile phenolics harder to detect and harder to remove from the wine. Enzymes in human sylvia are able to dissolve these bonds, releasing the smoke flavored volatile phenols directly to your taste buds.

As these chemicals are hard to deal with once inside the grape. It would be helpful to understand how damaging a specific exposure event was to your grapes, and if there is a way to prevent this exposure. Dr. Oberholster also touched on these subjects. She went into how smoke density does not always equate to volatile phenolic exposure and smoke taint. The ash particulates in smoke do not always represent the amount of volatile phenolics in that same smoke especially as that smoke ages. However, the presence of fresh (<24 hours old) thick smoke is highly likely to contain both ash and volatile phenolics. In a similar manner, washing ash from grapes does not affect the volatile phenolics already absorbed into the berry. However, ash that is fresh (less than 1 week old) does release small amounts of volatile phenolics that can be absorbed by the grapes, especially if they are in direct contact. Therefore, removing this ash can be beneficial to prevent additional smoke taint.

The ability to protect grapes from smoke exposure has had variable results, Dr. Oberholster explained. The first problem when trying to protect grapes is that protectants applied to the berries can normally only cover 30% - 40% of the berries surface. This leaves most of the berry’s skin free to still absorb the volatile phenolics from the atmosphere. Timing of protectant application is also a problem. If a protectant is applied too early, it can be worn off prior to the berry’s exposure. More interestingly, Dr. Oberholster talked about an application event that led to an increase in the effects of smoke taint. In this case the protectant was applied the day before a smoke event. It is believed that when the volatile phenolics from the smoke entered the vineyard after this application the berries were still wet from the previous day’s application. This water was a double-edged sword as it both increased the surface area of the grape from which absorption can happen; as well as present a surface which is more absorbent for the volatile phenolics. When asked about the ramifications of this work, Dr. Oberholster remarked that at this time it is probably best to avoid all spray applications when there is a worry about smoke exposure. Any liquid sprayed on the surface of a grape during a smoke event will increase the surface area for absorption and can possibly lead to higher levels of exposure.

In addition to the effect that volatile phenolics can have on grape and wine chemistry, the smoke can also affect additional components of grape production. These affects were felt across the San Joaquin Valley (SJV) during 2020, and in the remainder of this article, we will review how the smoke may affect:

1. Grapevine Physiology

2. Vine Water Status and Irrigation Management

3. Berry Ripening, Yield Formation, and Raisin Drying

4. Mitigation Strategy

Grapevine Physiology

Smoke in the SJV reduced sunlight and, in turn, also reduced ambient temperature (Image 1). Solar radiation has been significantly reduced during the middle of August and the beginning of September in comparison of ten years’ average in the Valley (Figure 1 B), and maximum daily temperature was also affected by smoke, although the influence was relatively mild (Figure 1 A). Changes in solar radiation have been well correlated to the recent smoke events. There were mainly two smoke events in the southern San Joaquin Valley: a series of lightning strike fires started on August 16 in Santa Clara, Santa Cruz and Monterey counties; and the early-mid September Creek Fire, near Huntington Lake. Among the two fires, the Creek Fire seemed to have more significant influence on grape production and there have been several reports on delayed wine grape ripening and poor raisin drying. During periods of heavy smoke, it was cloudy, hazy, and cooler than it had been before the smoke arrived. As a result, growers reported having a difficult time achieving the targeted Brix and get the raisin dried on time.

George Zhuang, the Viticulture Advisor for Fresno County, monitored the effect of smoke on vine physiology, water stress and berry ripening in SJV vineyards during the smoke events of 2020 (Picture 2). Heavy smoke decreased sunlight and, in turn, photosynthesis, and transpiration of grapes. During these smoke events, the photosynthetic active radiation (PAR) dropped from 1900-2000 µmol· s-1 ·cm-2, on pre-smoke days, to 300-400 µmol · s-1 · cm-2. Therefore, depending on the thickness of smoke, the decrease of sunlight on grapevine canopy could be as much as 80%. Grapevine leaves need approximately 1500 µmol · s-1 · cm-2 to achieve a maximal photosynthetic rate. At veraison, grape berries begin to rapidly accumulate sugar, anthocyanins, and other flavor compounds, and all these berry chemical components need carbon produced from photosynthesis to be produced. The severe reduction in sunlight limited grapevine photosynthesis for approximately two weeks. This period of suboptimal photosynthesis likely affected berry ripening, raisin drying and harvest yield.

smoke image 2.jpg — Image 2. Viticulture interns of UCCE Fresno were measuring leaf gas exchange during the recent smoke events in Fresno.

Vine Water Status and Irrigation Management

The main driving force for vineyard water use is sunlight. According to last 10 years’ average in the SJV plant water use, as tracked by crop reference evapotranspiration (ETo), is highly correlated with solar radiation and max daily air temperature (Figure 2). Other environmental factors influencing grape water usage include wind speed and vapor pressure deficit. With vapor pressure deficit also being highly dependent on ambient temperature. As a result, the reduced solar radiation and lowered ambient temperature resulting from smoke could reduce the grapevine water usage significantly.

As a general rule, grape growers intend to impose mild water stress on the vines after veraison to promote the sugar accumulation, anthocyanin biosynthesis, and manage other flavor compounds. The most common irrigation scheduling tool is crop evapotranspiration (ETc). A certain fraction of ETc is used to purposely stress the vines to achieve certain grapevine stress target. During smoke events, historical ETc, reference ETo from CIMIS station, and ET from satellite images are all impacted and did not provide the most reliable ETc information.

In short, the smoke might have reduced grapevine water demand, and the use of ET based on pre-smoke or clear days could have led growers to over irrigate the vines, which may have delayed berry ripening further. The combined effects of reduced sunlight and over irrigation might help explain why fruit ripened slowly in some vineyards. As such it is advised that growers use soil moisture, pressure chamber, visual assessment to adjust the irrigation during smoke events.

Yield Formation and Raisin Drying

Grape yield is dependent on the number of clusters per vine, berry number per cluster, berry size, and soluble solids (Brix). Number of clusters per vine is determined by pruning severity and node fruitfulness, both of which were established well before the recent smoke events. Berry number per cluster is largely determined by fruit set, which for the SJV is typically somewhere in early May varying across different cultivars. Thus, fruit set was also completed well before the smoke events. This means that by the time the smoke events occurred during the 2020 growing season in the SJV; berry size and soluble solids were the yield components still to be determined. Grape berry growth follows a double sigmoid curve and berry size achieves approximate 50% of final berry size just before veraison. The smoke events after veraison might affect berry growth and final berry size. However, the influence of smoke would only be on the final 50% of berry size after veraison. And considering how late into the raisin growing season these events happened, the effect would have been even more reduced.

Besides the effect on berry size, smoke also delayed the raisin drying. Berry temperature is the most important driving force in raisin drying, although low relative humidity and rapid air movement also promote drying. Dry-on-vine (DOV) grapes might normally take 4-5 weeks to dry, whereas Thompson Seedless grapes on trays normally take 2-3 weeks to dry. However, the smoke events appeared to delay drying. As for the traditional paper tray Thompson Seedless raisin, the high drying temperature is achieved by the absorption of radiant sunlight and heat accumulation at the soil and fruit surfaces during the day. The smoke blocked the sunlight reducing the solar radiation on the berry as well as lowered the ambient temperature decreasing the heat accumulation at the soil and fruit surfaces. The two weeks’ smoke event at the beginning of September has delayed the Thompson raisin drying for at least two weeks when the fruit was picked around Labor Day. As for DOV raisins, the drying might be further prolonged due to the extra leaf shading, high relative humidity, and less air movement without soil heat promoting drying.

Mitigation Strategy

Dr. Anita Oberholster’s presentation including her potential mitigation strategy for wine smoke taint were recorded and are available here (https://ucanr.edu/sites/viticulture-fresno/San_Joaquin_Valley_Grape_Symposium_Slides/). Mitigate strategizes to reduce the effect of smoke events on berry ripening, yield formation and raisin drying included:

Berry ripening and yield formation:

1. Maintain vine water stress target based on visual assessment (Picture 3 A), soil moisture (Picture 3 B), vine water status (Picture 3 C) without over-irrigating the vines.

2. Keep close monitoring on insect damage and bunch rot, since delayed harvest might increase the risk of insect damage (raisin moth) and bunch rot for certain varieties (like Zinfandel and Colombard).

3. Open the canopy and expose the fruit-zone if necessary, to decrease disease pressure and relative humidity with increased air movement.

4. Monitor berry ripening (Brix) and communicate with winery or processor to delay or reschedule the harvest.

5. Foliar spray of K might help the increase of berry Brix after veraison.

Raisin drying:

1. Lighter tray filling improves the drying rate and is less risky for late harvests.

2. Turning can be used to speed drying and reduce mold and rot under cool weather or when an excessive amount of green, undried berries are present.

3. Flop or cigarette rolls dry faster than biscuit rolls when tray rolling must be performed in anticipation of possible rain.

4. Remove the dried leaves on the DOV raisins to expose berries for sunlight to increase the drying rate.

5. Open the canopy on the DOV raisins to lower relative humidity and increase air movement to speed the drying.

In summary, smoke events can affect SJV growers beyond smoke taint issues. In 2020, the Valley experienced COVID disruption, record summer temperatures and prolonged smoke events. All of those have caused significant challenges for growers to achieve grape production and quality goals. This article briefly summarized the impact of smoke events on grape and wine chemistry, vine production, berry ripening and raisin drying.