After A Wet Winter: What You Should Do and Shouldn’t Do
George Zhuang, UCCE, and Matthew Fidelibus, UC Davis
Atmospheric rivers were in the headlines recently. There have been reports of flooding on farm or ag land after the winter storms, and we have had an excessive amount of rain this winter. But how much precipitation have we had since the start of the water year? Many weather stations, such as CIMIS and UC IPM, can provide the precipitation amount, and you can choose the closest station to provide the most accurate number to your farm. For myself, I used the CIMIS station at Fresno State (#80) to assess the winter rain and there have been 16.91 inches as of March 10th since October 1st of 2022. With a historical average of 10 inches on the east side of San Joaquin Valley (SJV), we will most likely double the historical amount since as the writing another atmospheric river arrived on March 10th. So, we can officially call 2023 a wet year.
Recently, I received a call from a grower asking me about what they should do after so much rain. It was an interesting question as we have started to treat drought as the new “normal”. After nearly 200% historical precipitation amount, it might be a good time to review what you should and should not do after several atmospheric river events have added a large amount of rain and moisture to the SJV vineyards.
Chart 1. Soil moisture content (water volumetric content measured as %) at soil depth of 12”, 24” and 36” along with precipitation and irrigation amount from September 2022 to February 2023 at UC IPM Caruthers weather station.
1. Check Soil Moisture
When you have abundant rain like this winter, you expect your soil moisture profile will be recharged. This moisture will help protect your vines from temperatures just below freezing, as well as support budbreak and strong early canopy growth. This is the reason during dry winters growers are encouraged to apply post-harvest irrigation, as well as winter irrigation to maintain a certain level of soil moisture during the dormant season. Currently, NRCS and Fresno/Madera irrigation districts even incentivize the growers to take flood water to apply on-farm groundwater recharge, which will not only benefit the soil moisture profile, and prevent possible flooding but also help with groundwater recharge. So how do you know if your soil profile is full? A soil moisture probe or a shovel will be useful to do that. You can use soil moisture either measured by content (%) or tension (centibar) to see the soil moisture increase or decrease by irrigation or rain event. You can certainly use a shovel to dig out a soil hole to feel the moisture by hand.
Chart 1 shows the data from a soil moisture probe at the Caruthers UC IPM weather station. In October multiple post-harvest irrigation events can be seen, as well as the resulting change in soil moisture at the 12”, 24”, and 36” depths. The precipitation events in December and January can also be seen really driving up the soil moisture at 12” and 24” depths. However, it is only the January rain events that really filled up the deeper 36” soil moisture. In summary, the rain events in December and January were beneficial to recharge the soil moisture profile that will protect the vines from freezing damage as well as provide the water for early canopy growth. Growers should check the soil moisture on site and make the irrigation scheduling based on that.
2. Check Soil Chemistry
Major rain events are also good for leaching soil salinity and other soil issues out of the root zone. This makes it a good time to check the soil chemistry to understand how much leaching has been done from all the winter storms. To do so take a composite soil sample from 2-3 locations below the drip system and blend the samples at each depth together separately. Samples should be divided by depth with each representing 1 foot of the soil profile. The top samples should be from 2” to 12”, the next should be 12” to 24”, the following from 24” to 36”, and lastly from 36” to 48”. Store the soil samples in the cooler before sending the samples to the commercial lab. Soil pH, electrical conductivity (EC), Sodium (Na), Chloride (Cl-), and Boron (B) are the main targets of soil analysis. If leaching has been successful, you should find that the salts were pushed from the top to the deeper soil, e.g., from the 2” to 12” sample down towards the 36” to 48” sample. The idea of leaching is to push the excessive salts below the root zone, which covers approximately the top 3 feet of the soil profile. This is because most grapevine roots stay in the top 3 feet of soil. The optimal soil chemistry range for a grapevine can be found in Table 1.
Photo 1. Actively growing shoot tips and tendrils indicate no water stress.
3. Holding Irrigation to Save Water and Energy Costs
When to start irrigation is a yearly question to be asked when managing a vineyard. With the December and January rain events affecting soil moisture down past 36” (chart 1), we are starting the 2023 growing season with full field capacity. But how fast is water in the soil profile depleted? It will vary depending on your vineyard and how the remainder of the rain season develops.
According to Dr. Larry Williams, at field capacity, SJV growers probably can start the first irrigation in mid-May or bloom. Irrigating the vines before they need irrigation will waste water and increase your energy bill from pumping that water. The decision to start irrigation should rely on at least one of the following: soil moisture; plant water stress; and visual assessment of the grapevine. I often recommend growers use soil moisture and visual assessment of the grapevines to trigger the first irrigation event. This is because plant water stress measurements are usually expensive and labor-intensive for SJV grape growing. The use of soil moisture data such as those seen in Chart 1. Growers will need to set up a threshold at a certain soil depth to start the irrigation, and I prefer to use the soil moisture at 24” since it will give the soil moisture indicator from neither shallow nor deep soil. For the majority of SJV vineyards with sandy loam soil, soil moisture at 24” ranging from 10-15% might serve as the threshold. However, growers cannot solely rely on soil moisture to guide their irrigation events. This is because measuring soil moisture in only one or two locations will not give a grower the full picture of the entire vineyard’s soil moisture profile. Visual assessment can really serve as a confirmation in addition to your soil moisture data. Looking at the tendrils and shoot tips (Photo 1) will give you an indication if there is any stress, and that can give you the confidence that soil moisture data are the true value.
Photo 2. Mechanical pre-pruned vines followed by final hand pruning.
4. Manage Disease, Pest, and Weeds Proactively
Abundant soil moisture is certainly encouraging and fruitful for SJV growers after successive drought years. However, the big concern from a wet winter might be the favorable conditions for pests and disease due to the larger vine canopy and higher weed pressure promoted by excessive soil moisture. Rapid shoot growth after budbreak will likely create dense canopies and well-shaded cluster-zone which favor fungal pathogens, such as powdery mildew and botrytis. This is not only because of increasing the relative humidity under the canopy, but also by decreasing cluster-zone light exposure and fungicide spray coverage. Growers can use the combination of the UC IPM powdery mildew risk assessment index (https://ipm.ucanr.edu/weather/grape-powdery-mildew-risk-assessment-index) and field scouting to schedule the spray interval. Canopy size can also be managed through irrigation management. Additional information on powdery mildew and botrytis fungicide efficacy and timing studied by Dr. Akif Eskalen can be found here: https://ucanr.edu/sites/eskalenlab/Fruit_Crop_Fungicide_Trials/
Another big concern after a wet winter is grapevine trunk disease which are fungal pathogens, such as Botryosphaeria and Eutypa. These pathogens enter the grapevine through pruning wounds under the condition of free water such as rain and dense fog. Although rain events were cheerful, they can significantly increase the risk of spreading grapevine trunk disease. To lower the risk of spreading trunk disease it is recommended to either practice delay pruning or double pruning (Photo 2), or to use fungicide spray right after pruning (Photo 3). Delay pruning and double pruning aim to expose the final pruning wounds to the warmer weather of February, rather than the cooler weather of December. This allows the pruning wounds to heal faster and avoid the pathogens’ invasion. Fungicides can serve as a barrier on pruning wounds to protect vines from pathogens’ attack. A study done by Dr. Akif Eskalen on fungicide efficacy and timing on grapevine trunk disease can be found here: https://ucanr.edu/sites/eskalenlab/Fruit_Crop_Fungicide_Trials/. However, right now it might be too late for some growers to implement either technique. It is still worth watching the vines, particularly in June or July when the temperature rises dramatically. Grapevine trunk disease typically shows the peak of the symptoms in the early or middle of the summer when the vine water demand is the greatest. Typical grapevine trunk disease symptoms are: 1. Dead arm or cordon; 2. Canker on the cordon/trunk; 3. Stunted shoot; 4. Leaf discoloration (Photo 4).
Photo 4. Left: dead cordon/arm. Middle: canker on the trunk. Right: leaf discoloration caused by Esca
Vine mealybugs have been the number one concern for many SJV growers in the past several years. This is not only because of the direct feeding damage on clusters but also due to its role as a vector of leaf roll virus-3 which can contribute to the sudden vine collapse. It is still unclear if the vine vigor might influence the vine mealybug population in the season. But it has been studied that high vine vigor increases the vineyard leafhoppers population (Daane and Costello 1998). Given that both leafhopper and vine mealybug feed on the grapevine to reproduce, it might be reasonable to assume that high vine vigor promoted by abundant soil moisture, like this past winter, also increases the vine mealybug population. Mating disruption combined with pesticide use can offer good protection from vine mealybug. Management strategies for sudden vine collapse include flagging, testing, removing, and replanting. Label the disease symptom vines, test positive in the lab, remove the virus-positive vines, and replant with less susceptible rootstock. Research is still ongoing at UC Davis to study the susceptibility of various rootstocks to sudden vine collapse, and currently, we know the rootstock Freedom is highly susceptible to sudden vine collapse. Following those steps can minimize the spread of the virus as well as sudden vine collapse.
Cited literatures:
Daane, K. and Costello, M. 1998. Can cover crops reduce leafhopper abundance in vineyards? California Agriculture. 52(5):27-33. https://doi.org/10.3733/ca.v052n05p27