Grapevine Red Blotch associated Virus (GRBaV) is the causal viral agent responsible for Grapevine Red Blotch Disease. This pathogen negatively impacts grape production by reducing fruit quality and yield. It can cause several physiological changes in vines, including changes in leaf color, reduced ripening speed and decreased fruit quality from reduced sugar and anthocyanin accumulation in the grapes. This reduction in fruit quality can lead to reduced market value, affecting the profitability of an impacted vineyard. The degree to which GRBaV will impact fruit quality and yield can vary from year to year within a vineyard but increases with the percent of infected vines contributing to the harvest. The exact mechanism by which Red Blotch reduces sugar accumulation in grape berries is still unclear; however, the available evidence suggests the virus affects the expression of genes involved in sugar transport and metabolism as well as in hormone signaling pathways that regulate sugar accumulation in grape berries.

Red Blotch disease was first discovered in Napa Valley, Calif. in 2008 when it was realized to be a new disease separate from leafroll which also causes similar red leaf symptoms as well as a reduction in fruit quality and yield. The similarities between the effects of Red Blotch and leafroll viruses likely played a role at masking the presence of Red Blotch virus until efficient virus screening techniques for GRBaV were commonly employed. At the point of its discovery, Red Blotch disease was not a newly emerged viral disease but rather a viral disease new to our recognition as a viral disease. Due to the misidentification of GRBaV in symptomatic grapevines, the virus may have been widely spread across California by the time a screening procedure was developed to identify its presence in plant tissues. To this point, Red Blotch virus was detected in herbarium samples collected several decades before the formal recognition of the virus. Red Blotch virus is the only member of the genus Grablovirus within the family Geminiviridae and has been separated into at least two clades of distinct strains of the virus. While originally recognized in California, it has since been found in several grape-growing regions worldwide. Grapevine Leafroll-Associated Disease, on the other hand, is caused by one of six viruses from the family Closteroviridae.

Grapevine Leafroll-associated Virus (GLRaV) is one of the most important viral diseases affecting grape production worldwide. Of the six viruses associated with leafroll disease, GRLaV-3 in the genus Ampelovirus is the most predominant; GLRaV-2 has also been noted as impacting large numbers of California’s vineyards.

Leaf Symptoms
Often described as beautiful fall color by visitors to the vineyard around harvest time, the development of red leaves in red grape varieties is a sign the vines are not healthy. The most prominent visual symptoms of Red Blotch are reddish-pink blotches or patches that appear on the leaves of infected red grape varieties. These blotches usually appear later in the growing season, usually after véraison, first appearing on older leaves near the bottoms of shoots and later developing on leaves higher up the shoot. These blotches are irregular in shape and often have a mosaic pattern. Foliar symptoms of GRBaV and GLRaV are visually similar, and both result in red- or yellow-toned tissue of the leaf blade depending on the grape cultivar. With Red Blotch, the leaf veins also turn red which is in contrast with leafroll in which the veins remain green. Another subtle difference is that Red Blotch does not distort the shape of the leaf causing the leaf blade to remain flat while Leafroll-associated viruses can cause some leaf blades to roll down along the margins, resulting in a downward curl. When a vine has Grapevine Red Botch Virus and a Grapevine Leafroll-associated Virus, leaf symptoms usually present as typically expected for Grapevine Leafroll associated Viruses with leaves showing green veins and some degree of leaf rolling. Towards the end of the season, symptomatic leaves from either virus may turn completely red, complicating visual diagnosis. In white varieties of grape, both viruses are much less conspicuous as the leaves do not turn red but may present in a subtle, yellow-chlorotic, patchy pattern.

Virus Spread Through Infected Material
Both viruses can be transmitted through propagation of infected planting material. The use of CDFA-certified virus-tested vines is an important initial step in excluding viruses in your vineyard. As there is currently no cure for vines once infected with Red Blotch or Leafroll, the use virus-free planting materials and the removal of infected vines is the first line of defense in virus management. Scouting and monitoring vineyards for red leaf symptoms in the fall is essential for early detection. PCR testing can be used to confirm viral presence in visually identified symptomatic vines if there is any doubt about the symptoms and is especially useful for confirming virus status of white grape cultivars. Once identified, infected vines should be rogued from the vineyard to reduce pathogenic inoculum and prevent them from serving as a source of virus that could inflect healthy vines. Vines identified in the late part of the growing season should be removed before the start of the next season to minimize the spread of the virus by insect vectors.

PCR-based testing methods are the standard for determining virus status of individual vines with a high degree of accuracy but becomes cost-prohibitive at commercial scales. New approaches to virus identification are being developed to allow for screening of vines rapidly and accurately within a whole vineyard. The use of drone-based hyperspectral imaging has been demonstrated to be more accurate at identifying vines infected with Red Blotch or Leafroll compared to visual scouting by experts when coupled with machine learning methods. Another approach in development involves the use of dogs trained to detect virus in vines. Other methods in development include non-destructive wavelength transmission and reflection to identify foreign organisms within the vine without damaging the plant.

Virus Spread by Insect Vectors
The three-cornered alfalfa hopper (Spissistilus festinus, TCAH) is currently the only confirmed vector of Red Blotch virus. It is an insect that can feed on many plant species including grapevines and is widely distributed throughout the U.S. Plants such as alfalfa and other legumes serve as the preferred hosts for TCAH; however, as these plants dry up over the season, TCAH will migrate onto less preferred hosts such as grapevine. This pattern of micro-migration may be particularly problematic in vineyards where annual cover crop decline occurs near the beginning of vegetative growth for grapevines. TCAH can acquire GRBaV from feeding on infected plants and subsequently spread the virus to healthy vines. However, studies investigating the vector-capacity of TCAH have found they are not good at spreading the virus. TCAH is a circulative and non-propagative host where the virus does not replicate within the insect but is transferred into the salivary glands. TCAH requires an extended acquisition period of about 10 days to uptake the virus while feeding on infected vines followed by an extended inoculation access period of about four days of feeding on healthy vines to spread the virus. As grapevines are not the preferred host, TCAH populations usually remain low within vineyards and are mainly driven by the presence of legumes in cover crops. Timely management of cover crops by tillage before TCAH reaches adulthood may help reduce population sizes and limit virus spread by the alfalfa hopper. Additionally, removing unmanaged grapevines from the perimeter of vineyard areas is helpful as these vines can act as virus reservoirs and are particularly at risk of infection in riparian areas where insect populations may be higher. The work of identifying and confirming insect vectors of Red Blotch is ongoing, and there are many other insects identified as potential vectors currently under investigation.

Due to the feeding nature of TCAH, alternative plant hosts of GRBaV should be identified in California and managed to reduce potential inoculum sources near vineyards. To date, few endemic plant species within California have been tested to see if they can harbor GRBaV. However, researchers tested 13 species of woody, herbaceous plants from riparian areas which tested positive for GRBaV and identified two positive hosts: Himalayan Blackberry and a wild grapevine hybrid (V. californica x V. vinifera). In certain regions of California, a higher number of inoculum sources and alternative hosts for GRBaV may lead to higher pathogen pressure in vineyards with Red Blotch symptoms. With the potential for other insect vectors being able to transmit GRBaV, it is important to understand which plants in vineyard-adjacent ecosystems can increase risk of Red Blotch transmission into grapevines.
For Leafroll, several species of mealybugs (family Pseudococcidae) and scale insects (family Coccoidea) have been shown to be competent vectors. However, the most important vector of GLRaV is the vine mealybug (Planococcus ficus), an invasive species first detected in California in the mid-1990s which can have up to seven generations in a single growing season. The vine mealybug requires a period of as little as 15 minutes of feeding on an infected vine to acquire Leafroll. Because of the efficiency of virus uptake by the vine mealybug and the rapid nature of its reproductive strategy, in areas where vine mealybug is present, Leafroll has the potential for rapid spread. The most effective management strategy for vine mealybugs involves an integrated approach which includes the use of biological, cultural and chemical control methods as well as managing ants which protect mealybugs from their natural enemies. The parasitoid wasp Anagryus pseudococci and other beneficial species have been shown to successfully reduce vine mealybug infestations to manageable levels. A detailed guide to this approach is available at ipm.ucanr.edu/agriculture/grape/vine-mealybug/.

In summary, both Red Blotch and Leafroll viruses can cause significant impacts to fruit quality and yield. Both viruses will cause symptoms of red leaves in red grape cultivars; however, slight differences between these symptoms can be visually identified and useful in distinguishing between the two. Once symptomatic vines are identified, PCR testing can confirm virus status. While both viruses can be vectored through the propagation of infected material, there’s great differences in the efficiency of virus spread by insect vectors. For Red Blotch, the three-cornered alfalfa hopper, requires an unusually long acquisition time of about 10 days of feeding on infected vines before it can spread the virus compared to the alarmingly quick uptake of Leafroll by the vine mealybug in around 15 minutes. Other potential insect vectors of Red Blotch have been identified and are currently under investigation. Additionally, the three-cornered alfalfa hopper usually doesn’t reach high populations within vineyards as the grapevine is not it’s preferred host plant; while the vine mealybug, on the other hand, due to its prolific reproductive strategy has the potential to reach very high numbers if left untreated in the vineyard. Once infected with Red Blotch or leafroll, there is no cure, and removal of the vine is the only sure way to prevent future spread to neighboring vines. Monitoring GRBaV symptomatic grapevines and removing them from the vineyard quickly may be the best approach to limiting the spread of Red Blotch at the moment. With research ongoing and without a clear understanding of how the virus can rapidly spread, reducing inoculum sources is a proven, preventative strategy.

Applying fertilizers or plant nutrients to foliage has a long history, and there is an extensive number of papers published on the subject. However, it may surprise some to learn that we still do not fully understand how nutrients applied to the foliage cross the leaf cuticle and enter the cytoplasm of plant cells where they can be used in metabolism. The cuticle is the main barrier to absorption of foliar applied nutrients.

Many Factors at Play
All aerial plant parts, including leaves and berries, are covered by a complex structure (a little like our own skin) known as the cuticle. The plant cuticle limits or regulates the transport of water and other substances between the plant and the environment, in addition to protecting plant cells from UV light and discouraging or sensing pests or pathogens. The cuticle is a hydrophobic layer formed just outside the cellulose cell wall that is composed mainly of the cutin polymer (a polyester) with waxes both embedded in it and covering its surface. However, numerous other chemical constituents beyond the scope of this discussion are also present in the cuticle and alter its properties with respect to the transport of any compound across this barrier. While there is still much debate about how different compounds (including plant nutrients) cross the cuticle, two major pathways of transport are known: the non-polar pathway or waxy pathway, and the polar pathway or aqueous pathway.

Compounds that are soluble in oil or an organic solvent, such as some herbicides or even smoke-related phenols, cross the cuticle easily through the waxy pathway because they are basically soluble in it. However, most plant nutrients are polar compounds or ions that are water-soluble, and transport of these substances relies on the aqueous pathway. This aqueous pathway exists because of ‘aqueous pores’ that occur within the waxy cuticle. While these pores are known to exist based on indirect evidence, they have yet to be seen. Aqueous pores in the cuticle become more open or connected as the relative humidity in the air increases. Therefore, transport of foliar-applied nutrient ions is much greater at higher humidity, and spraying foliar nutrients early in the day when humidity is still high is more effective than at midday or in the afternoon when humidity drops. Higher humidity also translates to a longer time that the spray droplets remain wet, thus keeping ions in solution longer, which also facilitates greater transport across the cuticle. Humidity also affects what is known as the point of deliquescence (POD) for different ions, which is the level of humidity needed for a given ion to attract water vapor and remain in solution on a given surface. POD varies for different nutrients, but higher humidity levels increase the chance that POD will be exceeded.

Another property of nutrients or ions related to aqueous pores and transport through them is known as the hydrated ion radius, which is the molecular size of the given ion and those water molecules tightly adhered to it when dissolved in water (Table 1). Many macronutrient ions have a larger hydrated radius when compared to micronutrient ions. A smaller hydrated radius allows for greater transport through aqueous pores estimated from various studies to have a diameter of about 0.3 to 2.0 nanometers. This is one reason that foliar application of micronutrients is more effective than foliar application of macronutrients. However, one must also consider that micronutrients are needed in much, much lower quantities to be in a healthy range. Indeed, it would be difficult, if not impossible, to supply enough nutrients for most macronutrients like N through the foliage alone.

Table 1 shows that the hydrated radius for boric acid is actually on par with the macronutrients. Yet, we know that boric acid and other sources of boron (e.g., Solubor) are used very effectively to alleviate B deficiency in grapevines. This is because uncharged molecules cross the cuticle far easier than charged ions. While it is still not entirely clear, some foliar nutrition experts think that relatively small molecules that are uncharged, such as boric acid and urea, cross the cuticle via the waxy pathway. Indeed, water molecules appear to be transported to some extent via the waxy pathway, allowing some plants to take up significant water directly through their leaves.

Cuticles on the upper surface of leaves (known as the adaxial surface) are generally thicker than the corresponding cuticle on the leaf bottom surface (abaxial surface). When nutrients were applied separately to upper versus lower leaf surfaces, more transport occurred across the bottom side of the leaf, up to four times more. Another practical consideration to increase nutrient absorption is to ensure that sprays are well deposited on the underside of leaves by having good airflow during spraying. This could mean increasing fan speed.
Another consideration important for nutrient transfer is the contact angle of the spray droplets that land on plant surfaces. This property is mostly dictated by cuticle properties, particularly the waxes on the outer surface, and by the physio-chemical properties of the spray solution. Different plant species, different tissues (leaves vs fruits) and even different ages of the tissues along with the prevailing environmental conditions affect cuticle properties that alter contact angles. Spray adjuvants or surfactants can also reduce the contact angle (i.e., help spread out the droplets) and increase transport across the cuticle. Numerous studies using isolated cuticles have shown the importance of spray adjuvants to increase foliar absorption of nutrients. In addition, adjuvants can delay the drying time of droplets and keep the nutrient in solution longer, allowing for greater uptake. It should be noted that there are also cases where the naked nutrients alone (without spray adjuvants) were absorbed just as effectively as when applied with an adjuvant.

What about chelates? This area gets a lot of marketing attention, but due to the proprietary nature of many chelates sold, it is often difficult to untangle advertising from evidence that specific chelates actually improve foliar uptake. While there are cases where chelates have proven more effective than nutrient ions alone, there are also cases where they have not.
Finally, another factor still unresolved regarding foliar uptake of nutrients is the role of stomates and other structures on the external surface of leaves, such as trichomes (leaf hairs) and specialized cells above veins.

Recent findings show that stomates can play a clear role in facilitating uptake of nutrients. For example, in a series of experiments with bean leaves where stomates were manipulated to be open or closed, uptake of nitrogen from urea, ammonium or nitrate was two times higher when the stomates were open. It should be noted that uptake via stomates does not just flow through the open stomatal pore; it is still a diffusion process along the edges of the stomatal guard cells that is more apparent when the pore is open. Additionally, recent studies using some new-age microscopic imaging methods showed that the base of trichomes was a key area for the uptake of foliar zinc in sunflower leaves, and other specific cell types directly above major veins was the primary uptake site of foliar phosphorus in barley leaves.

It is worth noting that much of what we know regarding cuticular transport of plant nutrients was derived from studies using isolated plant cuticles (enzymatically isolated from leaves.) Such a model system has its drawbacks, and artifacts can occur. For example, some studies comparing isolated cuticles to intact leaves did not yield consistent results for transport. This is why real-world studies with natural plant canopies under production conditions are needed to provide practical data on efficacy of foliar fertilizers. Numerous other factors not discussed here can also influence the success of foliar fertilization, which makes generalized recommendations difficult.

Foliar Nutrient Trials in Oregon Vineyards
My lab has conducted a number of foliar nutrient trials, and I will briefly share some of our findings. We have conducted three different trials using foliar urea-N for the specific purpose of boosting fruit or must YAN levels. Two experiments were conducted in Pinot noir and one in Chardonnay. These trials have shown that foliar urea is an effective tool to boost YAN (Table 2), and it has the advantage over soil-applied N in that canopy growth is not increased (a clear benefit for improving YAN in high-vigor sites.)

We also conducted a P trial years ago at two Pinot noir vineyards in the Willamette Valley. P was applied in three foliar sprays at a moderate dose, and leaf blade and petiole P status was improved at only one of the two sites. The increase in leaf blade P concentration at one site at veraison was only about 12%, and I concluded there was little benefit to spraying vines with foliar P (data not shown).

Currently, as part of the High-Resolution Vineyard Nutrition Management (highresvineyardnutrition.com/project), we are testing a foliar potassium treatment along with two rates of soil-applied K to correct a low K Pinot noir vineyard, and we are assessing two rates of foliar magnesium to correct a low Mg Pinot noir vineyard.

In the K trial, we are comparing a foliar K product (K-Metalosate, Albion Labs, Layton, Utah) at the highest label rate using three sprays per year (total = 3.3 pounds K/acre) with fairly high doses of soil K (K2SO4) at either 188 or 369 pounds K/acre applied in spring. Results from year 1 of this trial showed no increase in leaf or petiole K status at bloom or veraison, nor any impacts of K on growth or yield as well as must sugars, pH or TA. However, K levels in dormant-season canes (pruning wood samples) were slightly improved by all three K treatments compared to the no-K control. In year 2, leaf and petiole K status at bloom was increased in the soil K treatments, more so by the high rate, but not in the foliar K vines. Veraison leaf and petiole nutrients and pruning wood nutrients have yet to be analyzed for year 2, but similar to last year, no effects on growth or yield as well as must sugars, pH or TA were apparent due to K treatments (data not shown).

In the Mg trial, we are testing foliar sprays using MgSO4 (Epsom salt) because an earlier trial my lab conducted in the Willamette Valley with soil-applied MgSO4 at a high rate was unsuccessful. We are comparing foliar sprays of MgSO4 at 10 or 20 pounds/acre using three sprays (supplying a total of 3 or 6 pounds Mg/acre). The high rate of Mg increased leaf blade Mg levels at veraison in year 1 but did not alter petiole Mg levels as we found before. Both the low and high rates of foliar Mg reduced the number of leaves with Mg deficiency symptoms by more than 75% around harvest time in both years (Figure 1). However, Mg applications did not affect growth or yield as well as must sugars, pH or TA thus far.

In conclusion, foliar feeding of plant nutrients has its place in vineyard management, and certain foliar nutrients are effective in improving nutritional health of grapevines. This is especially true for micronutrients, notably B and zinc. Of the macronutrients, both Mg and urea-N have been effective in alleviating Mg deficiency or in boosting must YAN levels. Foliar applications of P and K have proven less effective so far. Caution is warranted when considering the myriad of foliar products that are being marketed to you as grape growers, and this is because we still have much to learn about how and when foliar-applied nutrients will be effective. A key question you should ask yourself when considering foliar nutrition is whether you have a deficiency or other problem that needs to be fixed. This is where money spent on tissue sampling for nutrients and tracking vine nutrient status over years will pay for itself in the long run. Prophylactic use of foliar fertilizers, except perhaps for B, is generally not a good idea.