With the passing of bloom, table grape growers are turning their attention from managing fruit set to maximizing berry size. One practice that increases berry size is girdling, removing a thin strip of bark from around the vine trunk. The girdle disrupts the flow of carbohydrates from the vine's canopy to its roots; the young berries grow fat on the extra sugar that would otherwise have been consumed by the roots.
Special double-bladed knives are used to make the girdles. Keeping the blades sharp makes the task go much easier. Some knives also have a single blade on one end of the handle. I prefer such knives as the single blade is a handy tool for removing loose bark that interferes with girdling. However, an experienced colleague prefers to remove the loose bark by hand.
Some trunks, especially on old vines, develop grooves and folds from which it is hard to remove the bark strip with a double-bladed knife.
Within five or ten minutes, any remaining phloem (sugar-transporting tissue of the bark) will become brown colored. If any such tissue is seen, it should be scraped away. However, the girdles should not cut any deeper into the trunk than is necessary to remove the phloem, as deep cuts into the wood can disrupt water supply to the shoots, which may injure or even kill the vine. Proper girdles, applied at the right time, can increase berry size by 10% to 30%.
Grapevines throughout California are now blooming.The flowers have a powerful and unique scent that reminds me of an herbal shampoo.Recently it was discovered that the characteristic odor of grapevine flowers is emitted from their pollen grains, unlike other flowers such as roses, whose scent arises from their petals.In addition to being unscented, grape flowers don't have much visual interest; the petals are green-colored and are fused together, forming a structure that resembles a cap.Until the caps fall, the flower clusters resemble broccolini.Cap fall exposes the yellow colored anthers and the ovary, which may develop into a berry.
Bloom is a critical period in grape production because physiological and environmental conditions at bloom affect the number of flowers that will set fruit. Grapes intended for the table must be carefully managed at this time to prevent excessive fruit set. If too many flowers set fruit, the grapes will become tightly packed as they grow. Tightly packed berries often become misshapen or crushed, resulting in clusters that are unsightly and predisposed to rot.
Grapevine powdery mildew is a pathogenic fungal disease of grapes, the management of which is a yearly challenge for California grape growers. Since the mid-80’s, multiple fungicides have been registered for grapevine powdery mildew management. The first of these was Bayleton, a demethylation inhibitor (DMI). It controlled mildew well for two seasons, after which growers noticed a sharp decrease in efficacy. Growers were experiencing powdery mildew fungicide resistance firsthand. Resistance to Bayleton prompted a survey which showed that mildew developed resistance to fungicides in many California vineyards. Since then, we have learned a lot about powdery mildew biology, fungicides, and resistance management.
What is fungicide resistance?
Mildew is said to be resistant to a fungicide after exposure to the fungicide fails to control the organism. Resistance implies that the mildew has developed the ability to detoxify the fungicide by changing either the intermediates within a metabolic pathway or by changing protein binding sites.
Currently grape growers (both conventional and organic) have a diverse chemical arsenal to control mildew, including several compounds with different modes of action. Compounds having a similar mode of action are said to be in the same class, which is to say that they “attack” or disrupt the same metabolic processes to affect the growth of the fungus. Repeated use of fungicides from the same class—one application after another—within a season and over seasons promotes the rapid development of resistant mildew populations. The development of resistant mildew forces growers to rely on the remaining fungicide classes that remain effective, which could lead to overuse of those fungicides, and the loss of another class due to resistance. The loss of any fungicide class also limits a grower’s ability to tailor control programs to their production systems, including cultivar, trellis type and end product. Thus, yield and quality can be compromised by the emergence of fungicide resistant mildew populations, especially on the most susceptible cultivars, such as Fiesta, Carignane, and Chardonnay. Thus, it is important to understand the different modes of action between classes to develop an effective mildew control program that minimizes the risk of resistance.
With the prediction of rain next Monday (April 6th), Valley growers should consider applying a fungicide to protect young succulent growth from Phomopsis infections. Spring showers create optimal conditions for Phomopsis viticola pycnidia to begin producing spores that can easily spread and infect susceptible cultivars such as Thompson Seedless, Flame Seedless, Redglobe and Grenache. Timely fungicide applications are paramount in managing this disease when spring rains are predicted.
Spring symptoms
Symptoms first appear on leaf blades as tiny dark brown-black spots with yellow margins (Fig. 1). Under optimal environmental conditions, spots may coalesce and kill portions of expanding leaves. Severely infected leaves become distorted over time, die and drop from the canopy. Tender green shoots that become infected display tiny black spots near the base of the shoot, which also begin to merge. Heavy infections result in blackened tissue that cracks and displays a scarred, scabby appearance at the basal end of the shoot (Fig. 2). Early, heavy infections will stunt shoot growth and development. Some buds may not emerge at all due to bud mortality. Thompson Seedless and Redglobe may experience poor budbreak after a season of heavy Phomopsis viticola infection. When shoots have developed 12-24” of growth, shoot breakage may occur during strong winds when heavy infections are present.
Optimal environmental conditions
Favorable conditions include temperatures between 60-75ºF and free moisture or humidity near 100%. Extended periods of rain will increase infection and may make it difficult to apply fungicides. Thus, it is important to protect young green shoots prior to forecasted rain. Spores are released in large quantities from pycnidia found on diseased wood at budbreak and splashed by rain onto the young growing shoots. Once growth has reached 15-18 inches the canopy forms an umbrella and helps prevent the splashing effect, therefore decreasing the need for fungicide applications.
Springtime management
Registered fungicides applied from budbreak through 12” of growth (mid March-May) act as foliar protectants and need to be applied to the foliage after budbreak (during the highest potential infection period). Foliar applications need to be applied prior to rain events in order to be effective and will need to be reapplied after significant precipitation.
The 2009 grape growing season has commenced—Thompson Seedless budbreak occurred on March 17th—and warm weather is predicted in the days ahead. The predicted temperatures are optimal for quick spring growth and for the development of powdery mildew. To improve powdery mildew control, Valley grape growers should consider incorporating the UC Davis powdery mildew risk assessment index (RAI) into their management programs. The advantage of using the RAI will be during periods of low mildew pressure when spray intervals can safely be lengthened thereby saving grower’s time and money as well as eliminating fungicide applications. When disease pressure is high, a grower following the RAI recommendations may apply more fungicides than a grower not following the model, but those following the model would be expected to achieve better mildew control. To get the most out of the RAI, growers should consistently monitor canopy temperatures, recognize differences among cultivar in susceptibility to mildew, and understand how trellis systems may affect mildew growth.
Using the RAI
Powdery mildew growth is temperature dependent and requires careful monitoring to optimize fungicide applications. The RAI helps determine grapevine powdery mildew growth based on optimal canopy temperatures (70-85oF). The RAI cannot be initiated until budbreak occurs and there are 6 continuous hours between 70-85oF for three consecutive days. Once these parameters have been met, growers have approximately 5-7 days to apply a fungicide. The RAI is calculated by adding 20 points each day in which there are least 6 continuous hours with optimal growth temperatures. If there are less than 6 hours or if the maximum temperature in the canopy that day is greater than 95oF, then 10 points are subtracted from the index. The RAI never deviates outside the 0-100 point range. Growers can calculate the RAI themselves, access the calculated RAI from the UC IPM website or follow us on Twitter (http://www.twitter.com/grapetweets) to receive weekly updates via the internet, your cell phone, or other mobile device.
The RAI can be used to determine disease pressure and how often fungicides need to be applied. Spray intervals can be shortened or lengthened depending on disease pressure, as indicated in the table.
Spray intervals based on disease pressure using the RAI
