Pierce’s disease is becoming a major threat to grape vines.
The bacteria responsible for Pierce’s disease, Xylella fastidiosa, was first seen on grapes in Southern California in the late 1800’s, when it was called Anaheim vine disease. By the 1930’s and 1940’s, it had spread to California’s Central Valley. By the late 1990’s, the disease had spread to several California counties. This increase is believed to be, in part, a result of warmer temperatures allowing more of the bacteria to survive the winter. According to CABI, Pierce’s disease is now found throughout the Americas, and in Italy, Iran, and Taiwan.
Pierce’s disease is carried by sap-feeding insects. Most commonly, this means sharpshooters, such as blue-green and glassy-winged sharpshooters. [Did you know that sharpshooters can consume hundreds, or even thousands, of times their body weight in sap in their short lives?] Spittlebugs have also been found to carry this disease. Whichever insect is chewing on your grape vines, they inject the bacteria into the vine’s vascular bundle as they feed, making them a disease vector. These bacterium then live and reproduce in the xylem, clogging the flow of nutrients and water through the plant.
Pierce’s disease can occur on a large number of weedy and ornamental crops, such as wild grape, California blackberry, periwinkle, stinging nettle, eucalyptus, live oaks, blue elderberry, and mugwort. These plants are not affected by the bacteria that cause disease in grapes. But they do provide a transitionary location for the insects that carry the disease to your garden.
Symptoms of Pierce’s disease
Infected plants exhibit leaf scorching and stunting. These symptoms start out as slightly yellow or red leaf margins (edges) of white or red grape varieties, respectively. Concentric areas of infected leaves may dry up. You may also see ‘matchstick’ petioles, ‘green islands’ on mature brown stems, raisined clusters of fruit, and dieback. These symptoms do not normally appear until spring, after temperatures are above 65°F.
There is no cure for Pierce’s disease. In some cases, the disease will disappear on its own and we don’t yet know how or why. It seems to be a function of temperature, the timing of the initial infection, and the variety of plant being infected. Generally speaking, a late season infection, one that occurs after June 1st, has a 95% chance of recovery. Water stressed plants are more likely to succumb to the infection. If a plant becomes infected early in the season, the bacteria have time to become firmly established. Once that happens, you will ultimately have to remove the vine completely.
Pierce’s disease control and prevention
This disease triangle consists of the host plant, the feeding insect carrier, and the disease-causing bacteria. Break the connection between any one of those three and you can reduce the chances of disease. The easiest ways to prevent Pierce’s disease is to keep host weeds out of the area and treat for the sap-sucking insect pests. Since insect-eating birds, such as bluebirds, along with several predatory insects, love to eat sharpshooters, keep your garden welcoming to these natural helpers.
Monitor your plants for signs of Pierce’s disease so that you can act quickly, reducing the spread of the disease. Most of the vector insects are low fliers, so physical barriers can be used to quarantine potentially infected plants. During the dormant season, remove any vines that have been infected for more than one year. They will not recover and they will spread the disease to other plants as vector insects feed on them and then move to nearby plants for more feeding.
Veraison refers to the time when grapes begin ripening.
If you grow grapes, you know that they start out as clusters of tiny spheres that don’t look anything like a grape. As they grow, they become more grape-like in appearance, but you wouldn’t want to eat one. An unripe grape is tough and very sour.
Grapes first grow to full size and then veraison begins. This is when acidity drops and sugar levels rise. This is also when grapes start to change color. Scientists do not know what triggers veraison, but now you know a garden word that most other people do not!
Pheromone traps use chemicals to trick insect pests.
We all know that smell is a powerful trigger for many species. Smell is why we are drawn to freshly baked bread, why we are soothed by lavender and vanilla, and why we wear cologne and perfume. The chemicals that stimulate these and many other responses are called pheromones.
Pheromones are chemicals that stimulate a response within a species. Since most insects only live for a single season, reproduction is critical to species survival. Insects frequently use pheromones to find each other. Pheromones are categorized according to the response they stimulate:
Most over-the-counter pheromone traps are of the sexual variety. They use synthetic female pheromones to attract males. Chemical signals loft into the wind and are picked up by sensory receptors on a male insect’s antenna. He follows the trail in search of love. What he ends up finding, in most cases, is a paperboard tent lined with a deadly stickum, right next to a little tab of luscious pheromones. In other cases, he may find a tray filled with powdered female pheromones. He joyfully rolls in it, finds nothing, and flies away - smelling like a female. Alas for him, in either case.
What pheromone traps can and cannot do
Pheromone traps interrupt mating. They do this by confusing the males with all the extra aromas and by trapping them on the sticky goo. Contrary to packaging claims and popular opinion, pheromone traps are not effective at eliminating a specific type of pest. In fact, they can do just the opposite! Your lone apple tree may have skirted a codling moth disaster until you hung a pheromone trap in it. Suddenly, every codling moth on the block wants to party at your house!
Proper use of pheromone traps
To be effective, pheromone traps are used to monitor specific areas for invasive or particularly destructive pests. Pheromone traps to help you to determine the best time for particular treatments.
How to use pheromone traps
Since pheromone traps are species specific, you must first identify the pest you are trying to monitor. Next, you need to learn the best time of year to employ pheromone traps. These tips can help you make the most of pheromone traps:
As the number of trapped insects changes, you can see become more aware of the rise and fall of breeding populations. This provides you with the information you need when deciding on which insecticide or other treatment plan is best suited to your garden or orchard. [If you feel like getting really technical, you can also calculate degree days for more effective information.]
Insects monitored using pheromone traps
Since reproduction is so important to short-lived species, pheromone traps continue to be an effective monitoring device for a large number of insects. Some of the most troublesome Bay Area insect pests that are monitored using pheromone traps (during specific time frames) include:
Crops best suited to pheromone trapping
Pheromone traps do not provide benefits to all crops. The most common applications of pheromone traps include:
Don’t be put off by this shift from quick-fix solution to data collection and informed decision making. The extra knowledge and effort can spell the difference between hosting a party of undesirables or producing an abundant crop of delicious food.
Blistered leaves and warty twigs are signs of eriophyid mites.
Eriophyid mites are a family of microscopic plant parasites that include blister mites, gall mites, bud mites, and rust mites. While you will probably never see them without a 20x hand lens, at 1/100” in length, they can be a translucent yellow, pink, white, or purple, with two pairs of legs up near the head.
Symptoms of eriophyid mite feeding include:
Blister mites commonly attack beech, boxelder, cottonwood, elm, maple, live oak, walnut, willow, poplar, roses, privet, and alder. Unfortunately, they also feed on tomatoes, peach, apples, pear, and grapes. I have also heard of them on plum trees in the Bay Area. Eriophyid mites spread on the wind, so there isn’t much you can do to stop them from showing up in your landscape. Luckily, the damage they cause is mostly aesthetic and poses no real threat to otherwise healthy plants. That being said, who wants warty leaves or scabby twigs? Also, eriophyid mites may also carry viruses that can cause real damage.
Insecticidal soaps and horticultural oils can be used on heavy infestations, but simply removing affected leaves, buds, flowers and twigs is really all that’s needed. Sulphur and neem oil have also been shown to be effective. Before you work too hard at getting rid of these minor pests, you may want to keep in mind that the eriophyid mite serves as a replacement food for predatory spider mites and other beneficials, when their foods of choice are absent.
Isn’t it nice to know that some problems in the garden are not big enough to worry about?
You reach in to deadhead a rose and a tiny white insect flies out from under a leaf. On the underside of that leaf is a strange white spiral. You have discovered whiteflies.
Whiteflies are not related to flies at all. Instead, they are close cousins to aphids, mealybugs, soft scale and armored scale.
Whiteflies got their name because most whitefly species look like tiny white flies. Adult bodies are actually yellow, but it’s hard to see. They have 4 white wings and are often no bigger than 1 mm. Some whitefly species are black or yellow. You can see a chart of the most common whiteflies found in California at the UC IPM Pest Note on whiteflies.
Whiteflies are quick and very difficult to see. It is much easier to tell if a garden is infested with whiteflies by noticing white spirals on the underside of leaves. These spirals are whitefly eggs. The adult walks a spiral path, laying eggs in a waxy trail that holds the eggs to the leaf. In warmer parts of California, whiteflies can breed year-round. Whiteflies go through four instars. Eggs hatch out nymphs (crawlers). The next three stages are nearly immobile, much like scale insects. The final instar is a pupal stage. At each stage, these pests are sucking plant juices, spreading disease, and leaving a behind drops of honeydew. Whitefly larval feeding can distort leaves and cause significant losses in some vegetable crops.
Whiteflies can reproduce at mind-boggling rates. They use piercing mouthparts to suck the sap from the phloem of a wide variety of garden edibles and ornamentals. Heavy infestations can cause leaf drop. The honeydew (sugary poop) left behind is host to sooty mold. Honeydew also attracts ants, which then protect whiteflies from their natural enemies. Some whitefly larva may transmit viral diseases, such as cucurbit yellow stunting disorder, and adults can also carry pathogens. Low whitefly populations are not a significant problem, but they can make citrus, pomegranate and avocado trees look shabby.
How to control whiteflies
Natural predators are the best defense against whitefly. Very often, population explosions are caused when gardeners apply broad-spectrum pesticides that kill off predators, or when ants are allowed free access to host trees. Ants can be prevented from protecting whiteflies by applying sticky barriers around the trunks of trees and shrubs. Dusty conditions can also put the odds in favor of whiteflies. Hosing off dusty plants can help reduce whitefly populations.
Heavy whitefly populations are very difficult to control. Any leaves with whitefly eggs should be removed and discarded. If the leaf looks particularly healthy, you can simply rub your thumb over the eggs to destroy them. Reflective mulches repel adult whiteflies, aphids, and leafhoppers. Also, yellow sticky traps can be used to trap and monitor whitefly populations.
According to companion planting lore, whiteflies can be discouraged from an area by planting basil, mint, thyme, and nasturtium. Whitefly predators, such as parasitic wasps and flies, and hummingbirds, can be attracted to an area by planting zinnias, bee balm, hummingbird bush, and pineapple sage.
California is famous for its wine grapes, but did you know it is easy to grow your own table grapes at home?
Not only will you get sweet, luscious grapes, but the vines can be trained over a patio or pergola, providing a nice shady spot in summer!
How to grow grapes
Pruning grape vines
The variety of grapes being grown determines which of two pruning methods to use. Grapes are either spur-pruned or cane-pruned. This is because different grape varieties produce fruit on different bud spurs.
Cane pruning leaves only the trunk and two to four shoots from the previous year’s growth to be trained along support wires. New buds will emerge from these canes to produce leaves and fruit. ‘Thompson Seedless’ and ‘Concord’ are cane-pruned grapes. One cane-pruning method is called the Four Arm Kniffen method and the University of Maine offers an excellent how-to video here.
Spur pruning leaves the bilateral cordons, or horizontal branches, permanently in place. In spring, new growth will emerge from this old wood. ‘Flame Seedless’, ‘Tokay’, and ‘Ribier’ are spur-pruned grapes. In either case, you will want to trim each cane to have no more than 14 spurs. Otherwise, all the plant’s energy will go into vegetative growth, rather than producing grapes.
While most of a grape vine’s roots are in the top 3’ of soil, some of those roots can go down as much as 15’ deep! Grapes perform best when they are watered deeply and allowed to dry out between waterings. The amount of water needed depends on the type of soil, the depth of the roots, and the weather. At the peak of summer, grape wines may need 8-10 gallons of water per day, if a drip system is being used. On average, a deep watering every 2-3 weeks, during summer, is adequate. During cooler or wet weather, little or no water is needed. Once your vines have bloomed, it is important to water regularly, rather than allowing the soil to dry out. As fruit develops, erratic watering can lead to water-stress and cracked fruit.
Feeding grape vines
If grapes are being grown in rich soil, nothing needs to be added. Too many nutrients can reduce or eliminate fruit production. Remember, in the plant world, it’s all about reproduction. Grapes are the reproductive part of the vine. If the plant doesn’t feel the need to reproduce, it won’t.
Assuming your soil isn’t perfectly rich, nitrogen and potassium can be added before berry set. (‘Berry set’ is when the grapes are 1/4” in diameter.) Zinc should only be added before the vines bloom. The only way to know if these additives are needed is to have your soil tested by a reputable lab.
To reduce the chance of pest problems, harvest grapes as soon as they taste ripe. Unripe grapes will not ripen off the vine. Grape clusters should be cut, not pulled, from the vine, and then cooled after being harvested. Do not rinse grapes off before storing them. Do that just before eating.
Pests & diseases of grapes
Spider mites, aphids, mealybugs, cutworms, thrips, click beetles, leafhoppers, branch and twig borers, and ants can infest grape vines. Diseases such as Eutypa Dieback and Pierce’s Disease can infect grape vines, and powdery mildew is a common problem. Birds and rodents can also wreck havoc on your harvest. Monitor grape vines regularly for these pests and diseases to ensure timely control. Contact your local County Extension office for information specific to your region.
While grape vines take some time to become productive, an established grape vine can produce fruit for 50-100 years! Only one or two vines are needed to provide a family with an abundance of grapes.
Get yours started today!
You can grow a surprising amount of food in your own yard. Ask me how!