- Author: Belinda Messenger-Sikes
After years of drought, we welcome rain in California. But we also recognize that rain can help spread a number of plant diseases. Rain and wind can splash bacteria and fungi from infected leaves, branches, and blossoms to uninfected parts of the tree. The fungal diseases anthracnose, peach leaf curl, scab, shot hole blight, and the bacterial disease fire blight can all be spread by rain splash. This ability to spread by water makes these diseases more common after a wet spring. With 2023 bringing quite a bit of rainfall and 2024 looking similarly wet, we want to focus on some common rain-dispersed diseases.
Anthracnose
Anthracnose affects many trees including almond, citrus, Chinese elm, and ash (Figure 1). In the spring, fungi produce spores on leaves and twigs that can be spread to new growth via water splashing. Under prolonged wet conditions, this cycle of spore production and spread can occur repeatedly. Anthracnose can defoliate trees, although it is not a lethal disease in most of its hosts. Once symptoms develop or become severe in a growing season, anthracnose can't be effectively controlled. It's best to rely on prevention of the disease by planting resistant varieties. For more information about identification and management of anthracnose, see Pest Notes: Anthracnose.
Fire blight
Pome fruit trees like pear and apple, and other related trees like pyracantha are often affected by fire blight. This destructive disease spreads in the early spring when rain splashes water from infected leaves and blossoms to healthy plant parts. Pollinators like bees can also spread the disease as they visit blossoms. Symptoms may not be noticeable until later when shoots and flowers shrivel and blacken. New growth is especially vulnerable to infection, so avoid heavy fertilization and pruning during mild wet weather.
The Pest Notes: Fire Blight contains much more detail on identification and management of this disease.
Peach leaf curl
Many home gardeners are familiar with the distorted, reddish leaves (Figure 2) caused by peach leaf curl, a disease that affects peach and nectarine trees. The fruit can also be damaged, becoming corky and cracked. Cool, wet weather favors the spread of the disease, but treatment must be started before the spring rains. Preventative sprays should occur after leaf drop but before flower buds swell. Resistant varieties are available to prevent this disease from occurring. Pest Notes: Peach Leaf Curl provides details about this disease and its treatment.
Scab
Various fungi cause the disease scab, which appears as spots and scabby blemishes on fruit and leaves. Apple scab is especially serious during wet springs and in the cool moist coastal areas of California, although these conditions are also conducive for development of other scab diseases. This disease can ruin the harvest of commercially grown apples and pears but in home gardens, some damage can be tolerated. Limit both apple and pear scab on backyard trees by removing and composting fallen leaves and fruit in the fall.
Information specifically about apple and pear scab can be found in the Pest Notes: Apple and Pear Scab. For more about other scab diseases, see http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/scab.html.
Shot hole blight
Shot hole blight, also known as Coryneum blight, is caused by the fungus Wilsonomyces carpophilus. It affects almonds, apricots, and other Prunus species, as well as English laurel. Symptoms appear in the spring as small reddish spots that turn brown and drop out, leaving holes in the leaves (Figure 3). Fruit and twigs can also be affected. Since this disease is worsened by continuous leaf wetting, avoid irrigating foliage. For more information, see the UC IPM page on shot hole blight at http://ipm.ucanr.edu/PMG/GARDEN/FRUIT/DISEASE/shothole.html.
While you can't control the rain from spreading these diseases, good plant care and sanitation practices can help limit the severity. See the publications above as well as other pages on the UC IPM website to find out more.
[Originally featured in the Spring 2024 edition of the Home & Garden Pest Newsletter]
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- Author: Kathy Keatley Garvey
Take stock of your flowers and pollinators; today is the first day of spring.
If Virginia stock, Malcolmia maritima, is blooming in your garden now, you'll likely be seeing honey bees, syrphid flies and other pollinators.
M. maritima, native to Greece and Albania, is a member of the family Brassicaceae, which includes mustards, broccoli, cabbages and cauliflower. It's self-sown. In its native habitat, it grows well in maritime sandy habitats, thus the "maritima" in its name.
The four-petaled flowers, quite fragrant, can range in color from lavender, white and pink to violet and purple.
In our garden in Vacaville, Virginia stock blooms profusely from spring to late fall, and can thrive in the winter.
Cheers to the first day of spring!
- Author: Lauren Fordyce
Spring is just around the corner and many types of fruit trees are starting to develop their leaves. If you are growing peaches or nectarines and are now seeing symptoms of peach leaf curl on new leaves, unfortunately it's too late to manage the disease.
According to University of California research, peach leaf curl can only be effectively managed during the fall and winter, after leaves have dropped but before buds swell in spring. After bud break (when new buds begin to open), you may notice symptoms on new leaves but at this point, there are no effective controls. Pesticides used against peach leaf curl prevent the disease as opposed to curing it. Currently there are no verified curative methods for controlling peach leaf curl during the growing season.
Peach leaf curl is a fungal disease that primarily affects peach and nectarine trees. Its spores overwinter on the twigs and flower buds, and germinate in the spring causing distorted, reddened leaves. If left untreated for several years, trees can decline and may need to be removed. In some cases, peach leaf curl can also affect fruit quality.
During the dormant season (fall and winter), spray affected trees with a fungicide containing the active ingredients copper or chlorothalonil. These active ingredients can be used alternately to limit copper buildup in the soil. Horticultural oils and other fungicides have not proven to be effective against peach leaf curl. Always read the label of the pesticide product you want to use and verify that it is labeled to treat the pest you are dealing with. To learn more about pesticide active ingredients and their potential toxicity, see the UC IPM Pesticide Active Ingredient Database.
To learn more about this disease and its management, visit the UC IPM Pest Notes: Peach Leaf Curl.
- Author: Jeannette Warnert
Weeds present a serious economic problem for farmers, a major headache for vegetable gardeners, and an unattractive appearance in landscapes, but herbicides are not the only solution.
“Studies have shown that more than 70 percent of weeds in lawns and ornamental plantings can be controlled based on good cultural practices,” said Dr. Clebson G. Gonçalves, a horticulture expert and farm advisor with UC Cooperative Extension in Lake and Mendocino counties. He spoke about springtime weed identification and management during a recent urban horticulture webinar offered by the UC Integrated Pest Management Program. “To control weeds, start with good soil; choose suitable plants; and mow, prune, water, and control pests to ensure dense and healthy turf and ornamentals.”
There are several definitions for the term “weeds,” but generally speaking, it can be defined as a plant out of place. Gonçalves used common bermudagrass as an example.
“Bermudagrass is one of the most desirable grass species for golf courses, sports fields, and residential lawns. But out of place, it's extremely difficult to control. It's adaptable to a wide range of environments and can reproduce by rhizome, stolon, and seeds,” he said.
When deciding how to control weeds in lawns and landscapes, Gonçalves suggests gardeners start by identifying the species. The Weed Research and Information Center at the University of California, Davis, offers a free online weed identification tool. Users input characteristics of the weeds – such as weed type (grasslike, broadleaf, woody), where the weed was found, leaf characteristics, stem characteristics, floral characteristics, life cycle, growth habit, etc. – and the tool offers potential species. The UC IPM Program maintains a weed photo gallery of species commonly found in California.
Knowing the species helps determine best practices for control. For example, nutsedge is a grass-like weed that develops nut-like tubers on the roots. The tubers are key to nutsedge survival. Dig deeply into the soil to remove the tubers on mature nutsedge roots. To limit tuber production, remove small nutsedge plants before they have 5 to 6 leaves.
Gonçalves suggests following the principles of integrated pest management to control the weeds by first considering practices that are effective and environmentally sound.
Preventative
- Avoid bringing weeds into your garden and landscape in the first place. Gonçalves suggests using weed-free soil and compost. “It's better to spend a little more money up front when you buy topsoil and compost, than spending even more money later for weed control,” he said.
- Inspect plants at nurseries before bringing them home to see if there are any weeds in or under the pot.
- Clean the mower between lawns. “This is especially important for landscaping companies that work in different locations on the same day. It is important to clean all the equipment before starting in a different location.,” Gonçalves said.
- Never let weeds go to seed. “A lot of species – such as dandelions, bermudagrass, and annual bluegrass – can produce a new seed head very quickly,” Gonçalves said. “If you mow every other week, that's enough time for these weeds to produce new seed heads and disperse seeds. It's better to mow once a week or even more often to prevent the production of seed.”
Cultural
- Soil aeration. “It's very important to alleviate soil compaction in established turf. You will have better water infiltration, greater nutrient availability and more oxygen underground, which promotes plant health and growth above and below the ground,” Gonçalves said.
- Don't leave soil exposed. Limit the area for weeds to grow by utilizing dense plantings. “When you have light and moisture, you have weeds coming up,” he said.
- Mow uniformly at a height of three inches or more. The tall and dense turfgrass canopy will shade the ground and prevent the germination of several weed species.
- Fertilize uniformly.
Mechanical
- Pulling weeds. “Every weed can be controlled by hand. If you have the time to pull by hand, that is recommended. There is some pleasure in pulling weeds. It can be used as therapy,” Gonçalves said.
- Mulches. Wood chips, stone, leaves or compost – especially on top of landscape fabric – can control weeds.
- Flaming. Very effective, primarily in the early stages when weeds are small in stature. But if they are aggressive weeds with strong taproots, such as dandelions and buckhorn plantain, they can grow back very quickly.
Herbicides
- Organic. Always use certified products. “Don't make homemade herbicides. They are still pesticides and can be dangerous,” Gonçalves said. Corn gluten meal is the only organic pre-emergent herbicide. Organic herbicides are available for consumers in different combinations and concentrations. “Vinegar-based herbicides can be very expensive, but other options such as citric acid-based, caprylic acid-based, clove-oil-based, or ammonium nonanoate-based can be more affordable and provide vinegar-like weed control,” Gonçalves said.
- Synthetic. “Many synthetic herbicides also are available for consumers. But the question is: Do you need it? Keep in mind that, all options presented above must be considered first before deciding to use herbicides,” Gonçalves said.
- In both cases, synthetic and organic herbicides are pesticides. Carefully read and follow the label directions.
Learn more:
- Prepared by: Terry Lewis
Tasks
- Inspect trees for die-back and weak limbs, which are common in drought-stressed plants, and remove them before they become a safety hazard.
- Leave frost-damaged growth on tender plants as protection until the danger of frost is past. Begin pruning as new growth emerges.
- Keep fallen camellia petals picked up to avoid petal blight.
Pruning
- Finish deciduous pruning. Chip debris for mulch.
- Wait to prune spring-flowering shrubs until after they bloom.
- Cut back scented geraniums to 18 inches.
Fertilizing
- Fertilize blooming ornamentals such as camellias and azaleas that have finished blooming.
- Fertilize cool-season lawns late in the month if fall fertilization was missed.
Planting
- Add permanent plantings of non-deciduous and needle evergreens.
- Annuals: fibrous begonia, twinspur (Diascia).
- Bulbs, corms, tubers: autumn crocus.
- Fruits and vegetables: cabbage, lemon grass (Cymbopogon).
- Perennials: Red-Hot Poker (Kniphofia), wallflower (Erysimum), blanket flower (Gaillardia).
- Trees, shrubs, vines: bird of paradise bush (Caesalpinia), beautyberry (Callicarpa), bottlebrush (Callistemon).
Enjoy now
- Annuals and perennials: calendula, larkspur (Consolida).
- Bulbs, corms, tubers: daffodil, iris.
- Trees, shrubs, vines: dogwood (Cornus), forsythia, lemon.
- Fruits and vegetables: chives, fennel, kohlrabi, leek.
Things to ponder
- Water may be scarce again this summer. Limit new plantings of annuals and bedding plants, and consider converting part of your landscape watering system to drip irrigation.