- Author: Lauren Fordyce
Aedes mosquitoes can transmit some of the most debilitating and deadly mosquito-borne pathogens to humans, such as Zika virus, dengue virus, chikungunya virus, and yellow fever virus. One Aedes species is also capable of transmitting heartworm to dogs. These invasive mosquitoes are now present in many regions of California, from Shasta County to San Diego County. View a map of Aedes distribution by county.
The yellow fever mosquito (Aedes aegypti), the Asian tiger mosquito (Aedes albopictus), and the Australian backyard mosquito (Aedes notoscriptus) are unlike most native mosquitoes. They prefer to bite during the day, especially around your legs and ankles. Adults are black with white stripes on their legs and back.
Many Aedes mosquitoes live in close contact with people and can breed in backyards and even inside homes. They are known as “container breeding mosquitoes” because they lay eggs in small containers, such as flowerpots or old tires. Eggs are laid in moist areas just above the water surface, can overwinter, and are resistant to drying so may remain viable for months or longer. Their entire life cycle can occur in as little as 7–8 days.
What can you do?
In many areas of California, public Mosquito and Vector Control Districts work to keep mosquito numbers down to tolerable levels. Since these mosquitoes primarily breed on residential properties, you can help by eliminating mosquito breeding sources around your home. This includes draining standing water in rain gutters, buckets, and watering cans. Change the water in pet dishes and bird baths weekly. Clean up clutter in the yard since these mosquitoes can lay their eggs in anything that can hold water. Standing water or ponds can be treated with Bacillus thuringiensis subspecies israelensis (Bti) or mosquito fish. Wear mosquito repellents and protective clothing outdoors. Keep mosquitoes out of your home by keeping fine mesh screens on windows and doors in good condition.
Learn more about invasive Aedes mosquitoes and how to protect yourself by visiting the following resources:
- UC IPM Pest Notes: Mosquitoes: https://ipm.ucanr.edu/PMG/PESTNOTES/pn7451.html
- Mosquito repellent information from the California Department of Public Health: https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/Mosquito-Repellent.aspx
- Interactive map of Aedes distribution in California: https://cdphdata.maps.arcgis.com/apps/webappviewer/index.html?id=57367199287a4d18a2cecf107854255b
- Invasive Aedes fact sheet from the Mosquito and Vector Control Association of California: https://acrobat.adobe.com/id/urn:aaid:sc:US:908f4b9e-8266-47b3-9e3d-ec165913fc7f?viewer%21megaVerb=group-discover
[Featured in the Summer 2024 edition of the Home and Garden Pest Newsletter]
- 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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Garden tools looking sad? Your hula hoe doesn't dance anymore? Got rust on your shovel?
Join the UC Cooperative Extension Master Gardeners next month for an experience that's sure to not be dull! Learn how to sharpen common garden tools such as hoes, shovels, pruners, loppers, and hula hoes so you can go home and sharpen the tools in your shed.
See live demos to help you better understand the process and get your questions about tool maintenance answered. Another important topic covered is how to keep tools clean and sanitized to prevent the spread of pathogens.
Where: Agricultural Center, Harvest Hall rooms AB&C
Address: 3800 Cornucopia Way, Modesto, 95358.
When: Saturday, January 13, 9:00 AM - 12:00 PM
Cost: $10
Instructors: Tim Long & Ted Hawkins, UCCE Stanislaus County Master Gardeners
Payment Link: https://ucanr.edu/tools/2023
Questions? (209) 525-6862
Peach leaf curl is a fungal disease that only affects peach and nectarine trees. The fungus causes distorted, reddened leaves to appear on the tree in the spring. While many peach and nectarine trees don't begin to show obvious symptoms of peach leaf curl until the spring, the time to manage the disease is in late fall and winter.
If left untreated for several years, trees can decline and need to be removed. In some cases, peach leaf curl can affect the quality of fruit too.
To effectively control peach leaf curl, spray an appropriate fungicide in the fall or winter, after leaf drop but before flower buds swell in the spring. The fungal spores overwinter on peach and nectarine twigs and flower buds. If left untreated, the spores will germinate in the spring, especially in years with heavy rainfall. In Northern California you can usually treat trees starting in late November. In Southern California you can usually start in early January. A single treatment while the tree is dormant is generally advisable, but in areas that get a lot of rainfall, you may want to spray again later in the dormant season.
Numerous copper-based fungicides are available to effectively treat peach leaf curl when used according to label instructions. Always read the product label before buying or using a pesticide. Use only the rate listed on the label to avoid fungicide resistance and avoid making applications before it rains.
For more management information, see the UC IPM Pest Notes: Peach Leaf Curl and Pesticides: Safe and Effective Use in the Home and Landscape.
- Author: Mike Hsu
Salinas Valley lettuce growers lost about $150 million in 2022 due to diseases
A stormy winter could portend another devastating year for the lettuce industry in the Salinas Valley, which saw approximately $150 million in lost gross revenue in 2022 due to INSV (impatiens necrotic spot virus) and associated diseases. Recent drenching rains might mean more weeds – overwintering “reservoirs” for the tiny insect, the Western flower thrips, that carries INSV.
Or the extreme precipitation could benefit growers, as thrips in the soil – during their intermediate stage of development – might be drowned in the waterlogged fields.
As with so many aspects of the INSV crisis, the ultimate effects of flooded fields on thrips populations remain unknown.
“We don't know if thrips are just so persistent and so stable in that pupal stage that maybe they will emerge unaffected,” said Kirsten Pearsons, University of California Cooperative Extension integrated pest management farm advisor for Santa Cruz, Monterey and San Benito counties. “There's just so much about their biology and ecology in the Salinas Valley that we just don't know.”
The mystery of thrips, INSV and soilborne diseases (namely Pythium wilt) is why UC Agriculture and Natural Resources assigned Pearsons to the area last November and hired Yu-Chen Wang in October as UCCE plant pathology advisor for the three counties.
“They're stepping in at a critical moment,” said Richard Smith, the region's UCCE vegetable crop production and weed science advisor who retired in January after a 37-year career. “They've gotten grants funded already – and that's just incredible. They're hitting the ground running.”
Experienced in disease diagnosis and collaboration with growers and industry partners, Wang said her pathology background – paired with Pearsons' entomology expertise – will be crucial in addressing INSV and other diseases.
“It is important for Kirsten and me to work together and provide different insights for the vector and the pathogen, respectively,” Wang said.
‘It's going to take everything to get a crop'
One priority is untangling the dynamics of INSV and Pythium wilt co-occurrence – the subject of ongoing research by JP Dundore-Arias, a plant pathologist at California State University, Monterey Bay. While the vegetables may tolerate one disease or the other, their one-two punch often deals the lethal blow.
“The challenge is – which is why it's great to have Yu-Chen and Kirsten – is that we have so many problems now, whether it's Fusarium (wilt), or Verticillium (wilt), or Pythium, or INSV,” said Mark Mason, pest control adviser for Nature's Reward, which primarily grows lettuces on 5,000 acres across the Salinas Valley.
Mason said that co-infections on his crops (sometimes with three or four diagnosed diseases) make it difficult to assign monetary damages to a specific pathogen, but he noted he has seen fields with “100% loss.” According to the Grower-Shipper Association of Central California, about 11,500 acres were deemed not harvestable in 2022, representing 12% of lettuce industry acreage.
Given the gravity and complexity of the disease dilemma, Pearsons said she has been fielding calls from growers seeking new and better solutions – ways to improve existing tools, techniques borrowed from other crop systems, and additional biological or chemical means of control.
And although there are a couple of pesticides that manage the disease-carrying thrips reasonably well, growers and researchers are worried about their diminishing efficacy due to overuse. Plus, they only constitute a short-term fix.
“Managing the thrips will only reduce the amount of INSV that can get transmitted,” Pearsons explained. “You can kill 99.9% of the thrips, but you get one thrips that has INSV that enters a field, and now you have an infected lettuce plant. All of the thrips are going to come and they can spread it from there; pesticide slows things down, but it's not going to eliminate it.”
Finding disease-tolerant lettuce cultivars is a more sustainable approach. Trials conducted last year by Smith, Wang and others identified several varieties that appeared to hold up well to Pythium and INSV. While additional research could maximize their potential benefit, Wang said even the hardier cultivars will lose their resistance over time, and a multi-layered INSV strategy with “integrated management tools” is crucial.
“We realized, when this thing started happening, that we cannot spray our way out of this problem,” Mason said. “We need varieties; we need management practices; we need pesticides…it just seems like it's going to take everything to get a crop.”
Weeds key to disease control
An all-hands-on-deck approach helped control thrips-harboring weeds last winter. With fields drying out from January storms, Smith said communities must get back to weed management – with a focus on prominent weed hosts for INSV and neglected areas adjacent to farms. Hotspots of infection last year were traced to industrial lots that were overlooked during the weeding process.
“People can't lose sight of the fact that we still need to be controlling the weeds in key areas, because that's the reservoir of the virus during the winter,” Smith said. “We have to stay on task with that.”
Yet despite the diligent weed abatement, crop damage from INSV and Pythium was widespread in 2022, and Smith said it's “very possible” that high heat during the summer was a contributing factor to especially prevalent disease in fall. Thrips populations tend to thrive in warmer weather, Smith said, but much more research needs to be done to understand the basic biology of the insect, including how they acquire the virus and how they spread it.
High hopes for future
Pearsons cited the work of Daniel Hasegawa, a research entomologist with the U.S. Department of Agriculture, who leads teams in monitoring thrips populations in several locations across the Salinas Valley. Currently the counting of thrips on sticky card traps is done manually, but Pearsons and Mason mentioned the possibility of using AI and machine learning to expedite that process.
Mason said that the grower community is excited about the new technologies and ideas that Pearsons and Wang are bringing to the region. As a participant in the search for candidates to fill the advisor positions, Mason said “they were, in my opinion, by far the best fit for what we were looking for.”
“I hope they stay here for 30 years,” he added.
The new advisors both noted the palpable energy and cooperative spirit in the Salinas Valley to proactively meet the challenge.
“Looking to the past, there have been other outbreaks and diseases that they've managed to overcome,” Pearsons said. “These farmers are resilient and creative and I fully believe that lettuce will still be growing here for years to come – it might look a little different, and it might take a little bit of a painful period to get to that point, but I think that we're going to be able to come up with some solutions.”
And while there are concerns that some lettuce growers might decide to leave the region, Wang said she also believes in the industry's strong roots and rich history.
“Salinas Valley has had a beautiful climate for lettuce for so many years; there are some undeniable advantages here,” she said. “This is still the best place in the United States – and maybe the world – to grow lettuce.”
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