- 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: Belinda Messenger-Sikes
Have you been seeing a lot of defoliated sycamore trees recently? Sparse foliage and early leaf drop on sycamore trees might be due to anthracnose. The cool, wet spring in many parts of California provided the perfect conditions for this disease. Anthracnose is a common fungal disease sometimes called leaf, shoot, or twig blight. It can cause twisted, distorted branches in American sycamore, some varieties of London plane trees, and California sycamore trees. Sycamore anthracnose is primarily an aesthetic concern since it usually doesn't kill established trees.
Symptoms
Take a close look at the fallen leaves for the characteristic irregular blotches caused by this disease. Anthracnose can cause leaf spots, cankers, and wilting of young leaves in the spring (Figure 1). Emerging leaves turn brown and die. On older leaves, irregular spots or brown blotches begin on either side of the main leaf vein. The lesions spread, covering much of the leaf surface. Extensive twig or shoot blight occurs when young, growing shoots are killed. New shoots grow back from lower on the branch, eventually causing the branch to look gnarled with a “shepherd's hook” appearance (Figure 2).
Springtime disease
The fungus overwinters on infected sycamore twigs and dead leaves., The disease is most severe during wet years when temperatures during the leafing-out period are below 55°F. The disease is not a problem in hot, dry weather. While the first flush of leaves might be lost to the disease, the canopy will regrow as the second flush of growth matures.
Management
Sycamore anthracnose is best managed by cleaning up fallen infected leaves to prevent the spread of the spores to the following year's new growth. Rake and dispose of fallen leaves and twigs during the growing season and in the fall. Prune and dispose of infected twigs and branches in the fall or winter. Avoid sprinkler irrigation that keeps trees wet. When planting sycamore trees in gardens and landscapes, choose resistant cultivars.
Fungicides do not reliably control sycamore anthracnose in California. By the time you see symptoms, it's too late to treat since fungicides effective against the disease are preventive (before the symptoms appear), not eradicative (after the disease symptoms appear). In landscape and residential settings, chemical spray applications are usually not practical due to the size of the trees and the high potential for pesticide drift.
Most sycamore trees will fully recover from defoliation caused by anthracnose, so the best option could be to just rake up the fallen leaves to reduce disease inoculum for the following year.
For more information about anthracnose, see the UC IPM Pest Notes: Anthracnose.
[Originally published in the Summer 2023 Issue of the UC IPM Green Bulletin Newsletter]
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by Penny Pawl, U.C. Master Gardener of Napa County
I recently discovered bunchberry (Cornus canadensis) and could not resist.I admit I am a plant collector but that is another story. The bunchberry is a ground-hugging member of the dogwood family.The leaves resemble the leaves on bigger dogwoods but are much smaller. It only grows about eight inches high but has the same showy white blossoms
When I looked for information on this little guy, I discovered that the dogwood family is quite large. Dogwoods are found within the temperate zones of the Northern Hemisphere. Some are native to the West Coast. The bunchberry also is found in Greenland.
The bunchberry thrives in moist, shady areas, so if you decide to plant one, know that it has specific needs. Plant it in an area that stays cool and shady. Instead of a tap root it has many fibrous roots that grow from a rhizome or mass of roots, which can send up new plants.
The bushberry's small blooms produce edible berries in the fall. I transplanted mine to a larger pot and it has since almost filled the pot. Some of the outside branches are now creeping across nearby soil.
Dogwoods like to grow under the shade of other trees. I recall driving through Virginia in April years ago and seeing the beautiful blooms floating in the shade of the overhead trees. The Western dogwood is native to the redwood forests near Mendocino. Most give a beautiful display in May.
The ones most popular in Napa County gardens are the Eastern dogwood (Cornus florida), Western dogwood (Cornus nuttallii) and red twig dogwood (Cornus alba ‘Elegantissima'). I especially love a small bush variety that was given to me many years ago by a nurseryman. I have taken many cuttings off the plant, but I have not been able to identify it. It grows about three feet tall and every spring puts out many blooms that look like a spray of stars.
Another dogwood growing in my garden is Cornelian cherry (Cornus mas), also known as European cornel or Cornelian cherry dogwood. I kept expecting the big white flowers on this tree but what finally appeared were small yellow flowers. None of these flowers have ever turned into fruit although I have read that it does produce small red berries. This tree grows vigorously and takes well to an annual severe pruning which I often do during the summer months.
Dogwoods do not do well in either flooding areas or dry soils. They like fertile soil and will slowly die if not given the right conditions. Root rot from excess soil moisture is one of the most frequent causes of death. If soil does not drain properly, the roots rot and little can be done to correct this situation.
Diseases that bother dogwoods include anthracnose and crown canker disease. Both are fungal diseases. Avoid damaging the tree trunk as that is often how diseases enter the tree. Some borers also attack dogwoods. However, in all the years I have had dogwoods in my garden, the worst threat I have encountered is some scale on the newer growth. I scrape those off with my fingernail rather than try to remove with chemicals.
In warmer climates, the bark can sunburn, which may cause the tree to decline and die. This bark damage may also invite borers, which can kill the tree. If you find borers in branches or limbs, cut them off below the infestation and dispose of the wood. If the borers are in the main trunk, the tree may be a total loss. Some dogwood hybrids are resistant to some of these diseases and pests, but they can only be reproduced by grafting and cuttings, not by seed.
In the fall, dogwoods produce a blaze of red leaves ranging from pink to burgundy-orange to blood-red.
UC IPM Pest Note on Anthracnose updated May 2020:
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7420.html
Crown canker of dogwood:
http://plantclinic.cornell.edu/factsheets/dogwoodcrowncanker.pdf
The UC Master Gardeners of Napa County are volunteers who provide University of California research-based information on home gardening. To find out more about home gardening or upcoming programs, visit the Master Gardener website (napamg.ucanr.edu). Our office is temporarily closed but we are answering questions remotely and by email. Send your gardening questions to mastergardeners@countyofnapa.org or leave a phone message at 707-253-4143 and a Master Gardener will respond shortly.
Symptoms of anthracnose vary by plant host and weather conditions. High humidity and dense canopies can exacerbate this common disease. Management relies on planting resistant cultivars of landscape plants along with careful maintenance of susceptible cultivars, such as pruning and removal of fallen leaves and twigs.
Authors Jim Downer (UCCE Ventura County), Steven Swain (UCCE Marin County), and Amanda Crump (UC Davis Plant Sciences) recently revised Pest Notes: Anthracnose which includes a list of anthracnose-resistant trees as well as more information about the life cycle of Anthracnose and how it spreads, and an updated section on chemical control.
Subject: Control of Anthracnose on Sycamore Trees
Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
MGCC Help Desk Response to Sprays for Control of Anthracnose on Sycamores: Thank you for contacting the UC Master Gardener Program Help Desk with your question about treating anthracnose on Sycamore trees. You have done a lot of research on this problem, so I am not sure we can tell you anything you do not already know. Sycamore, full-grown in suburbia is a large tree, and were a common tree planted usually several decades ago, but smaller lots and today's suburbia limit their planting these days. While they are often prized for their majesty (and heigh 60-100') they can e a problem from dropping branches, debris from leaves and branches, and anthracnose, especially if planted to close to a house and other trees.
As you know, anthracnose is a fungal disease which can cause unsightly changes to Sycamore trees. The effects on the tree can be brown spots on infected leaves to distortion of branches, cankers and dieback. Unless signs consistently occur every year, trees are usually not seriously harmed; they just may not be aesthetically pleasing. The fungi survive over winter on infected leaves and twigs or dead leaf litter under the trees. Rain in the spring splashes the fungus onto new growth and starts the infection cycle again. Infections are worse in years with heavy spring rain than those with dry springs. The best management mechanism is to remove infected twigs, branches, nd leafs during the fall or winter to decrease the re-infection of the trees in the spring. If there is a wet spring, spraying new growth with fungicides can be protective: however, University of California does not recommend fungicide applications for trees other than ash. I believe you have already reviewed this document but the UC web link to the Integrative Pest Management (IPM) on anthracnose is below. http://ipm.ucanr.edu/PMG/PESTNOTES/pn7420.html
I reviewed the pesticide labels of the two products we discussed - Agri-Fos and Reliant. If there is a difference in the products, it was not apparent to me. It does appear that Agri-Fos is not labeled for use on sycamores for anthracnose and Reliant is allowed to be used as a foliar spray and as an injection into the tree trunk in sycamores for anthracnose. Why these products, with the same active ingredient in the same concentration, are labeled differently is not apparent to me. So, we forwarded this part of your question onto our local Urban IPM advisor to see if he can provide clarification on this…. And we heard back from our Urban IPM Coordinator. Here is his response:
“Product registrants can seek wide or narrow spectrum labels, depending on targeted markets, costs, and efficacy information. In this case, the manufacturers of Reliant have included sycamore anthracnose while the manufacturers of Agri-Fos have not. It seems that Reliant is an older product that is registered for use against a very wide range of agricultural pathogens, so maybe it was cost-effective to include anthracnose. It's also likely that the inert ingredients of these two products are different. Agri-Fos, for instance, is usually injected or otherwise introduced internally, while Reliant is sprayed.
The more important consideration here is that phosphonates (active ingredients in both these products) are not considered effective sprays as per the UC IPM 'Anthracnose'
Pest Note: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7420.html
I think the last paragraph is the most important for this discussion, that phosphonates are not considered effective sprays against anthracnose."
Finally, as we discussed on the phone, you may try consulting another certified arborist to see what their recommendations for management would be. We cannot recommend a specific company, but would recommend you find a certified arborist through the following website. https://www.treesaregood.org/findanarborist
Best of luck and we will let you know if we find out more Please do not hesitate to contact the Help Desk again if you have more questions.
Help Desk of the UC Master Gardener Program of Contra Costa County (SEH/SES)
Notes: Contra Costa MG's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/. MGCC Blogs can be found at http://ccmg.edu/HortCoCo/ You can also subscribe to the Blog.