- 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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Winter is the time of year when many people prune their deciduous fruit trees. However, did you know that now is not the right time to prune apricot and cherry trees?
In the past, home gardeners were advised to prune stone fruit trees (cherries, apricots, plums, peaches, and nectarines) after trees lose their leaves and go dormant. However, cherry and apricot trees are more likely to be attacked by certain diseases that can be spread by rain.
Gardeners who prune cherry and apricot trees during winter create wounds that may be invaded by fungal and bacterial canker diseases. Symptoms show up in spring and summer, when infected tree limbs wilt and suddenly die with their leaves still attached, or when bark becomes discolored and limbs produce an amber-colored ooze.
This year, wait to prune apricot and cherry trees until late spring or early summer. For more information about fruit trees, visit The California Backyard Orchard.
- Author: Igor Lacan
[From the August 2016 issue of the UC IPM Green Bulletin]
Symptoms
A single wound or bark crack located on the trunk or a large branch may be observed that is actively oozing or bleeding. The oozing liquid is usually clear and may be sour-smelling in the case of slime flux, or frothy (Figure 1) and “fermented-smelling” in the case of alcoholic flux.
The bleeding is often vigorous, and the liquid stains the bark below dark brown or black, whereas the bark above remains completely unaffected (the stain often resembles a comet with a long “tail” extending below the bleeding wound) (Figure 2).
Differential diagnosis
Flux problems can affect any tree species, but appear most commonly on mulberry, elm, and oak. The flux point occurs singly, may be isolated on an otherwise-healthy trunk, branch, or stub, is often associated with pruning wounds, and is usually not too close to the ground (usually above one meter in height or so).
In addition, the volume of the fluxing liquid is usually substantial, wetting the bark for some distance below (in contrast to canker diseases that produce only a few droplets of liquid or none at all).
Importantly, there will be no entry holes or other evidence of insect infestation, thus differentiating the fluxes from beetle-vectored infections such as foamy bark canker of oaks or the Fusarium infection carried by the polyphagous shothole borer (these two insects also invade trees en masse, creating multiple weeping points).
Causes
The two flux problems are thought to have different causes. Slime flux is associated with bacterial wetwood (Figure 3), a condition in which the heartwood and parts of sapwood become soaked with liquid containing high levels of bacteria. The bacteria ferment the liquid, increasing its pressure until it oozes out through a bark crack or wound.
The differences in bacteria and the location of infection (wood vs. cambium) likely cause the two liquids to differ in smell and appearance.
Management
Despite their prominence, both types of flux are thought to be minor problems in landscape trees (in contrast to within some fruit or nut trees, where they can be more serious). Neither disease affects tree structure, and slime flux only occasionally causes branch dieback.
Foamy flux is typically also benign, as it usually dries up with the onset of cool weather in late fall. Importantly, no treatment has been shown to consistently result in tree improvement, and chemical treatments are ineffective.
Slime flux has occasionally been treated with scribing (excision) of the margins of the bleeding canker, but this is supported only by anecdotal evidence. On the other hand, such “surgery” may risk interfering with the tree's own process for compartmentalizing and sealing-over the damaged area. Because of this concern, installation of drainage tubes is also no longer recommended.
Instead, provide appropriate cultural care—which may mean providing water—and avoid wounding the tree. The bark staining can often be washed off with water.
Additionally, continue monitoring the tree, as other problems (such as Phytophthora or other canker diseases like Armillaria) could occur on the same tree and should not be allowed to go unnoticed as they can resemble the flux diseases, but are distinguishable upon closer examination.
Visit the UC IPM Website for more information about bacterial wetwood (or slime flux) and alcoholic (“foamy”) flux.
/h3>/h3>/h3>/h3>/span>Cherries, apricots, and a few related species are particularly susceptible to fungal and bacterial canker diseases, including Eutypa dieback, Botryosphaeria canker, and bacterial canker. Pathogens can be spread by rain or tree wounds – such as pruning wounds – during wet weather; subsequent infections spread through the wood for several years and may eventually kill the tree.
Late summer is the best time to do final pruning, but with precautions to prevent the tree from sunburn.
To learn more, see the article “Avoid Pruning Apricots and Cherries in the Cool Season” from the November 2014 issue of the UC IPM Retail Nursery and Garden Center IPM News.
Find out about common pests of cherries and apricots on the UC IPM web site and learn more about caring for cherry and apricot trees on the UC California Backyard Orchard web site.