Chapter 54 Identifying and Treating Common Fungal and Bacterial Diseases

Full disclosure: part of this chapter draw heavily from an article that Michael Persiano posted to the rec.arts.bonsai news server in 1997.

54.1 Most Fungi and Bacteria Are Beneficial, Not Harmful

As a group, both bacteria and fungi are VERY diverse. North Carolina is known among biologists as one of the great hotspots for fungal biodiversity. They have evolved to live in many different habitats, and to take advantage of a variety of available materials to grow and reproduce. Bacteria are no less varied, living in every place you can imagine.

Fungi and bacteria are essential to life as we know it. For instance:

  • The process of decomposition recycles nitrogen and other nutrients caught in dead plants and animals and returns them to the soil for use by other organisms. Fungi produce the necessary enzymes to start the process, then bacteria complete it. Without this recycling process, living organisms would have long ago used up all the available phosphorus. **Trees and shrubs have beneficial fungi and bacteria growing on their roots that help them extract soil nutrients and water; without these partners, the tree grows more slowly or even dies.
  • Soil bacteria convert non-available forms of nitrogen into nitrate, nitrite, and ammonium ions that plant roots can absorb.
  • A complex community of bacteria in soil communicate with plant roots, helping plants resist and adapt to heat and other stresses. Beneficial bacteria in soil also help prevent pathogenic bacteria from attacking the roots.

While the vast majority of fungi and bacteria are harmless or beneficial, some do feed on landscape trees and plants.

54.2 Knowing Your Enemy

There are literally thousands of possible plant pathogens, and distinguishing between them often requires a specialist. Fortunately there are just a few main diseases we need to worry about. Fungal plant diseases are much more common in this region, so the discussion will focus more on fungi than bacteria.

To start, remember that diseases do not occur randomly. Pathogens have distinct temperature and moisture levels at which they grow; that is why they appear in some seasons but not others. If conditions are not favorable one year, many pathogens form highly resistant spores that remain dormant through repeated winter freezing. This makes seasonal conditions the biggest factors determining which trees are at risk. For example, cooler than usual spring weather combined with heavier spring rains create ideal conditions for fire blight. A consistently warmer and drier spring tends to keep fireblight in check. The onset of summer heat and humidity fosters different diseases like downy mildews. In less humid summers, mildew pops up less often.

Other diseases are restricted to certain hosts. For example, cedar-apple rust alternates between red cedars or junipers, and apples or crabapples. This particular pathogen rarely infects other conifers or members of the rose family. If you do not have crabapples in your yard or collection, you are less likely to see infections of cedar-apple rust on your junipers.

Still other pathogens, particularly bacteria, are more or less likely to invade depending on the health of the host tree. Healthy, actively growing trees will mount a strong resistance response to the invading bacteria and wall them off. Drought-stressed trees, trees with open wounds, and trees that are growing poorly are less able to resist initial infection.

54.2.1 Diseases Caused By Fungi

Soft rots are fungal decay due to excessive moisture. Root rot is the most common type of soft rot, but jin, shari, and exposed deadwood are also susceptible.

Mildews, anthracnoses, rusts, and molds attack foliage. Leaves or needles that have a powdery residue are the most common symptoms. Other fungi cause discolored or dead spots on leaves

54.2.2 Diseases Caused By Both Bacteria and Fungi

Blights and wilts often are the most devastating tree diseases because they fill and block the xylem and phloem that carry water and nutrients. As a result the tree loses vigor and slowly dies. Most are due to fungal infections, but fireblight and a few others are due to bacteria. Generally trees with true blight or wilt cannot be saved, and should be removed immediately, bagged up, and disposed of in the garbage to prevent the disease from spreading. Never compost or chip debris from trees with blight or wilt, and sanitize any tools you use on them.

54.2.3 Strictly Bacterial Diseases

Bacterial cankers are sunken, dead areas of bark that develop in spring and early summer, often accompanied by a gummy ooze. Canker also can caude small holes in leaves, called ‘shothole.’ If the infection spreads all round the branch it will die rapidly.

Slime flux is a foamy or oozing, foul-smelling liquid flowing out from the bark of a tree.

54.3 Don’t Over-React to Harmless Bystanders

  1. 2.

Two kinds of saprophytic fungi. 1. Mushrooms coming out of the soil are harmless. 2. Brackets or mushrooms emerging from a branch or trunk are signs of internal rot. Link to original images: image 1; image 2.

Saprophytic fungi feed on decaying matter in the soil or decomposing wood. The most obvious symptoms are white thread-like mycelia in the soil, possibly with small mushrooms growing on the soil. They can be unsightly, but are not an immediate worry. Very few mushrooms feed directly on living tree roots. Rather, they are digesting the decomposing bark and other organic matter in the soil. If a few small mushrooms appear, simply break them off. Reduce watering slightly to discourage regrowth, but do not try to kill the mycelium in the pot, as it will also kill the beneficial mycorrhiza on the tree roots.

Mushrooms or bracket fungi on the tree are much more serious. Bracket fungi on a tree mean that part of the trunk is decaying. Orangish-yellow mushrooms on the tree may be honey fungus, a major parasitic disease that can kill the tree.

54.4 Identifying and Treating Common Local Diseases

In the sections below I’ve described the tree and shrub diseases we are most likely to see here. About half are diseases I have had direct experience with; the rest are diseases that NC Cooperative Extension lists as routinely present.

For each I listed the susceptible tree species (focusing mostly on species used for bonsai), typical symptoms, and possible controls going from least to most risky to use. The chemicals I name have separate entries in the final section of this page.

54.4.1 Root Rot


Brown root rot. 1. Unpotted stock showing white healthy new roots, lignified brown roots, and black roots that are dying back. This level of decay is not immediately dangerous, but could advance quickly. 2. Individual seedlings uprooted to show stubs left behind by root rot. Links to original images: image 1; image 2.

Commonly Affected Trees: ANY tree with roots which are continuously wet is susceptible.

Symptoms: Roots turn black instead of white or reddish brown, and become soft or waterlogged. Woody parts become soft and black, and no longer dry out between rains.

Cultural Controls: Routine monitoring and repotting are the best ways to prevent root rot. Watch that your soil does not stay constantly wet.

54.4.1.1 Michael Persiano’s Treatment for Root Rot

Posted to rec.arts.bonsai on May 19, 1997

If you suspect your bonsai has root rot, I recommend that you promptly remove it from the pot and examine the roots. Should you find rotten, soft mushy roots with black cores, execute the procedure for the treatment of root rot.

  1. Remove the specimen from the pot.
  2. Cut away the rotten roots until white core appear.
  3. Rinse away the soil and re-inspect for rotten roots which might have been concealed.
  4. Soak the specimen in a solution of triforine/Funginex for 5 minutes. This should be a full-strength solution prepared according to the manufacturer’s instructions. A diluted mixture will not do the job.
  5. Scrub the pot and then repot the specimen using fresh, quick-draining soil. (The goal here is to provide the minimal amount of water required to sustain the life of the tree.) Be careful to work the soil into the root pad with a chopstick to insure optimal root-to-soil contact.
  6. Water the specimen into the soil with a solution of Superthrive. Water with Superthrive every time you water for the next 2-3 weeks. (Insert a chopstick into the soil and use it as a watering gauge. When the stick is on the dry side, water thoroughly)
  7. Place the repotted specimen in indirect light for a period of 2-3 weeks, and then gradually reintroduce the bonsai to direct light.
  8. In 7-10 days, submerge the potted specimen (to the level of the soil) in a bath of one of the above-mentioned fungicides for a few minutes to complete the fungicidal treatment.

This method will not harm your bonsai: it will restore the specimen to its original state of health.

 

54.4.2 Powdery Mildew


  2. powdery mildew

Powdery mildew is a fungal disease that affects a wide range of plants and trees. Infected plants have white powdery spots on the leaves and stems. Links to original images: image 1; image 2.

Commonly Affected Trees: apple, crabapple, cherry, crape myrtle, dogwoods, euonymus, maples (including Japanese maple), oaks, pears, tropicals.

Symptoms: The lower leaves are the most affected, but mildew can grow on any above-ground part of the plant. It grows best at moderate temperature and relatively low humidity.

Dozens of species of fungus can cause powdery mildew. Each affects only a few host plants, but collectively they can attack a wide range of landscape shrubs and trees.

Powdery mildew fungi need a living host. They are not transmitted easily from dead materials. Spores usually are brought to new host trees stuck to woolly aphids or other sucking insects.

Cultural Controls: Routine monitoring is the best way to catch mildew in its early stage. Good airflow will reduce both the likelihood and severity of a breakout.

Chemical Controls:

First Tier: Neem oil effectively manages powdery mildew on most trees by interfering with the fungus’ metabolism and terminating spore production.

Milk diluted 1:10 with water can be sprayed on susceptible plants at the first sign of infection, or as a preventative measure, with repeated weekly application often controlling or eliminating the disease. Milk is as effective in treating powdery mildew on roses as some synthetic fungicides. Part of its effectiveness is due to ferroglobulin, a protein in whey that produces fungus-damaging oxygen radicals when exposed to sunlight.

Second Tier: Copper fungicides are an effective way to manage powdery mildew disease on plants.

Last Resort: Effective synthetic fungicides include triadimefon, propiconazole, hexaconazole, myclobutanil, and penconazole. They rarely are needed though.

 

54.4.3 Sooty Molds


Sooty mold on two different trees. Link to original images: image 1; image 2.

A black, powdery coating adhering to leaves and small twigs. Severe infections can spreads to the benches beneath infected trees.

Commonly Affected Trees: azaleas, blueberries, camellias, crape myrtles, gardenias, poplars, and tropicals. Plants located under pecan or hickory trees are susceptible because honeydew-secreting insects often inhabit these trees.

Symptoms: sooty molds are caused by at least eight different genera of fungi. Most cases of sooty mold are secondary to infestation with either whiteflies, scale insects or aphids. These pests secrete sugary honeydew along with feces, producing a sticky surface on which the fungi feed and grow.

The fungus itself does little harm on its own but correcting the underlying cause is important.

Cultural Controls: routinely checking the undersides of leaves can uncover aphids, scale, or whiteflies that produce honeydew before the population gets too large.

Chemical Controls: spray insecticidal soap on the tree, let it sit for 10-15 minutes, then rinse thoroughly to wash off existing mold. Then spray a second time with a mix of neem oil and insecticidal soap to keep it from returning. This treatment also works well to kill and control aphids and whiteflies that lead to the mold.

 

54.4.5 Leaf Spot


Examples of leaf spot disease. 1. What we see most often. 2. Severe necrotic leaf spot. 3. Tar spot discoloration. Link to original images: image 1; image 2; image 3.

Leaf spot is a general name for both fungal and bacterial diseases that discolor and disfigure leaves on a variety of trees and plants. If they occur in clusters or rows, they may be called anthracnose or canker. There are many species that cause leaf spot; most broad-leaved trees are susceptible to at least one species.

Commonly Affected Trees: Most broad-leaf trees can be affected by one or more spots. Particularly common on apple and crabapple, dogwoods, oaks, maples, azalea, rhododendron.

Symptoms: Leaf spots come in a wide variety of shapes, sizes and colors. Leaf spot diseases usually are seen first on the lower and inner branches where humidity is higher and leaves are shaded.

Bacterial leaf spots tend to have angled edges and a yellow halo. Fungal leaf spots usually are rounded with brown to black halos.

Concentric rings or dark margins around the spot may also be present. Over time the spots can group together to form blotches and sometimes they are formed in angular patterns on the plant. Infected leaves may yellow and drop prematurely.

Most leaf spot diseases affect only a small percentage of the tree’s overall leaf area, and are a minor stress on the health of the tree.

In years with very high humidity or frequent rain events, leaf spot pathogens can spread throughout a tree or shrub’s canopy resulting in severe disease.

Leaf spots often mature in one to two weeks. Smaller leaf spots are younger infections. Larger leaf spots are older infections. At the center of larger leaf spots, it is possible to see signs of the pathogen such as fungal spores or spore-producing structures. The infection and spore production cycle repeats whenever leaves stay wet for 12-24 hours.

Infections that kill a majority of leaves or recur several years in a row can weaken a tree enough to allow other diseases to pests to take hold, or stunt growth.

Cultural Controls: Keep leaves as dry as possible. Water plants at the base instead of the leaves.

Fungal leaf blights can survive in dead debris. Remove all diseased leaves or branches throughout the season and once again before winter. Remove all diseased leaves and canes from the plant and surrounding areas. Do not compost; put in garbage or send out for disposal.

Chemical Controls: Sulfur or copper fungicides change the pH balance of the plant surfaces, creating an environment where leaf spot and other blights cannot thrive.

Fungicides are not necessary unless a tree has lost all of its leaves several years in a row. They are preventive, so need to be applied before symptoms appear on the leaves. They cannot control existing infections.

Successful control usually requires two to three spray applications. In general, the first spray is applied at bud break and the second seven to fourteen days after that. A third spray might be necessary, particularly during rainy periods.

 

54.4.6 Fire Blight


Fire blight on pear (1) and apple (2). Link to original images: image 1; image 2.

Fire blight is caused by Erwinia amylovora, a Gram-negative bacterium that is spread by wind, rain, insects, and birds. It infect leaves through the stomata, or enters wounds in terminal branches and buds. Fire blight kills leaves, blossoms, fruits, shoots, and branches of infected trees. Severe infections can kill a tree in a single growing season.

Commonly Affected Trees: It affects apples, crabapple, pears, quince, hawthorn, cotoneaster, pyracantha, and other members of the Rosaceae.

Symptoms: The severity of fire blight varies from year to year. Some seasons there are few or no trees affected; in other years nearly every susceptible tree shows some burned branches. The strongest predictor of severity is the weather. Cool dry springs limit infections. The disease is more severe when spring temperatures during pre-bloom and bloom are warmer than average. Warm rainy springs encourage infection of the blossoms. Twigs can get infected in late May through June by windy rain storms. Hot summer weather generally slows or stops further spread.

Infections start in early or mid-spring about the time flowers break. Initially leaves on the tips of shoots take on a dull, gray-green appearance, then shrivel and turn brown or black. Often shoots curl into a hook shape as they die. Dead shoots look as if they have been burned in a fire, hence the name.

If untreated, the initial infection of the blossoms and shoots can spread back through the vascular system to larger tree limbs then the trunk. Branches darken and become spongy. Wood under the bark develops black streaks. When the infection reaches the roots and/or graft junction, the tree usually dies.

Cultural Controls: Inspect susceptible species regularly from spring to mid-summer. Once trees are infected, the only treatment is to prune off and destroy dead branches. On large trees the recommendation is to prune branches a full 12 inches back from the dead zones. This is not possible on most bonsai, and it may not be possible to save a heavily infected tree.

Chemical Controls:

First Tier: Bordeaux mixture has some limited benefit in preventing fire blight.

Second Tier: Streptomycin is the only consistently effective preventative measure; all susceptible trees should be treated if there are infected trees in the area.

 

54.4.7 Slime Flux/Bacterial Wetwood

Slime flux on a linden tree. Link to original image.

Commonly Affected Trees: Slime flux primarily hits elm, maple cottonwood, poplar, boxelder, ash, aspen, fruitless mulberry and oak.

Symptoms: A wound to the bark, caused by pruning, insects, poor branch angles or natural cracks and splits, causes sap to ooze from the wound. Bacteria may infect this sap causing it to darken and stain the bark, eventually taking on a foamy appearance and unpleasant odor. This slimy ooze becomes toxic to the bark and eats into the tree. Additionally, the fermented sap attracts insects like flies, ants, and maggots.

Slime flux will not kill a tree, but is a sign that there has been an earlier injury. In some cases, as the outer wound calluses over and the internal methane production decreases, the slime flux may stop in a year or two. In other cases slime flux may reoccur year after year.

Cultural Controls: There is no cure or treatment other than managing symptoms. The bark of a tree is like skin. The wound should be disinfected with rubbing alcohol or a household bleach solution of one part bleach to nine parts water. Excess sap should be wiped from the tree to discourage hungry insects. With prompt and continuous treatment, the tree should survive.

 

54.4.8 Bacterial Canker


Bacterial cankers. 1. Canker break in a branch. 2. A terminal branch with gummy sap oozing from the canker. Link to original images: image 1; image 2.

Commonly Affected Trees: Prunus are the main targets, especially plums and cherries, but also apricots, peaches and ornamental species.

Symptoms: sunken, dead areas of bark develop in spring and early summer, often accompanied by a gummy ooze. Emerging shoots may either fail to emerge, or start to grow normally in spring then dying back rapidly. Canker also can cause small holes in leaves, called ‘shothole.’ If the infection spreads all round the branch it will die rapidly.

The causal bacteria Pseudomonas syringae pv. morsprunorum and P. s. pv. syringae are members of a large genus of plant-pathogenic bacteria.

Cankers develop when bacteria enter wounds or leaf scars when the leaves fall. The bacteria remain dormant until spring, when the infections spread rapidly, killing the bark.

Shotholes form in leaves when the bacteria invade during wet weather in spring or early summer. As the leaf matures the infected spots fail to expand, forming small patches of dead tissue that tissues pull away from the dead patch, which drops out, leaving a ‘shothole.’

Cultural Controls: Prune out affected branches, going several inches into clean wood. Destroy clippings by burning them or sending them to the landfill. Where possible, carry out all pruning in July or August when tissues are most resistant.

There are no treatments available to amateur gardeners for the control of bacterial canker.

 

54.5 Identifying and Treating Less Common Local Diseases

54.5.1 Oak Wilt

Oak wilt. 1. Symptoms of oak wilt. 2. Red staining underneath the bark of a tree, adjacent to a fungal spore mat. Link to original images: image 1; image 2.

Oak wilt is a lethal disease of oaks (Quercus spp.) caused by the fungus Bretziella fagacearum. This disease is prevalent throughout the eastern U.S., and mostly kills members of the red oak family (e.g. red oak, black oak, pin oak, etc.) Mats of fungus form under the bark, attracting beetles and other insects that carry the spores to other trees.

Cultural Control: avoid pruning oak trees during spring. Destroy nearby trees that have the disease.

Chemical Control: treat infected oaks with a systemic fungicide like mycobutanil or propiconazole.

 

54.5.2 Cryptomeria/Passalora Blight

1.
2.

Branches damaged by Cryptomeria blight. Link to original images: image 1; image 2.

Cryptomeria blight (Pestalotiopsis funerea) causes the foliage to first turn yellow and then brown starting at the tips of the needles.

Commonly Affected Trees: Cryptomeria, Leyland cypress, eastern red cedar, bald cypress.

Symptoms: Blight is caused by a fungus that affects stressed and weakened plants. Blight causes foliage to turn yellow, then brown, beginning at the tip. The lower, older foliage is most affected. It is most noticeable in the summer when the tree should be a healthy, rich, dark green. Inner leaves turn brown first, giving the tree a thinning, unhealthy appearance. Left untreated, older twigs will turn gray and black cavities develop on the upper surfaces of infected leaves.

Cultural Controls: If possible, situate trees so they get as much morning sun as possible. Remove affected foliage using sterile pruning and dispose of all plant materials in a sealed bag or by burning. Continue to monitor the tree and remove any newly symptomatic foliage. Be sure to dispose of all debris completely and remove any plant material from beneath the tree to prevent reinfection.

Chemical Controls: If the symptoms persist, you can treat the problem with a fungicide. Start with potassium bicarbonate, then move up to Bordeaux mix or synthetic fungicides if needed.

 

54.5.3 Needle Cast Fungus



Needle cast fungus on jack pine (1) and Douglas fir (2); close-up view (3). Link to original images: image 1; image 2; image 3.

There are several species of needle cast fungi. Here we see spruce needle cast, and pine needle cast; the latter mainly affects 5-needle pines.

Commonly Affected Trees: mugo, Eastern white, loblolly, slash, shortleaf, Virginia, and Scotch pines are particularly susceptible species. Blue, black, and white spruces are the most susceptible spruce species. Frasier fir also can be affected. Full size trees usually recover, but seedlings and bonsai-sized trees can be severely damaged.

Symptoms: Needle cast diseases are often difficult to diagnose. The symptoms are similar to low pH, poor nutrition, fertilizer or chemical burn, and even root rots.

Generally, new needles are infected in the spring or summer. The fungi colonize the needle tissue, turning it yellow and later brown. Fruiting bodies are formed in these brown areas, which produce spores that are spread during wet weather to reinfect new needles on other trees.

Symptoms include yellow or brown spots or bands on needles, needle tip discoloration or dieback, needle death, and premature shedding of needles. Sometimes affected needles can have a red or purple tinge. Typically, the affected needles are the previous year’s needles. The current year’s needles may look healthy, even though they too may be infected.

Mild temperatures and prolonged wetness encourage invasion of emerging needles in the spring. Symptoms may develop later in the growing season or the following year. When trees are stressed and weakened, symptoms of infection tend to develop more rapidly.

Cultural Controls: Once needle cast has been positively identified there are four control strategies to prevent it from spreading.

  1. Cut and burn or remove problem branches from trees, seedlings or saplings that are infected, or nearby trees that are diseased. Usually only a few branches or trees are initially diseased and need to be removed. Remember that there may be trees which are infected but haven’t shown symptoms. These may need to be culled the following year when symptoms show.

  2. Improve air movement around trees so foliage does not stay wet as long, move individual pines further apart, and keep tall weeds or foliage away. These changes help foliage dry faster and

  3. Maintain proper soil fertility and pH. Too-low soil pH and lack of macronutrients can stress trees and make them more susceptible.

  4. Use and time fungicide spraying appropriately. Removing diseased wood, improving air flow, and correcting soil fertility usually is enough to control fungus. If fungicides are needed, time spraying when the needle cast is actively sporulating to protect neighboring plants from infection. Good coverage of current needles is essential for prevention of further infections.

Chemical Controls: Broad-spectrum fungicides such as chlorothalonil, mancozeb and copper salts of fatty acids can suppress transmission of the fungus, but cannot clear an existing infection.

 

54.5.4 Pine Fusiform Rust

1.
2.

Pine fusiform rust. Link to original images: image 1; image 2.

Fusiform rust (Cronartium quercuum f. sp.fusiforme) creates galls or cankers on the main stem and/or branches. Branch cankers within 12-18" of the stem may grow into stem cankers. Main stem cankers can girdle and kill the tree. This is likely on smaller trees and almost assured on nursery trees infected with fusiform rust.

Commonly Affected Trees: Loblolly and slash pines are the most affected tree species. Oaks are the alternate host, especially water, willow, and laurel oaks.

Symptoms: Spindle-shaped swellings or galls develop on branches or main stems. On older trees, infections are somewhat depressed on one side. The fungus produces orange spores on pine galls in the spring. On oaks, orange spores appear on lower surface of leaves. Infected areas may grow for many years until they eventually girdle and kill the branch or stem, or they may become inactive.

The rust must pass through both pine and oak hosts to reproduce successfully. In late March to mid-April spores blown by wind infect emerging oak leaves. The fungus growing on oak leaves quickly forms new spores that transmit the disease back to pines from late April to mid-June.

The timing of the life cycle is tied to temperature. When air temperatures are higher, the annual infection cycle runs earlier in the season.

Cultural Controls: Spores that infect pines move long distances by wind, but infected oaks nearby account for most pine infections. Oaks are not severely damaged by the rust, but cant lose some leaves. Avoid keeping your susceptible pines near oaks, and vice versa. It is best to cull and destroy small trees that become infected, as they will be stunted and grow poorly.

Chemical Controls: Topical fungicides have minimal effect on infections. The systemic fungicide Bayleton provides excellent control in forest tree nurseries with 3 to 5 sprays per season.

 

54.5.5 White Pine Blister Rust


White pine blister rust. 1. On a tree. 2. Close-up view. Link to original images: image 1; image 2.

Cronartium ribicola is the species of rust fungus that causes white pine blister rust. It is native to China, and was subsequently introduced to North America. Some European and Asian white pines (Macedonian Pine, Swiss Pine, Blue Pine) are mostly resistant to the disease, having co-evolved with the pathogen.

Commonly Affected Trees: Western white pine, sugar pine, whitebark pine, and limber pine. Other pines can be infected as well. Alternate hosts are currants and gooseberries (Ribes spp.).

Symptoms: Living trees infected with white pine blister rust frequently have dead tops and scattered dead branches. Newly infected needles display yellow and red needle spots.

Sometimes the remainder of the crown looks healthy and green, but severely infected trees have yellowing foliage and poor needle retention. Infected trees have cankers that form rough textured, spindle-shaped swellings on branches or the main stem. Clear to amber sap-like material oozes from the margins of cankers in spring, followed by the appearance of small cream-colored to yellow-orange pustules in cracks on the cankered areas in spring and early summer.

Environmentally, Cronartium ribicola prefers cooler temperatures coupled with moist conditions in low-lying areas, especially during the late summer and early fall. The low temperatures and humidity are an essential part of the pathogen’s spore germination and dispersal requirements. The combination of cooler temperatures and a high humidity create an environment at which dew can easily form. The free water helps loose spores adhere to a leaf’s surface and promotes spore germination.

Cultural Controls: The rust cannot complete its life cycle without passing through two hosts: pines, then currents and gooseberries, then pines again. Eliminating the second host from areas near your pines breaks the infection cycle.

Pruning out and destroying infected branches is the only method for eliminating active infections from pines. There are no effective chemical controls.

 

54.6 Available Weapons

Controlling bacterial and fungal diseases is not easy. For one thing they are extremely tough. In addition, fungi are more closely related to plants and animals than to bacteria. It is much harder to find chemicals that can kill fungi that are not also toxic to humans, pets, or plants. This is why there are fewer fungicides available to the public than bacteriocides, herbicides and insecticides.

No fungicide is effective against all fungal infections. USDA divides fungicides into two types: protective and curative; some provide both functions. Protective fungicides prevent infection, but do little once a tree is attacked. They tend to work on the surface of a tree, and only directly where they are applied. Curative fungicides are systemics that are absorbed into the leaves, roots, and wood. However, they still should be applied at the first sign of disease; wait and the infection may be so severe that it cannot be cured.

A big problem when buying any insecticide, bacteriocide, or fungicide is knowing what the various brand names mean. Fortunately Federal law requires manufacturers to list the registered name of every active compound and its concentration. That is why I always recommend reading labels to find out the active ingredients. The chemicals listed below are all currently available to consumers without a special license. They are listed with the registered name first, with some common brand names in parentheses.

 

54.6.1 First Line Treatments

Neem Oil is a hydrophobic extract of neem seeds. The oil smothers aphids and overwintering insects under the bark, prevents fungal spores from germinating, and slows growth of existing fungi. One of the easiest ways to prevent fungal diseases is to routinely spray all of your trees with neem oil just before leaf-out. This also kills overwintering insects. A mixture of neem oil and insecticidal soap can be applied 3 times each growing season to keep fungus and nuisance insects at bay.

Potassium Bicarbonate is very effective for fungi including anthracnoses, blights, leaf spots, gray molds, powdery mildew, rusts, and scab. Most trees tolerate it well.

Elemental Sulfur works as a preventive fungicide against powdery mildew, black spot, rusts, and other diseases by preventing fungal spores from germinating. It must be applied before the disease develops for effective results.

Do not use sulfur within a month of applying any oil spray. Likewise, do not use sulfur when temperatures are expected to exceed 80°F to reduce the risk of plant damage. Finally, never use on Japanese flowering apricots or other trees with known sensitivity.

Powdery Mildew Spray is a mixture of 10 g baking soda, 10 ml canola oil, and 1 drop of dishwashing detergent in 1 quart of water. Spray trees repeatedly at intervals of several days. This is largely a physical preventative; baking soda raises the pH of the leaf surfaces to a level where the mildew does not grow.

Use sparingly and be careful not to let the spray wash into the soil. Unlike potassium bicarbonate, excess sodium harms root systems.

 

54.6.2 Second Line Treatments

Copper-Based Fungicides & Bacteriocides

There are three main forms of copper used as fungicides and bacteriocides: copper sulfate, copper octanoate (also called copper soap or copper fatty acids) and copper hydroxide.

All copper compounds will damage new foliage unless used very carefully. Do not use copper fungicides in cool wet weather, or if frost is predicted. Spray needs to dry quickly to avoid burning leaves. Do not use acidic water, because it makes the copper more toxic.

Copper sulfate is the active ingredient in Bordeaux Mixture, along with lime (calcium hydroxide) to neutralize the acidicity of the copper sulfate. Bordeaux mixture prevents both fungal diseases (mildews and anthracnoses) and bacterial diseases (leaf spots and to some extent, fire blight.)

Dilute Bordeaux mixture to 50% of recommended concentration when treating new, unhardened spring foliage. Don’t use Bordeaux mixture on crabapple (Malus spp.), pear (Pyrus spp.), plum (Prunus spp.), azalea and rhododendron (Rhododendron spp.), and juniper (Juniperus sp.) They cannot tolerate it.

Three-Way Dust is a mixture of elemental sulfur and copper sulfate plus pyrethrins (an insecticide).

Copper Diammonia Diacetate kills both bacteria and fungi. It is not acutely toxic to people or other mammals, but is toxic to fish and aquatic animals. Do not use it where it could run off into streams or ponds. Do not use it when frost is possible; it kills bacteria that help trees protect young leaves from late frosts.

Copper Hydroxide is found in some commercial fungicides, but is unsafew for bonsai. Check chemical labels, and don’t use products containing it.

Lime Sulfur (calcium polysulfide) is effective in preventing powdery mildew. Lime sulfur burns foliage so cannot be used on evergreens or after trees have leafed out. Remember that undiluted lime sulfur solution is as caustic as lye drain cleaner. Full strength lime sulfur solution is only used to protect exposed wood. It MUST be diluted before it is sprayed on trees.

 

54.6.3 Heavy Duty Synthetic Compounds

Mycobutanil (Immunox) is a systemic broad spectrum fungicide taken up by leaves. It works both as a preventive/curative for a variety of rusts, leaf spot, and powdery mildews.

Triforine (Funginex) is another broad-spectrum agent that is particularly effective for powdery mildew, scab, rust, and other diseases of fruits, vegetables, cereals, and ornamentals.

Thiomyl/Thiophanate is a low-toxicity general fungicide that does not pose a high risk to terrestrial or aquatic organisms. It is relatively short-acting, because it breaks down very quickly over a few days in soil and water, or on foliage.

Daconil/Chlorothanonil is a broad-spectrum general fungicide. It is toxic to fish, so watch out for runoff.

Propiconazole is a systemic fungicide used for a variety of crop and ornamental diseases. Mixed with permethrin it can be used as a wood preserver. It is fungistatic; it prevents fungus from synthesizing a key molecule needed for growth. Propiconazole does not appear to injure soil microbes, and does not migrate in soil. It persists in soil and water for several weeks.

Streptomycin is the only reliable treatment for preventing fireblight. It is applied starting at early bloom and repeated every 3 to 4 days. Once the disease spreads from the blossoms, streptomycin and other chemical sprays have little effect.

In addition to controlling fire blight in apples and pears, streptomycin can be used to control other bacterial diseases. It has no effect on fungal diseases.

Streptomycin was used as a clinical antibiotic for more than 10 years before it was registered as a pesticide. EPA has found it is practically non-toxic to birds, freshwater invertebrates and honey bees, and only slightly toxic to cold and warm water species of fish. Streptomycin is toxic to algae but not vascular plants, so does not pose unreasonable risks or adverse effects when used as indicated on the label.

 

54.6.4 Treatments of Last Resort

Triadimefon (Bayleton) and Tebuconazole (Bayer All-in-One) are not well-suited to bonsai. They work on just a few pathogens that mainly attack grasses.

Mancozeb is a contact fungicide sold as Dithane, Manzeb, and several other names. It interferes with energy production and protein metabolism of fungal cells. It is classified as a low-risk fungicide, but it breaks down into a toxic compound that can damage your thyroid gland. Avoid it except in dire situations.

Captan is a broad spectrum fungicide used for leaf spots and scab diseases. It is very toxic to bees and fish. It also can cause eye damage. Treat this one as the VERY last resort.

 

54.7 Is Routine Preventive Spraying Appropriate?

Trees that are affected at random only need to be treated as needed based on the pests or diseases that are present. If certain trees in your collection develop the same disease or are attacked by the same pests year after year, consider putting that tree (but not necessarily the whole collection) on a routine preventive spray regimen.

Before you start a preventive spray plan, try clearing away any dead leaves or debris where the resting stages of fungal and bacterial disease overwinter. Many times just removing the reservoir of overwintering spores can break the cycle of re-infection.

If cleanup fails to control routine infections, spray BOTH susceptible and uninfected trees at the start of the season with neem oil. (Spraying non-affected trees prevents them from being reservoirs of spores.) If neem oil does not help, try potassium bicarbonate. If that fails, move up to triforine/Funginex, or Bordeaux mixture. BE SURE to check compatibility before using Bordeaux mixture; many trees do not tolerate it.

 

54.8 Preparing and Using Fungicides and Bacteriocidesfor Use

Every manufacturer sells different formulations, and the margin of safety on fungicides and bacteriocidal chemicals can be small. When preparing them, follow the directions on the label TO THE LETTER! If you need measuring spoons, you can get a set at the Dollar Store.

Always make spraying your last chore for the day. Wear goggles and a face mask, and take a shower immediately after cleaning up. Also, never dump leftover spray where it might run off into water, as many of these chemicals are very toxic to fish.