Skip to content
Elisabeth C. Miller Library

Keyword: Fungal diseases of plants

Honeydew and sooty mold

This is both a plant and insect question. I manage a property that has a small stand of planted English and Garry oaks. This summer, there has been a very large population of bees and yellowjackets attracted to these trees. There isn’t a nest or hive on the 5 acre property that I have found, so these bees seem to be coming in from a distance. Indicators that I am observing are:
Many sap (sucker) holes on these trees, one in particular.
A heavy black dusty coating on most of the leaves.
The sound of droplets, not humming or eating.

 

Based on your description, my first thought is that the coating on the leaves sounds like sooty mold. First comes the sticky honeydew left by leaf-sucking insects such as aphids, and then the sooty mold develops on the sticky substance. It would make sense that bees and wasps might be attracted by the honeydew, but might also be preying on the smaller insects that cause it.

This article, which comes from a beekeeping perspective, may also be of interest. Here is an excerpt:
“Although it may be off-putting to think of eating insect excrement, honeydew honey is prized in parts of Europe and New Zealand and often fetches higher prices than floral honey. It is rich in mineral content, amino acids, and may have stronger antibacterial properties. When a psyllid insect or aphid ingests the plants sap, it digests the small amount of protein present and expels the rest of the water, sugars, tannins and other indigestible material as honeydew. If you have ever parked your car under a tree and found it covered in a sticky substance the next morning, chances are the tree is infested with sap-sucking psyllids or aphids. For many other insects including ants, wasps, and bees, this is a valuable food source.”

Your poetic description of “the sound of droplets, not humming or eating” might be the sound of transpiration from the leaves, or just the honeydew (part tree sap, part insect excrement) seeping from them.

, ,

Tulip fire

The leaves and petals of my tulips are shot through with little holes. Is this some kind of insect or a disease? And what can I do to solve the problem?

 

Your tulips may have a fungal disease called Botrytis. According to Cornell University’s Plant Disease Diagnostic Clinic, Botrytis tulipae is specific to tulips and lilies. A more colorful name for this disease is ‘tulip fire.’ Cool and damp spring or summer conditions favor the development of the disease:
“Tulip fire infections cause malformations and/or large, light tan patches on tulip leaves. These patches are most noticeable on light-colored varieties. On leaves these infections are somewhat sunken, yellow to light tan, and surrounded by a water-soaked area.
On colored petals the spots appear white and on white petals they appear brown.”

Since the fungus can overwinter in plant debris, good garden hygiene may help:

  • Clean up any diseased leaves and petals, but not when they are wet.
  • Make sure you don’t water your tulips from above (of course, you can’t stop the rain–just don’t aid and abet it).
  • Avoid congested plantings–provide air circulation around your plants.
  • Ultimately, you may want to dispose of infected plants (don’t compost).
  • Rumor has it that using a grit mulch around your tulips can be helpful as a preventive measure.

The Royal Horticultural Society advises that gardeners not plant tulips in a location where Botrytis has been present for three years. According to University of Illinois Integrated Pest Management, there is no point using fungicide where the fungus is already present, but they do describe preventive uses. Always look for the least toxic option. There are some Neem-based fungicides available for home gardeners.

An article in the Telegraph by garden writer Sarah Raven gives an excellent overview of the problem and how to manage it. She highly recommends not hesitating to remove infected plants and bulbs at first discovery, the better to keep the disease from spreading. Early tulips are less susceptible, and according to Raven, the most susceptible in her garden are Darwin tulips, while Viridiflora hybrids are unaffected. Avoid cross-contaminating lilies, fritillaries, and Juno irises by planting them at a distance from your tulips.

,

Pear rust and lack of fruitfulness

I have a ‘Rescue’ pear which has gotten pear rust. It is about 3 years old. I also have an ‘Orca’ pear tree that so far this season does not have rust. Last year we had terrible rust. We thought maybe it came from a secondary host, because there were Juniper bushes. Now those bushes are all gone and I did clean up the leaves from last year to try and avoid contamination from the rust.

Also, I have never gotten any pears on either tree. The Orca tree is a bit older, about 5 years old. They both were bought from Raintree Nursery.

Sorry to hear about your pear with rust, and about the lack of fruit. Washington State University’s HortSense website says there are two types of rust that affect pears in our area:

“Two pear rusts which occur in Washington are Pacific Coast pear rust and pear trellis rust. Both require an alternate host. The rust fungus causing Pacific Coast pear rust is also found on hawthorn, apple, crabapple, serviceberry, quince, and mountain ash. The alternate host is the incense cedar (Calocedrus decurrens), which develops witches’ brooms. Infected fruits of pear are deformed and drop prematurely. On the surface of the fruit, yellowish spots with cup-shaped pustules develop. Leaves and green shoots may also be infected. Symptoms are most obvious after flowering and before July. Pear trellis rust may also infect pears, causing reddish to orange blotches on leaves. The alternate host is juniper, which develops elongate, swollen galls along branches.”

The only controls they recommend are cultural:

  • Avoid susceptible varieties such as ‘Winter Nelis’.
  • Collect and destroy fallen fruit beneath trees.
  • Plant resistant varieties such as ‘Bartlett’.
  • Prune out and destroy rust-infected tissues in pears and alternate hosts.
  • Remove alternate hosts in the vicinity of pear trees, when practical.

Here is an article from British Columbia Ministry of Agriculture and Food on managing this disease in the home orchard.

It sounds as if you are doing everything you can to prevent a recurrence. The web resource above also states that fungicide is probably not a worthwhile approach to managing rust on pears or junipers.

As for the lack of fruit on your trees, Raintree’s pollination chart shows that ‘Rescue’ and ‘Orcas’ should cross-pollinate. It is possible that the ‘Rescue’ pear is not mature enough, or that its bout with disease slowed it down.
Here is an article about failure to produce fruit, from University of Maine. It mentions possibilities such as immature tree(s), lack of sun, and frost damage to flower buds.

I have an ‘Orcas’ growing without other pears in the garden, and yet it produces fruit, so I wonder if something else may be happening. Do you have a good number of bees and other pollinators in your garden? Do you or nearby households use pesticides that might interfere with pollinators? Here is information on protecting and encouraging pollinators, from U.S. Fish and Wildlife.

I also recommend contacting Raintree to see if they have any advice.

Dark leaf spots on Photinia

I have a 75-foot long Photinia x fraseri hedge that is losing leaves. The leaves have dark spots on them. I wonder if it is a mold or fungus. I have not removed trimmings in previous years and air circulation has not been optimum. Some plants have lost almost all their leaves but there is new growth there too. The hedge is about 4 feet tall and is about 20 years old. I was thinking it could use some sort of fertilizer and mulch after I clear out all the old prunings. Any suggestions?

There are a few possible reasons your hedge is dropping leaves. In my own experience with Photinia, they were constantly dropping leaves throughout the year. Some of the leaves had dark spots, but some were just older foliage, reflecting this plant’s tendency to shed older leaves. According to Washington State University’s Landscape Plant Problems (2000), spots on Photinia leaves can be caused by a fungus called Entomosporium mespili, but they can also occur in fall and winter with no disease present. The fungal spots tend to have purple black edges and ash grey centers. Virginia Cooperative Extension has descriptions and images of this fungal disease.

There is another leaf spot of unknown origin, simply called ‘physiological leaf spot,’ described by Washington State University Extension as follows:
“Physiological leaf spot occurs on Photinia in western Washington. The symptoms resemble those of early fungal leaf spot infections. Small red to purple spots appear on the leaves, but do not develop the dark centers characteristic of fungal leaf spot infections. This problem is more common than fungal leaf spot. It causes little damage to plants, although some leaf drop may occur. While the cause of this problem is unknown, leaf spotting appears to be more severe on plants in low-lying or shady areas. Cold temperatures appear to be involved.”

If you want to know for certain what the spots are, you should bring samples to your local county extension agent so that they can identify the cause.

Improving air circulation and removing and disposing of all fallen leaves are always good garden hygiene practices. Avoid getting the leaves wet when you are watering the garden, and avoid summer pruning. The American Horticultural Society book Pruning & Training (DK Publishing, 1996) recommends pruning in spring, though hedges will need to be clipped two or three times a year.

I don’t recommend adding fertilizer, as it could cause your hedge to grow more quickly than you want to prune it. Mulching lightly with compost would probably be beneficial. In general, Photinia is a tough and vigorous plant, and the leaf spots are unlikely to cause its demise.

rose bush buds won’t open

I have several roses that bloom just fine but one particular rose bush produces buds that never open. Why is this happening?

 

It is possible that your rose has a problem with insects like thrips, which can cause buds not to open. If you see tunneling in the buds (holes in the petals), it could be caused by beetles. There is also a possibility that a disease is causing the problem. Fungal infections like botrytis blight can result in buds which do not open, but you would probably notice signs of the fungus during warmer temperatures, such as gray-brown fuzzy growth, and blotched petals or drooping buds. The Northwest Center for Alternatives to Pesticides provides information describing various rose problems, and organic solutions.

Here is a description of botrytis blight from University of California, Davis Integrated Pest Management. This is a brief excerpt:
“Botrytis blight, caused by the fungus Botrytis cinerea, is favored by high humidity. Affected plants have spotted flower petals and buds that fail to open, often with woolly gray fungal spores on decaying tissue. Twigs die back and large, diffuse, target-like splotches form on canes. Reduce humidity around plants by modifying irrigation, pruning, and reducing ground cover. Remove and dispose of fallen leaves and petals. Prune out infested canes, buds, and flowers. Botrytis blight is usually a problem only during spring and fall in most of California and during summer along coastal areas when the climate is cool and foggy.”

The Olympia Rose Society also has information on these potential causes of failed buds. Below is their description of thrips:
“Buds do not open, or flowers are deformed. Petals have brownish yellow streaks and small dark spots or bumps. White and pastel roses are particularly susceptible. Thrips (are) tiny orange insects with elongated bodies. Thrips feed at the bases of rosebuds and on the petals of open flowers. They seem to be attracted to light-colored blossoms.”

The Organic Gardener’s Handbook of Natural Insect and Disease Control edited by Barbara Ellis (Rodale Press, 1996) suggests encouraging natural predators of thrips and, if the infestation is severe, spraying weekly with a safer insectidical soap or pyrethrin-based product. This same resource suggests that if your roses have botrytis blight, you will see the buds turn brown and decay instead of opening, and you should pick off and dispose of any diseased buds. They recommend spraying with sulfur once a week during the growing season.

A few things that are always a good idea when growing rose:

  • make sure there is good air circulation around your plants
  • don’t water from above the plants (keep the leaves dry)
  • always clean up around the plants–don’t let leaf debris or any diseased buds lie on the ground under the rose bushes

This (now archived) site has many pictures of rose pests and diseases for you to compare with what you are seeing on your plant. Since I cannot diagnose the problem without seeing the plant, I recommend that you take samples of the affected buds to a Master Gardener Clinic.

on low hazard pesticides

I heard somewhere that the fungicide Rose Pride was less toxic to beneficial insects than plain baking soda. Is this true? I’d like to continue to use it in my garden.

Rose Pride is the chemical Triforine. I was not able to find any articles which suggest it is safer for beneficial insects than baking soda. Pesticides Action Network’s Pesticides Database indicates it is toxic in varying degrees to some forms of aquatic life. It is on the PAN List of “Bad Actor” pesticides, which means it belongs to a group of pesticides classified as most toxic (because they are known or probable carcinogens, reproductive or developmental toxicants, etc.). The Extension Toxicology Network also has a profile for this pesticide. Here are excerpts:

“In the United States, triforine is marketed for use on almonds, apples, asparagus, blueberries, cherries, hops, ornamentals, peaches and roses. Triforine is a ‘restricted use’ pesticide (RUP) with an EPA toxicity classification of I (highly toxic). Check with specific state regulations for local restrictions which may apply. Products containing triforine must bear the Signal Word ‘Danger’ on their label.

Triforine and the formulated product Saprol are considered of low hazard to honeybees and to the predatory mite Typhlodromus pyrii. It is also of low hazard to earthworms at recommended dose rates.”

My comment would be that “low hazard” is not the same as no hazard, and since there are many other areas of concern with this highly toxic product, it would be best to find an alternative. Locally, the Woodland Park Rose Garden converted a pesticide-dependent landscape to an organic one, and the roses look better than ever. (See an article about the garden from the Seattle Times.) Many gardeners are learning to live with a bit of black spot on their roses, and manage the disease by maintaining good garden hygiene. Don’t leave fallen leaves on the ground. Give your roses good air circulation, and keep the leaves dry when you water your plants. Mulching with wood chips can help, too, since they may prevent water from splashing up onto the leaves.

When deciding whether to treat a garden problem with pesticides, the “Precautionary Principle” provides an important perspective:
“When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.”

fungal growth on trees

I have a 50-year-old Italian plum tree. The limbs have oyster-shaped growths on them. These growths will not come off–they are hard. Is it a fungus or disease? Or is the tree just getting old?

Hard fungal growths on trees are called conks, a type of bracket fungus, and they are not a good sign. According to The Sunset Western Garden Problem Solver, these growths “indicate a hazardous condition from decay inside.” Usually by the time they are seen, decay inside the tree is substantial and you may need to get an arborist’s help to keep the tree from breaking and dropping branches, and to assess whether it is a danger to structures or people. The Pacific Northwest Chapter of the International Society of Arboriculture provides a listing of local certified arborists. You can also get an arborist referral from Plant Amnesty.

honeysuckle and common diseases

I noticed my honeysuckle, which is intertwined
to look like a topiary bush with the greens and flower all bunched
up at the top, to have yellowing of the leaves and drop off. Why
are the leaves yellowing? It smells lovely and is green on the
outside, but if you look under the canopy you can see many yellow
leaves. Is it a disease? Should I use a fungicide?

 

There are a few possibilities. It might be a kind of leaf blight, as
described by Iowa State University Extension.

Leaf blight is a fungal problem, but the control methods described above are not
nontoxic, so you may want to look for a safer fungicide (example here), and also
try to prevent the ideal conditions for fungus. Avoid wetting the leaves
of the plant, and make sure there is good air circulation around the
plant (by siting it properly, and by pruning to keep the plant’s shape
open).

Yellowed leaves could also be caused by scale, which is an insect. Do you
see small bumps on the leaves and stems? If so, here are recommendations
from The Organic Gardener’s Handbook of Natural Insect and Disease
Control edited by Barbara Ellis (Rodale, 1996):

“Minor infestations can be controlled by scraping the insects off the
plant with your fingernail, and by pruning out the most infested parts of
the plant. You can also use a soft brush and soapy water to scrub scales
off the stems, or you can apply dormant oil to the trunk and stems of the
plant just before growth begins next spring, and use superior oil during
the growing season.”

Because I’m not certain which type of problem your honeysuckle may have,
you should bring a sample to a Master Gardener Clinic.

managing mildew on Euonymus

I am considering using Euonymus ‘Green Spire’ for a hedge. My experience with Euonymus japonicus is that every year it seems to get mildew and drop leaves. Is this likely to happen with the ‘Green Spire’ as well? Do you have any suggestions about how to treat the mildew or avoid it?

 

‘Green Spire’ is a variety of Euonymus japonicus. This plant can suffer from a fungus, Oidium euonymus japonici, which occurs only on Euonymus japonicus, and is found wherever the host grows. Clemson University Extension says that fallen leaves and heavily affected branches should be disposed of. Plant in a sunny site which is not overcrowded, and do not water from above.

According to University of California, Davis’s Integrated Pest Management site, variegated forms of Euonymus are less susceptible to mildew. Preventive measures are the first step, but if your plants already have mildew, this resource lists less toxic fungicides, such as Neem oil, jojoba oil, baking soda spray, potassium bicarbonate, and biological fungicides: “With the exception of the oils, these materials are primarily preventive, although potassium bicarbonate has some eradicant activity. Oils work best as eradicants but also have some protectant activity.”