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Plant Answer Line Question

Keywords: Plant care, Malus, Plant diseases

I am looking for a Malus (crabapple), not necessarily native, but is decorative in terms of blooms and foliage. I am also interested in plant diseases. I am hoping for a tree that will mature to about 20 feet with a 20 foot spread. Growing conditions are half shade, half sun, behind a semi-dense fence. We live in the San Juan Islands where the soil is not great and the tree will not get much water past establishment.


Here is what I found about the culture of flowering crabapples from the book Flowering Crabapples, the Genus Malus, by Fr. John L. Fiala (1994), p. 45:

"Crabapple trees luxuriate in full sunlight in deep rich soils that are well drained. Soils with a pH range of 5.0 to 7.5 suit crabapples well, but the ideal pH range is from 5.5 to 6.5. Even if gardeners are fortunate to have ideal soil conditions, they may not be able to allocate the best part of the garden to crabapples. Flowering crabapples, however, are not greedy and will accept almost any soil that is not waterlogged or overly dry. As long as the soil has a reasonable amount of nutrients and water, crabapples manage to do very well.

"Like most plants, crabapples prefer rich sandy loams, but even in heavier clay soils they do better than many other trees and shrubs and seem to bloom well once they are established. They will accept slightly wetter soils than lilacs, for example, but in these heavier soils they should have excellent drainage as they will not grow in waterlogged, swampy areas nor in soils inundated for long periods of time."

Regarding particular trees you might like that would be disease-free, I found a couple of crabapples that were listed in The Pacific Northwest Gardener's Book of Lists,by Ray and Jan McNeilan (1997). This is from page 24:

1. Malus 'Prairiefire' has red foliage when young that matures to deep green, has bright pink/red blossoms and deep purple-red fruit. It grows to 20 ft x 20 ft and has excellent resistance to scab and mildew (Pacific NW scourges).

2. From the book Flowering Crabapples, the Genus Malus, by Fr. John L. Fiala (1994), p. 147: Malus sieboldii 'Calocarpa' (trade name, Redbud crabapple) is a dense, upright to spreading tree, 15 ft high and as wide... buds deep red, opening to single, white to pink-white flowers 1.4 in across; fruit 0.4 in diameter, bright red to red-orange... A reliable, abundant, annual bloomer... One of the most beautiful of all the ornamental crabapples both in bloom and in fruit. Birds relish the small fruit which never is messy.
From The Pacific Northwest Gardener's Book of Lists, p. 25, I found that this tree is rated 'excellent' in terms of resistance to both mildew and scab.

3. Malus 'Strawberry Parfait' is a "vase-shaped, spreading tree 18 ft high and 20 ft wide; leaves red-purple, turning green with maturity; buds red, opening to single, pink flowers in clusters; fruit yellow with red blush, 0.4 inch in diameter. Excellent disease rating but not rated for fire blight [bacterial disease]. Not very ornamental."
From The Pacific Northwest Gardener's Book of Lists, p. 25, I found that this tree is rated 'excellent' in terms of resistance to both mildew and scab.

[Note: fire blight appears to be more the issue in the midwest and eastern U.S.]

Date 2017-12-09
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Plant Answer Line Question

Keywords: Acer palmatum, Plant diseases

I have a lovely, 3-ft. Japanese Maple (Acer palmatum) that has access to full sun. I am guessing it is over 3 years old at least. The tree is leafing beautifully, but last week I noticed there are "pustules" all over the stems and branches. They are yellowish-brown in color and somewhat mottled looking. They form in irregular clusters along the branch. Each pustule is about the size of a ladybug; in fact, at first I thought they were beetles, but they do not move and when I removed one, it was liquid-y inside and left a thin, white streak along the branch. I am a beginner/ homeowner, so I do not know what this is. Do you have any ideas? What can I do to treat this? I would hate to lose my Japanese Maple.


The best way to determine if your tree is diseased is to bring a sample to a Master Gardener Diagnostic Clinic and ask a Master Gardener to diagnose it for you. This service is free to home gardeners.

What you describe sounds like several quite different problems (canker, or scale, for instance), which is why having a hands-on diagnosis is so important. Below is general information about maple diseases which you can compare with your tree. Additionally, I recommend the book Japanese Maples by J.D. Vertrees and Peter Gregory (Timber Press, 2009). It has sections on the most common pests and diseases affecting Japanese maples.

My Garden Guide from Bartlett Tree Experts has a document about Japanese Maples including extensive information about diseases.

Clemson State University has a factsheet on Maple diseases and insect pests (read the entry on scale).

Try searching for "maple" in Pacific Northwest Guide to Plant Disease Control, and compare the descriptions to see if any ring true with what you are seeing. Ultimately, though, the best thing is to get a hands-on diagnosis from the Master Gardeners, as mentioned above.

Date 2017-06-09
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Plant Answer Line Question

Keywords: Arborist, Trees in cities, Roots--Wounds and injuries, Plant diseases, Fungi and mushrooms, Arbutus menziesii

We recently bought a house on San Juan Island with lots of beautiful madronas (Arbutus menziesii) on the property. Two of them show no signs of life... others have the occasional dead branch here and there. We have been advised that this is likely caused by a fungus and that it can spread rapidly. We have been shown blackened excavated areas on the trunks of the dead trees.. and similar though less extensive areas on some of the others. What can be done to save our beautiful madronas?


It is possible your trees are suffering from canker fungus (Nattrassia mangiferae), or some other type of fungal disease. Here is a link to a file called "The Decline of the Pacific Madrone" edited by A. B. Adams (from a symposium held here at the Center for Urban Horticulture in 1995).

You may want to call a certified arborist to look at the trees, determine the extent of the disease, and help you decide whether the trees can be salvaged. (Search the Pacific Northwest Chapter of the International Society of Arboriculture for a local arborist.)

Below is a response to a question similar to yours from the University of British Columbia Botanical Garden and Centre for Plant Research:

"What you describe are the classic symptoms of 'Arbutus decline,' which is postulated in the literature as being caused by mostly naturally-occurring, weakly pathogenic fungi, made more virulent by the predisposition of Arbutus to disease, caused by urban stresses, especially root disturbance." (see also: "Arbutus Tree Decline" from Nanaimo. B.C.'s Parks, Recreation and Culture department)

Nevertheless, I am convinced that much of the die-back we are seeing on established Arbutus trees stems not from disease, but primarily from the complications of damage, competition, shading and especially, drought stress (we have had a run of very droughty summers). Typically, the most affected natural stands of Arbutus are very dense, with poor air-circulation, internal shading and intense competition for resources (characteristic of rapid growth after clearing). And because this region is becoming increasingly urbanized, with more vehicular and marine traffic (marine traffic evidently accounts for a huge proportion of the pollution in the Fraser Basin air-shed), I would not discount atmospheric pollution as a contributor to the decline (one more stress).

I think the reason your shaded trees are not as affected is that their roots are probably deeper and less exposed, and there is reduced evaporative demand on the leaves. However, as the shade increases, these plants, or at least their shaded branches, will succumb.

What to do? I do not think there is anything you can do to save the existing trees, except, perhaps, to minimize human influence around them. You should avoid both disrupting roots and damaging above-ground portions of the trees (with pruning, for example), as any wound is an open invitation to disease-causing micro-organisms. Interestingly, a friend of mine who kayaks has seen black bears foraging for fruit in the tops of Arbutus trees on Keats Island (he should have told them they are not helping the situation any).

Irrigation of established plants is nearly always counter-productive because it encourages surface rooting (which is typically short-lived and considerably less resilient than deep rooting), and summer irrigation is worse, as Arbutus are well adapted to our conditions (at least, where we find them growing naturally) and normally somewhat dormant in summer. You can plant more Arbutus, as a previous correspondent in this thread has, to replace what you are losing, but there is no guarantee that these plants will survive the next drought or indeed, your well-intentioned meddling. (I suspect his plant was lost for the same reason most young Arbutus are lost--by root damage from saturated or compacted soil conditions). The natural succession on your island is probably (as elsewhere in similar places along the coast) tending toward open Douglas fir forest with a few scattered Arbutus in the more inhospitable places. In other words, you can plant what you will, but the larger the Douglas firs, the fewer Arbutus will be able to survive around them. Neither species is particularly shade tolerant and resources are pretty limited on rocky ground, where both prefer to grow locally. Expect change.

Date 2018-09-08
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Plant Answer Line Question

Keywords: Passiflora, Plant diseases--Diagnosis, Plant diseases, Master gardeners

I have a passion flower that I brought in for the winter. It has lost all leaves and has sticky little brown spots on it . How do I get rid of the brown sticky things? And how do I get the leaves to grow back. Would putting under a grow light for the winter help.


I read up on Passionflower cultivation and pests in the book Passiflora: Passionflowers of the World by Torsten Ulmer & John M. MacDougal, Timber Press (2004).

Under the section, Overwintering, it says that in winter, passionflowers suffer from lower temperatures, shorter days, and low light, and therefore this season is the most critical period for these plants. Before night temperatures drop below 10 degrees C, the more sensitive container plants, such as Passiflora quadrangularis and P. vitifolia, should be taken indoors. Depending on their resistance to cold, other species will need to be taken to their winter quarters later on; for now, though, these plants should just be cut back and thoroughly scrutinized for pests. Unlike many other decorative plants, passionflowers keep their foliage in winter, with the exception of certain herbaceous species such as P. incarnata, P. lutea, and P. bryonioides. Do you know if your plant is any of those three (so we will know if its loss of leaves is normal and not a sign for alarm)? [This is from p. 49.]

About the brown sticky spots, it is extremely difficult to make a diagnosis or suggest a treatment, sight unseen. If your plant was not supposed to lose its leaves, and the leaf drop is a sign of severe stress, then those spots could be the result of the plant's health being poor (as in low resistance to disease). The spots could be bacteria, viral, fungal or even from some insect (although I read through the list of these and could not tell what it might be).

Your plant would be a great candidate for the Master Gardeners to whom the public can take their plants for advice and diagnosis of problems. I found a Master Gardeners of Ontario, which you can search for your particular region.

Date 2018-04-11
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Plant Answer Line Question

Keywords: Plant diseases, Phytophthora

Will the heavier than normal rainfall this fall and winter create an increased problem with Phytophthora this year? Is there anything we can do now or in the Spring to prevent a Phytophthora problem? Is there a chemical we should spray, and when?

We have numerous Rhododendrons, Azaleas, Heather, Camellias, etc. There are areas of our gardens where puddles have formed during, and remain for the day after, heavy rains. Some areas probably have a clay lens of soil underneath that is preventing good drainage, and others may have a mass of roots preventing the good drainage.

We've had Phytophthora before in two groupings of Rhododendrons (2 different varieties). We removed those plants, drenched the soil with the recommended chemical, waited a year, then replanted the same varieties as before. It seemed that things have been fine for at least the past five years, but now, seeing puddles, I'm concerned. Suggestions?


Phytophthora is a fungus which favors our cool, wet conditions and also tolerates heat and drought, so you may be correct that the heavy rainfall will intensify the problem. Here is a link to a September 2011 article from the journal Digger by Niklaus Grunwald, which discusses the origins and spread of the disease.

Here are some links to PDF documents with further information:

Oregon State University information about Phytophthora ramorum (sudden oak death) for forest managers and nursery operators

Toxic-Free Future (formerly Washington Toxics Coalition) has some information on Phytophthora root disease

You can ask when buying plants from local nurseries if their stock has any known problems with this fungus, and you can avoid purchasing affected stock, or planting especially susceptible host plants. If you have walked in an area where the disease is present, clean your shoes before walking in unaffected areas.

The Pacific Northwest Landscape Integrated Pest Management Manual published by Washington State University Extension (3rd ed., 2002) recommends the following methods of managing the root-rot (rather than leaf damage) manifestation of Phytophthora in Rhododendrons and Azaleas (your message does not indicate how your plants were affected by Phytophthora).

1. Plant resistant species and hybrids.
2. Avoid drought stress or flooding, high salinity, or poor drainage. The drainage issue sounds like one you should address in your garden.
3. Remove and destroy diseased plants.
4. Use clean water low in salts, and avoid splashing soil on plants when watering.

More information, from University of California, Davis's Integrated Pest Management web site, indicates that the best way to prevent the disease is to provide good drainage and practice good water management. Here is more information from U.C. Davis.

Date 2017-08-04
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Plant Answer Line Question

Keywords: Plant diseases, Holly

I purchased a gallon size Japanese Holly (Ilex crenata 'Sky Sentry') 5 years ago and put it in a 12" diameter container. It has not grown much, and has been looking bad lately, so I thought it was probably root bound. To my surprise, when I took it out, there were no new roots--the root ball was about 3" deep and 6" across. Is this a normal root for the Ilex? What does it need to thrive?


Since you mention that the plant is not looking healthy, I wonder if it may have root rot.

According to North Carolina Cooperative Extension, Ilex crenata is highly susceptible to this fungal problem.

It is possible that there are nematodes feeding on the roots and diminishing the plant's ability to get water and nutrients from the soil.

Another North Carolina Cooperative Extension site provides descriptions of several problems affecting hollies.

The Organic Gardener's Handbook of Natural Insect & Disease Control by Barbara Ellis (Rodale, 1996) says that holly roots grow close to the surface, so perhaps the size of the root ball is not abnormal.

Missouri Botanical Garden has general information on this plant.

To determine what exactly is causing the plant's ill health, you may want to bring pictures and samples of the affected parts of the holly to a Master Gardener Clinic.

Date 2017-07-18
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Plant Answer Line Question

Keywords: Prunus lusitanica, Plant diseases, Integrated pest management, Phytophthora

I have two Portuguese laurel shrubs. One has large reddish-purple leaf spots and the leaves on part of the shrub have dropped. It looks like the fungus is spreading to part of the other shrub. Do you have any suggestions? I have raked up as much of the dropped leaves as I can. Would Daconil be safe for Portuguese laurels? I also have Bonide multi-purpose fungicide (contains chlorothalonil), but wanted to see if you thought it would be safe for laurels. Thank you for any suggestions you may have.


While I cannot diagnose the problem remotely, I will offer several possibilities of what may be causing the leaf spots on your Prunus lusitanica (Portuguese laurel). The causes might be bacterial, fungal, or environmental. According to Oregon State University's Plant Disease database, English laurel (and other types of laurel) can suffer from leaf spots and shothole. (Search plant list under the letter P, for Prunus laurocerasus.) Excerpt:

Shothole symptoms are commonly observed on Prunus sp. and can be caused by a variety of factors. The bacterium Pseudomonas syringae pv. syringae and several fungi including Cercospora sp., Blumeriella sp., and Wilsonomyces carpophilum (Coryneum blight) can cause leaf spots and shothole on cherry laurel (English laurel, Otto Luyken, or 'Zabeliana'). Copper spray injury and boron toxicity can also cause leaf spotting and shothole. When symptoms are advanced, it is not possible to identify the cause specifically.

Cherry laurels (English laurel, Otto Luyken, or 'Zabeliana'), P. laurocerasus and sometimes other Prunus sp. including cherry and plum, commonly show shothole symptoms resulting from cultural or environmental stress. Research has failed to identify what specific stress is responsible. Both container- and field-grown laurel can develop symptoms.

Symptoms: Necrotic leaf spots with circular to irregular margins. Bacterial spots are brown surrounded by a reddish border with a yellow halo. Abscission layers develop around necrotic leaf spots causing the injured tissue to drop away, leaving holes and tattered areas in the leaf (as if someone fired a shotgun at the leaf-thus the name shothole). After tissues drop, most often it is difficult to determine specifically what caused the initial injury. Observations of early symptom development, signs, and symptoms on other areas of the plant may help make an accurate diagnosis. Note the holes in the leaves.

Cultural control: No management practices have been shown to help reduce physiological shothole. For disease-induced shothole, try the following cultural practices.

Avoid overhead irrigation.

Remove and destroy fallen leaves.

Do not plant near other flowering or fruiting Prunus sp.

If the problem is physiological shothole, this is an environmental disorder which cannot be controlled with chemicals, and infected parts of the plant should be removed and destroyed.

Your description does not sound like bacterial blight, which in laurels usually affects only the leaves, but this link, from University of California, Davis's Integrated Pest Management site, may help you see if the symptoms match your plant.

Prunus lusitanica can also suffer from Phytophthora, which may be seen in affected leaves as reddish or purplish discoloration.

It would be best to find out for certain what is causing the problem before attempting to treat it. I suggest bringing samples of the affected leaves to one of the Master Gardener Clinics in our area.

I cannot recommend using pesticides such as Daconil or Bonide (which both contain chlorothalonil), as I do not have a pesticide handler's license. Also, the information linked here, from Northwest Coalition for Alternatives to Pesticides, indicates that there are many more concerns (human health, environmental) about chlorothalonil than simply whether it will harm the Portuguese laurel. If you do choose to use pesticides, you must follow the directions to the letter. Another reason to find out the specific cause of the leaf spot and leaf loss is that it is against the law to use a pesticide on a pest or problem for which it was not intended.

An alternative approach would be to prune the plants severely to rejuvenate them. Portuguese laurel is a good candidate for this type of renovation. Here is more information on how to do this type of pruning. Scroll down to the section on renovating evergreen shrubs. Excerpt, from the Royal Horticultural Society:

Aucuba, Buxus, Choisya, Euonymus, Ilex x altaclerensis, Ilex aquifolium, Prunus laurocerasus, Prunus lusitanica, Taxus, and Viburnum tinus all tolerate severe pruning. Many evergreens are best renovated over several years, removing one-third to half of shoots to ground level, and reducing all other shoots by one-third in the first year. Over the next couple of years remove half of the older shoots to ground level.

Date 2018-06-27
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Plant Answer Line Question

Keywords: Plant diseases, Arbutus

I was told by a local arborist that Arbutus 'Marina' in the area are all dying of some disease. I have 5 that were adversely affected or died over last winter but at the time I thought this was due to a cold winter. Is there a disease going around, or is it safe to get some new plants of this species and try again?


I know that the City of Seattle has listed Arbutus 'Marina' as an alternative to the disease-prone Arbutus menziesii, because it is supposedly less susceptible to the fungal and bacterial problems affecting our native madronas. However I imagine it is not immune, and perhaps what you have observed indicates that conditions for disease development were just right this year. You might want to talk to the city arborist and ask how the city's plantings of this tree are doing. You could also talk to tree expert Arthur Lee Jacobson to see if he has any thoughts on this.

University of British Columbia Botanical Garden and University of Washington both have information about diseases that affect the genus Arbutus. A commercial operation, San Marcos Growers, comments on disease in Arbutus 'Marina:'

"In the initial release of Arbutus 'Marina' the Saratoga Horticultural Foundation noted that the tree was a fairly pest free and disease resistant; we in fact started calling it the garden tolerant Madrone. Through years of growing and gardening with Arbutus 'Marina' we have discovered that it in fact has a few pests that feed upon it and a few diseases that can cause it harm. The new growth is occasionally attacked by aphids, which will cause the associated sooty mold. Ant control seems to be the best preventative for this. On occasion we also see Greenhouse thrip and soft scale. For these pests, the pest preasure on our garden plants have never reached a threshold that required us to treat with a pesticide. We treat our nursery plants as necessary to assure that they are pest free.
For many years we thought that Arbutus 'Marina' was resistant to plant diseases but in conditions that promote the disease we have found that Arbutus 'Marina' is susceptible to at least 2 plant pathogens. Phytophthora root rot is the most serious of these diseases but when planted correctly in well drained soils and not over irrigated this disease has not appeared to be a problem. In most cases we have seen this problem when the tree is being overwatered or has been planted too deep or in compacted soil. Unfortunately this disease seems to eventually kill the infected plant.
More recently we have seen several established trees losing lower leaves and small twigs inside the canopy. A laboratory analysis has proved this to be caused by the fungus Botryosphaeria. This fungus is opportunist in nature and usually only attacks plants that are under environmental stress of some form. A tree infected with Botryosphaeria may appear vigorous and healthy at its growth tips yet have twig die out within the canopy. The information for controlling this disease on ornamentals is limited but the general consensus is to reduce the stress to the plant and avoid wounding the plant unnecessarily. When pruning infected branches, do so well below all discolored wood and dispose of dead plant material. Clean pruning tools between cuts with a dilute solution of household bleach (1 part bleach to 9 parts water)."

Date 2018-09-08
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Garden Tip

Keywords: Insect pests--Identification, Plant diseases--Diagnosis, Plant diseases

National Gardening Association's Pest Control Library is a pictorial guide to diagnosing pests and diseases. Every article has a color picture to help confirm if the pest in question looks like the bug eating your plant. Major plant diseases are also included and organized by the part of the plant it infects: leaf, fruit, roots or all parts.

Date: 2007-07-13
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