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Gardening Answers Knowledgebase

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: 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: Quercus, Phytophthora, Nuts

I have a few acres in South Kitsap where I am creating pasture by cutting down many of our firs and pines. We will have herds of goats and sheep and swine. I like the idea of acorns for swine and the increased btu of oak for firewood. From my research, it seems that Garry oak and White oak grow too slowly, while Red oak and Pin oak are fast and produce a lot of acorns.

So, I'm leaning towards Red and Pin Oaks, particularly Pin Oaks, but is the fact that these aren't native a problem? Would these trees grow well in the Pacific Northwest, in sandy loam with low nitrogen a pH of 5.3. The soil has pretty good organic matter, Potassium and Magnesium. The trees will be planted in a full sun to mostly sunny area, but depending on the angle of the sun, the surrounding firs throw a pretty big shadow.

Are there other oaks with good acorns I should consider?

Also, is Sudden Oak Death a problem in the Puget Sound?


I'll start with your last question first. Washington State University has a Sudden Oak Death information page. A summary of the work researchers are doing may be found in a 2013 edition of WSU's online newsletter, On Solid Ground.

I have certainly heard that it is present in our region. If you are concerned, you may want to purchase from nurseries with certifications from the USDA Plant Health Inspection Service saying that they are free of the disease. Here is a list of nurseries that have such a certificate. Interestingly, the plants on which Phytophthora ramorum has been detected (in nurseries) in our state are not oaks, but Rhododendron, Viburnum, Camellia, Kalmia, and Pieris. Outside of nurseries, the pathogen has only been found on salal.

The USDA Plant Health Inspection Service has a list of other plants which are hosts of SOD.

Returning to your questions about species of oak, Oregon State University's Landscape Plants Database has information about Quercus rubra (Red oak)and Quercus palustris which confirm that they prefer sunny sites. Red oak will produce acorns in two years. Pin oak is one of the fastest growing oaks, and its acorns (also produced after 2 years) are small.

Local tree expert Arthur Lee Jacobson has this to say about Quercus rubra in his Trees of Seattle (2006):

"New Jersey's State Tree proves to be Seattle's fastest-growing oak, on average. [...] the safest bet if you want an oak in a hurry is to plant a Red oak--and then stand back!" Here is what he says about pin oak, Quercus palustris: "Seattle's most abundant oak. [...] Pin oak is slender in all respects: trunk, limb, branch, twig, leaf--only the tiny squat acorns belie the name. Rapidly growing, it can attain up to 135' x 23 1/4". Inexperienced tree-watchers must be careful not to confuse it with Scarlet oak, which is less common, less slender, makes bigger acorns [...]."

The only aspect of your site description that concerns me, as far as these two oak choices, is the sun exposure--you might want to see how far the shadow of the firs extend, as these oaks will do best in sun. Unless your goal is to plant native species only, I do not see a problem with adding these non-natives to your landscape. You can certainly add lots of native shrubs and perennials if you feel that would be advantageous. There are some excellent resources for selecting native plants:
Washington Native Plant Society's landscaping guides
King County's interactive Native Plant Guide

As far as other good acorn-producing oaks which are also large trees, I can see that Arthur Lee Jacobson mentions English oak (Quercus robur) for its large acorns--and this is also potentially a very large tree (150' x 40+'). Scarlet oak (Quercus coccinea) has notably large acorns too. Burr oak (Quercus macrocarpa) has the largest acorns, but does not do very well in our area. The native Garry oak (too slow a grower for your needs) Quercus garryana does produce acorns.

If you want to diversify the source of nuts (in case of Sudden Oak Death), you might want to consider adding some hazels and filberts (Corylus species) which should do well here. (I don't know if pigs and goats will eat these, though.)

Date 2018-04-11
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Plant Answer Line Question

Keywords: Wood chips, Phytophthora

I have two 40' trees diseased with Phytophthora lateralis that I am having removed tomorrow. Can I chip the branches and spread them in my garden or will this spread the disease to other plants?


Although the following information from Washington State University Extension refers to a different species of Phytophthora, I imagine that the same precautions hold true.
"P. ramorum can be spread to other hosts through air, water, rain, soil and plant debris. People can move it via plants, plant material, soil, plant products, wood, woodchips, dirty shoes, and water. P. ramorum does best in cool, wet climates (like ours)."

A resource from Oregon State University confirms this for both commonly found species of Phytophthora fungus:

Generally, one does not need to worry about plant diseases being spread by wood chips, because "they cannot compete well with wood-decay fungi. Uncomposted plant materials can, however, carry two important diseases of woody plants: Port-Orford-cedar root rot, caused by Phytophthora lateralis, and Sudden Oak Death (Phytophthora ramorum). Many diseased Port-Orford-cedar trees are disposed of by chipping, and mulch made from these chips can spread disease to healthy plants."

Date 2017-12-09
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May 31 2018 13:14:08