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Elisabeth C. Miller Library

Keyword: Trees--Diseases and pests

diagnosing maples and dogwood anthracnose

I live in Kitsap and my 50-year old maple is dying — what should I do?

Also, my Dogwood trees seem to be infected with anthracnose. Can you give me some information about this disease?

 

To get some information about your maple, you can consult with a Master Gardener at a WSU Kitsap County Extension Diagnostic Clinic.

Regarding dogwood anthracnose (Discula destructiva), it is a shame that so many of these beautiful trees are infected. You may be somewhat reassured to know that although the disease often causes tree death in the northeastern U.S., here in the Pacific Northwest, many trees survive. Douglas Justice, Curator of Collections at University of British Columbia Botanical Garden, discusses this on the UBC discussion forum.
Excerpt:
“Weather is probably the deciding factor in infection. A cool, wet period seems to be most conducive to infection, and such factors probably have to coincide with a specific time of tissue susceptibility. In other words, the conditions have to be “just right” for the disease to take off and become established. However, it is well documented that stress predisposes plants to disease susceptibility. Stressors for Cornus nuttallii (Pacific dogwood) would include compacted soil, poor drainage, full exposure with overly dry soil (C. nuttallii is adapted to a summer drought regime, but let’s be reasonable!) and wet soil in summer (e.g., irrigated soil — see previous comment).

Nearly all the local anthracnose-affected dogwoods recovered, including the wild natives and the even some of the more severely affected C. florida (eastern dogwood). Anthracnose has visited us subsequently, but mostly only on C. florida and urban C. nuttallii. This suggests that the there isn’t much anyone can do to prevent the disease from occurring and that as along as trees aren’t overly stressed, they will eventually recover. ”

A U.S. Forest Service article entitled How to Identify and Control Dogwood Anthracnose includes images which may help you to determine if your tree has anthracnose.

Master Garden Products.com provides a short article about dogwood anthracnose that contains a What to Do list.

University of Maryland College Home and Garden Information Center’s Integrated Pest Management Series HP #12 offers information about dogwood diseases and pests, including anthracnose.

Oregon State University Extension’s Online Guide to Plant Disease Control describes cultural controls for anthracnose. There is also an extensive list of chemical controls, which you may choose to ignore after reading Douglas Justice’s comments from the UBC discussion forum mentioned earlier:
“The application of fungicides is probably a waste of money and also likely counter-productive, particularly with a systemic such a benomyl, which will kill most of the good fungi, but probably not the target pathogen. Common fungal pathogens frequently develop resistance to this fungicide.”

 

managing brown rot on ornamental cherry trees

We have a mature ornamental cherry or plum tree that suffered
from brown rot last year. We removed all affected branches and leaves. We were told that we might need to do something else this winter or spring–spray the tree with something, possibly. Can you advise us on how to keep our tree healthy?

 

I consulted The Organic Gardener’s Handbook of Natural Insect and Disease
Control edited by Barbara Ellis (Rodale Press, 1996), and this resource
recommends doing what you already did, by removing and destroying
affected parts of the tree. At the beginning of the growing season (early
spring) you can spray sulfur to control this fungal disease on blossoms.
If you were growing fruit, you would spray again later in the season to
protect the fruit, but since this is an ornamental tree, it isn’t
necessary. Copper sprays are also used to control the disease. Washington
State University Extension recommends preventive measures, such as
avoiding wounding trees (damaging bark with string
trimmers/weed-whackers/lawnmowers, or making bad pruning cuts). Avoid
wetting the blossoms and leaves, and keep the tree pruned for good air
circulation in the canopy. Avoid excessive nitrogen fertilizer. While the
tree is in bloom, check it frequently for symptoms, and destroy any
diseased parts as soon as you notice them.

I found sources for less toxic (but still not hazard-free) versions of
these fungicides from Peaceful Valley Farm & Garden Supply, but they may
be available at your local garden center as well. Some of these require a
pesticide handler’s license.

Lime Sulfur Fungicide

Copper Sulfate

powdery mildew resistant dogwood hybrids

Where can I find information about dogwood hybrids, especially crosses between Cornus kousa and C. florida? Won’t these trees be more resistant to the mildew affecting many dogwoods?

 

In addition to powdery mildew, many dogwoods can suffer with anthracnose. In his book Dirr’s Hardy Trees and Shrubs (Timber Press, 1997), Michael Dirr mentions Rutgers Hybrids (which are a cross of the kousa and florida species of Cornus). These trees were developed at Rutgers University by Elwin Orton, and are resistant to dogwood anthracnose. This article from North Carolina State University Extension discusses powdery mildew resistance. Scroll to the second table at the end which charts cultivars and their resistance or susceptibility to powdery mildew.

Oregon State University provides information about each of the six hybrids of C. florida x C. kousa. Two of the trees on this list are resistant to powdery mildew.

Clemson University Extension offers further information about the insects and diseases affecting dogwoods.

diseases affecting fir trees

Can I attempt to diagnose a diseased tree online? We’re getting more brown spots on our grand fir and I would like to try to figure out what is wrong.

 

You can attempt it, but you will not know for certain based solely on a comparison of symptoms. You can certainly get an idea of what the potential problems could be. Try the Pacific Northwest Plant Disease Management Handbook online—it has excellent photos. Search for fir.

There are several possibilities with brown spots as symptoms, especially:
*needle casts (there are 3 kinds)
*rust
*web blight
*current season needle necrosis
*shoot blight
*Grovesillea canker
*interior needle blight

The best way to diagnose a problem is to bring photos of the affected tree along with samples (if you can reach them) to your local county extension agent or Master Gardener diagnostic clinic.

control of cypress tip moth on cypress trees

I haven’t been able to find much about control of cypress tip moth on true cypress (Cupressus). I’m looking for a non-toxic control instead of the WSU recommendation of Orthene. Would Neem possibly work? Spinosad? Both are registered for leaf miners (fly larvae), but this is a moth larvae. Bt (Bacillus thuringiensis) won’t work because the larvae are inside of the foliage. What’s the best timing for a non-toxic? WSU recommends controlling the adults in July-August.

 

University of
British Columbia Botanical Garden’s forum has this to say:

“In the west, cypress tip moth sometimes infests cypress (Cupressus and
Cupressocyparis) and false cypress (Chamaecyparis); those are also
sometimes called cedars. In the east, cedar often refers to Juniperus
(red cedar), Thuja or Chamaecyparis (white cedar); all are subject to
bagworm infestations and various tip-miners. In the west, timely shearing
is the most effective way to control cypress tip moth, and this may also
be a tactic in other parts of North America.

“In many cases, infestations occur because there are few natural enemies
about to reduce pest levels. Sometimes, pests are attracted to plants
that are already weakened by stress. Healthy plants and diverse
plantings, together with a reduction in pesticide use, will over time,
increase beneficial organisms which will in turn reduce pest levels.
Spraying to reduce pests generally affects beneficials to a greater
degree than the actual target pest. This is because pest species often
have a greater capacity to rebound — they often reproduce faster, have a
greater tolerance for pesticide residues and have a greater capacity to
become resistant to pesticides.”

Oregon State University’s IPM site only mentions chemical controls.

From an online forum, ‘Horticulture Guy:’

Q. I have a row of 16 – three year old “Emerald Green” arborvitaes. I
suspect they have arborvitae leafminer (cypress tip moth). I have
noticed the moths before, but now there are more and I just recently
noticed brownish-yellow tips on a couple of the trees. All of them have
lots of needles falling from the interior. My problem is that I have
received conflicting reports about the proper time to spray for them, and
is there anything I can do in the meantime to lesson the damage? Thank
you! Linda Brieger – Tacoma, WA

A. The way to gain control over any pest population is to know its life
cycle. Spraying is geared toward eliminating the adult form of the
insect, which is a moth as the second of the two common names indicates.
The most likely reason you may see conflicting reports on when to spray
the moths is because of varying times the moth may emerge in different
regions where they are present. They are generally active in our area
from April to June with a peak of activity in May. The moths lay their
eggs during this period and the eggs hatch and then burrow into the
needles of the host plant. According to the WSU extension the adult
moths are silver-tan and approximately 1/4″ in length. External sprays
won’t have an effect on the larvae once they burrow so you need to spray
weekly during this period to catch the larvae as they hatch. Systemic
insecticides are able to kill the larvae once they are in the host. You
can limit systemic insecticide spraying to one application near the
beginning of the activity since they generally remain effective for some
time (see labels for instructions). As far as “in the meantime” a
sprayless solution is to prune out and destroy infected parts of the host
now so that there are less moths in the spring. You can also keep an eye
out in the spring for the white cocoons that form after the larvae exit
the host to become adult moths. You can remove these as well.

University of California Integrated Pest Management suggests that proper
cultural care and removal of susceptible plants is the answer. Excerpt:

Provide proper cultural care to keep plants vigorous. Prune out and
dispose of foliage infested with immature leafminers to restore the
plant’s aesthetic appearance and provide some control. Consider replacing
plants especially susceptible to the cypress tip miner. High populations
and damage can be reduced on established plantings by applying a
broad-spectrum, persistent insecticide such as acephate on susceptible
varieties when adult moths are active. Beginning in early spring, examine
foliage tips for the cocoons. When these appear, vigorously shake foliage
and watch to see if silvery tan, tiny moths fly up then settle back on
the foliage. One application to foliage can be made when a large number
of tip moths appear, between March and May in California. This reduces
browning next season.

You could try using the Neem oil (instead of the more toxic alternatives)
although I did not find any information specifically suggesting this as a
control for cypress tip moth. The WSU book, Pacific Northwest Landscape
IPM Manual (2002) suggests natural parasites which attack this species of
insect, but they do not specify the identity of these predators. They
state that there are no “biorational pesticide management options” for
this pest.

Garden Tip #69

Gypsy moth is often in the news and with it comes the promise of aerial spraying of Btk by the department of agriculture. While the idea of the government spraying pesticides over an entire neighborhood may be frightening, a gypsy moth out-break would be devastating to the trees of the Emerald City or any city. Gypsy moths defoliate over 500 species of trees, both deciduous and evergreens.

Bacillus thuringiensis var. kurstakiis a bacterium that affects only caterpillars. It is considered an acceptable pesticide by organic gardeners, provided it used only when really needed. The major caterpillar pests in our area include:

  • the larvae stage of the gypsy moth;
  • cutworms that feed in winter and spring on primroses, chives and other perennials;
  • tent caterpillar often seen later in the spring on apple trees;
  • keep in mind that sawfly larvae which can strip a flowering red currant bare in a few weeks are not caterpillars, and Btk will not control them.

Btk will kill caterpillars of butterflies, which is why it must be used with caution only when pest populations are high or the potential damage is intolerable. Btk is typically sold as \”caterpillar killer\” where other pesticides are sold.

Trees are Good

Sponsored by the International Society of Arboriculture, this site offers factsheets on tree care and selection as well as a database for finding professional arborists anywhere in the world.

Tree decay: an expanded concept

A classic handbook by Alex Shigo, tree expert and former pathologist for the US Forest Service. The handbook demystifies how and why trees decay and how decay can be slowed or accelerated.

Garden Tip #93

A reference book is available to help gardeners solve pest problems. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide (University of California, $42.00) diagnoses common diseases, insects and environmental stresses with color photos and suggests appropriate solutions. This book also has a chapter on how to get your plants off to a healthy start with proper planting techniques.

The Integrated Pest Management approach tell us the most important fact to remember about plant problems is that poor growing conditions like soggy roots or bone-dry roots inevitably leads to pests and diseases. Select the right plant for the garden conditions to avoid problems later.