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.