{"id":2874,"date":"2019-05-18T00:06:42","date_gmt":"2019-05-18T07:06:42","guid":{"rendered":"https:\/\/depts.washington.edu\/hortlib\/pal\/pal-question-583\/"},"modified":"2023-08-04T10:13:28","modified_gmt":"2023-08-04T17:13:28","slug":"pests-and-diseases-affecting-laurel-shrubs","status":"publish","type":"pal","link":"https:\/\/depts.washington.edu\/hortlib\/pal\/pests-and-diseases-affecting-laurel-shrubs\/","title":{"rendered":"pests and diseases affecting laurel shrubs"},"content":{"rendered":"

I have two Portuguese laurel shrubs. One has large<\/strong>
\nreddish-purple leaf spots and the leaves on part of the shrub have<\/strong>
\ndropped. It looks like the fungus is spreading to part of the other<\/strong>
\nshrub. Do you have any suggestions? I have raked up as much of the<\/strong>
\ndropped leaves as I can. Would Daconil be safe for Portuguese laurels?<\/strong>
\nI also have Bonide multi-purpose fungicide (contains<\/strong>
\nchlorothalonil), but wanted to see if you thought it would be safe for<\/strong>
\nlaurels. Thank you for any suggestions you may have.<\/strong><\/p>\n

 <\/p>\n

While I cannot diagnose the problem remotely, I will offer several
\npossibilities of what may be causing the leaf spots on your Prunus
\nlusitanica (Portuguese laurel). The causes might be bacterial, fungal, or
\nenvironmental. According to Oregon State University’s Plant Disease database<\/a>, 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:<\/p>\n

\n

Shothole symptoms are commonly observed on Prunus sp. and can be caused
\nby a variety of factors. The bacterium Pseudomonas syringae pv. syringae
\nand several fungi including Cercospora sp., Blumeriella sp., and
\nWilsonomyces carpophilum (Coryneum blight) can cause leaf spots and
\nshothole on cherry laurel (English laurel, Otto Luyken, or ‘Zabeliana’).
\nCopper spray injury and boron toxicity can also cause leaf spotting and
\nshothole. When symptoms are advanced, it is not possible to identify the
\ncause specifically.<\/p>\n

Cherry laurels (English laurel, Otto Luyken, or ‘Zabeliana’), P.
\nlaurocerasus and sometimes other Prunus sp. including cherry and plum,
\ncommonly show shothole symptoms resulting from cultural or environmental
\nstress. Research has failed to identify what specific stress is
\nresponsible. Both container- and field-grown laurel can develop symptoms.<\/p>\n

Symptoms: Necrotic leaf spots with circular to irregular margins.
\nBacterial spots are brown surrounded by a reddish border with a yellow
\nhalo. Abscission layers develop around necrotic leaf spots causing the
\ninjured tissue to drop away, leaving holes and tattered areas in the leaf
\n(as if someone fired a shotgun at the leaf-thus the name shothole). After
\ntissues drop, most often it is difficult to determine specifically what
\ncaused the initial injury. Observations of early symptom development,
\nsigns, and symptoms on other areas of the plant may help make an accurate
\ndiagnosis. Note the holes in the leaves.<\/p>\n

Cultural control: No management practices have been shown to help reduce
\nphysiological shothole. For disease-induced shothole, try the following
\ncultural practices.<\/p>\n

Avoid overhead irrigation.<\/p>\n

Remove and destroy fallen leaves.<\/p>\n

Do not plant near other flowering or fruiting Prunus sp.<\/p>\n<\/blockquote>\n

If the problem is physiological shothole,<\/a> this is an environmental
\ndisorder which cannot be controlled with chemicals, and infected parts of
\nthe plant should be removed and destroyed.<\/p>\n

Your description does not sound like bacterial blight, which in laurels
\nusually affects only the leaves, but this link,<\/a> from University of
\nCalifornia, Davis’s Integrated Pest Management site, may help you see if
\nthe symptoms match your plant.<\/p>\n

Prunus lusitanica can also suffer from Phytophthora,<\/a> which may be seen in affected leaves as reddish or purplish discoloration.<\/p>\n

It would be best to find out for certain what is causing the problem
\nbefore attempting to treat it. I suggest bringing samples of the affected
\nleaves to one of the Master Gardener Clinics in our area.<\/p>\n

I cannot recommend using pesticides such as Daconil or Bonide (which both
\ncontain chlorothalonil), as I do not have a pesticide handler’s license.
\nAlso, the information linked here,<\/a> from Northwest Coalition for
\nAlternatives to Pesticides, indicates that there are many more concerns
\n(human health, environmental) about chlorothalonil than simply whether it
\nwill harm the Portuguese laurel. If you do choose to use pesticides, you
\nmust follow the directions to the letter. Another reason to find out the
\nspecific cause of the leaf spot and leaf loss is that it is against the
\nlaw to use a pesticide on a pest or problem for which it was not
\nintended.<\/p>\n

An alternative approach would be to prune the plants severely to
\nrejuvenate them. Portuguese laurel is a good candidate for this type of
\nrenovation. Here<\/a> 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:<\/p>\n

\n

Aucuba, Buxus, Choisya, Euonymus, Ilex x altaclerensis, Ilex aquifolium,
\nPrunus laurocerasus, Prunus lusitanica, Taxus, and Viburnum tinus all
\ntolerate severe pruning. Many evergreens are best renovated over several
\nyears, removing one-third to half of shoots to ground level, and reducing
\nall other shoots by one-third in the first year. Over the next couple of
\nyears remove half of the older shoots to ground level.<\/p>\n<\/blockquote>\n","protected":false},"excerpt":{"rendered":"

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…<\/p>\n

Continue reading pests and diseases affecting laurel shrubs<\/span><\/a><\/div>\n","protected":false},"author":4,"featured_media":0,"menu_order":0,"template":"","keyword":[156,755,290,520],"class_list":["post-2874","pal","type-pal","status-publish","hentry","keyword-integrated-pest-management","keyword-phytophthora","keyword-plant-diseases","keyword-prunus-lusitanica"],"acf":[],"_links":{"self":[{"href":"https:\/\/depts.washington.edu\/hortlib\/wp-json\/wp\/v2\/pal\/2874"}],"collection":[{"href":"https:\/\/depts.washington.edu\/hortlib\/wp-json\/wp\/v2\/pal"}],"about":[{"href":"https:\/\/depts.washington.edu\/hortlib\/wp-json\/wp\/v2\/types\/pal"}],"author":[{"embeddable":true,"href":"https:\/\/depts.washington.edu\/hortlib\/wp-json\/wp\/v2\/users\/4"}],"wp:attachment":[{"href":"https:\/\/depts.washington.edu\/hortlib\/wp-json\/wp\/v2\/media?parent=2874"}],"wp:term":[{"taxonomy":"keyword","embeddable":true,"href":"https:\/\/depts.washington.edu\/hortlib\/wp-json\/wp\/v2\/keyword?post=2874"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}