Pacific Northwest 1998 An Online Guide to Plant Disease Control

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Prunus laurocerasus (Prunus laurocerasus) -- Leaf Spots and Shothole
 
Cause: 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.

  1. Avoid overhead irrigation.
  2. Remove and destroy fallen leaves.
  3. Do not plant near other flowering or fruiting Prunus sp.
Chemical control: Use only if a bacterial or fungal cause has been identified.

  1. Daconil Weather Stik at 1.4 pints/100 gal water is registered for fungal leaf spots. Daconil can be used in home gardens. 12-hr reentry.
  2. Fixed coppers may help against both fungi and bacteria but can cause leaf injury and defoliation. 24-hr reentry.
  3. Junction at 1.5 lb/100 gal water. 24-hr reentry.
  4. Spectro 90 WDG (chlorothalonil plus thiophanate-methyl) at 1 to 2 lb/100 gal water. For fungal problems. 12-hr reentry.
  5. Zyban WSB at 24 oz/100 gal water for fungal problems. Not to be confused with the smoking cessation drug. 12-hr reentry.
References:
De Boer, S.H. 1980. Leaf spot of cherry laurel caused by Pseudomonas syringae. Canadian Journal of Plant Pathology 2:235-238.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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