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Elisabeth C. Miller Library

Keyword: Bacterial diseases of plants

managing lilacs with bacterial blight

I have a ‘Palibin’ lilac that appears to have a bit of bacterial
blight. I have pruned out the diseased branches. Is it too late to
spray to control the disease? I didn’t do a dormant spray this year,
and haven’t done any preventive spraying to this point, either. If it
isn’t too late, what spray product would you recommend? What else can I do to keep the blight under control?

 

There are cultural methods of dealing with bacterial blight you should try before using any spray. The information below should help.

Washington State University Extension’s HortSense website recommends:

  • Avoid injuring plants to reduce possibility of infection.
  • Avoid overhead irrigation.
  • Maintain proper plant nutrition. Healthy plants resist disease better.
  • Plant disease-resistant species such as Syringa perkinensis, S. microphylla, or S. vulgaris vars. ‘Alphonse Lavallec’, ‘Crepuscule’, ‘Floreal’, ‘Guinevere’, ‘Jeanne d’Art’, ‘Lutece’, ‘Maud Notcutt’, ‘Mrs. W.W. Marshall’, ‘Rutilant’, or ‘William Robinson’.
  • Prune and destroy infected tissues as soon as they are noticed.
  • Space plants properly and prune to provide good air circulation. This will slow down spread of the disease.

Here is more information from University of California, Davis’s
Integrated Pest Management site. Excerpt:

“Bacterial blight is promoted by prolonged rainy springs. Symptoms may be
more extensive in wetter areas. Prune branches showing dieback and severe
blight. Space plants to provide good air circulation. Prune during the
dry season when infection is less likely to occur. Do not wet foliage
with overhead irrigation; do not overfertilize. Small plants can be
protected to some degree by keeping them covered by plastic (or moved
under plastic). Plant resistant species if available. If the disease is
systemic or cankers appear on the trunk, the tree will probably die and
should be removed. If the disease is confined to leaves, damage is not
usually serious and trees normally recover. Sprays do not give reliable
control.”

diseases affecting English laurels

I have some laurel bushes that are developing black or
dark brown leaves. It starts at the top and then works down.
Trimming them off seems to help but then another bush develops the
problem. I want to take care of this before it gets out of control.
A neighbor had an entire laurel die–it was probably 15 feet tall.

 

While I cannot diagnose a plant problem via e-mail,
it might be a bacterial or fungal problem, or an environmental
disorder. I am assuming your laurels are English laurels (Prunus
laurocerasus), not Mountain laurels (Kalmia). According to the
Oregon State University Extension’s Plant Disease database, English
laurel can suffer from leaf spots and shothole. Excerpt:

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.

  • Avoid overhead irrigation.
  • Remove and destroy fallen leaves.
  • Do not plant near other flowering or fruiting Prunus sp.

If the problem is shothole, this is usually an environmental
disorder, and infected parts of the plant should be removed and
destroyed.

Here is more information from U.C. Davis Integrated Pest Management on bacterial blight, which in laurels
usually affects only the leaves.

If you want to be sure of what the problem is, I suggest
bringing samples of the affected leaves to one of the Master
Gardener Clinics in our area.