Archive for the ‘Landscape Disorders’ Category

Oak Leaf Blister

Oak Leaf Blister. Closeup view of leaf tip. Photo: UF Laura Sanagorski

INTRODUCTION ~ Oak Leaf Blister is not a disorder of great concern and is primarily aesthetic.  Oak Leaf Blister is caused by a fungus (Taphrina caerulescens).  Generally this aesthetic disorder is noticeable in the springtime – especially when we have cool, wet weather – the perfect condition for this fungus’ development.

HOSTS ~  Oak Leaf Blister affects Oak trees (Quercus spp.).

Oak Leaf Blister. Photo: UF Laura Sanagorski

SYMPTOMS / IDENTIFICATION ~ The first sign is small chlorotic (light-colored) spots on new growth of various oak species.  The spots will continue to develop into blisters.  The blisters may eventually fall out of the leaves, leaving holes behind.

Oak Leaf Blister. View from underside of leaf. Photo: UF Laura Sanagorski

LANDSCAPE MANAGEMENT ~ Treatment is not particularly warranted.  If you feel the need to treat for this blister, fungicides can be helpful, but should be used prior to spring bud break.

Download a printer-friendly fact sheet here: Oak Leaf Blister

Remember, the label is the law; be sure to use products only in a manner consistent with the manufacturer directions on the labels.  Please use pesticides safely.

Armillaria Root Rot

Armillaria Root Rot. Photo: Orli Zimmerman

INTRODUCTION ~ Armillaria root rot is a common disease of many trees in Florida.  Although it is better known as a forest disease, it can also be found in urban landscape trees.  It is often a secondary disease that takes hold after a tree is stressed or begins to decline.  It is common to see trees in groups with this disease because it spreads by rhizomorphs, which are shoestring-like fungal growths that can spread underground from tree to tree, as well as through wind-borne spores and direct root-to-root contact.

Mushrooms indicating Armillaria Root Rot (Older mushrooms). Photo: Orli Zimmerman

HOSTS ~  Armillaria root rot affects a wide host range.  Two of note in South Florida are the many ficus and oak species we commonly use in the landscape.

SYMPTOMS / IDENTIFICATION ~ Armillaria might not be particularly noticeable until the disease has significantly progressed.  A general decline, wilting, or dieback may be the noticed first.  This can be confused with or compounded by a number of other disorders.  The disease is better identified by the presence of the shoestring-like rhizomorphs and tan or honey- colored mushrooms near the base of the tree.  The rhizomorphs look like black shoestrings, and may even be confused with the root system of the tree.  Armillaria rhizomorphs are vegetative parts of the fungus; their function is to transport nutrients and increase the fungus’ size.  The rhizomorphs are easily confused with ficus trees’ many aerial roots.  Armillaria mushrooms may only persist for a few days.  White mycelium may also be present under the bark; mycelia are another vegetative portion of the fungus.  Their purpose is to absorb nutrients and decompose dead vegetative matter.

Armillaria Root Rot. Photo: Orli Zimmerman

LIFE CYCLE of the DISEASE ~ In an urban landscape, Armillaria root rot is often a secondary condition that affects a stressed tree.  Drought, disease, pest infestation, cold injury, over-pruning, and improper fertilization are some of the stresses that may initially affect trees.  Armillaria can be spread by airborne spores that come from mushrooms at the base of the trunk, or by the rhizomorphs.  Spores may enter a tree’s canopy through open wounds and rhizomorphs affect the tree by contact with its roots.  Spores may persist in dead tissues for well over ten years, should any part of an infected tree remain in the landscape.

LANDSCAPE MANAGEMENT ~  As with many landscape disorders, the most appropriate management technique is the avoidance of infection.  Maintain healthy trees by ensuring that proper pruning, fertilization, irrigation, and pest management techniques are used.  Commit to planting a more diverse landscape; diverse landscapes tend to better withstand pests, diseases, and even severe weather events.

Trees in the landscape affected by Armillaria should be removed in order to reduce the continued spread of this disease.  There are no fungicidal cures for Armillaria, however there is some evidence the fungicidal treatments may reduce the level of infection.  Fungicidal soil treatments can be used at planting to reduce the likelihood of infection in newly planting trees.  As always, disinfect pruning tools between plants to reduce the possibility of transmitting diseases.

Click here for a pdf version of this fact sheet: Armillaria Root Rot (2012) Sanagorski, L.

References and Further Reading

Cox, K.D., Scherm, H., Beckman, T.G. Armillaria root and crown rot.  Available at:

Florida forest diseases: Armillaria Root Rot.  Available at:

Williams, R.E, Shaw, C.G III, Wargo, P.M, & Sites, W.H. Armillaria Root Disease.  Available at:

Managing Downy Mildew on Impatiens Workshop!

Managing Downy Mildew on Impatiens Workshop! ~ for Nursery and Landscape


Final stages of downy mildew on impatiens - nearly complete drop of flowers and leaves - only stems remain. <Photo: UF / IFAS Laura Sanagorski>

  1. Downy Mildew has been a difficult to control pathogen on Impatiens walleriana in the past season.  Read all about it here.
  2. The latest University of Florida research results for management of Downy Mildew on Impatiens in the nursery
  3. Current University of Florida recommendations for managing Downy Mildew in the landscape

WHEN: 9:00AM – 10:00AM: Tuesday March 6, 2012

WHERE: Palm Beach Extension – Exhibit Hall A, 559 North Military Trail, West Palm Beach, FL

COST: Free of charge, but seating is limited.

REGISTRATION: Accepted via e-mail, mail, or fax:  Download the Managing Downy Mildew on Impatiens Flyer and Registration Form here.

CEUs: Pesticide Applicator and FNGLA CEUs requested

Downy Mildew on Impatiens

Discolored upper leaf surface and downy mildew growth on lower leaf surface. <UF Laura Sanagorski>

updated 1/31/2012

Laura Sanagorski, Environmental Horticulture Extension Agent

Bill Schall, Commercial Horticulture Extension Agent

Downy mildew on impatiens is currently a concern in Palm Beach County.  High humidity paired with cool nights created the perfect conditions for disease development.  Downy mildews are caused by a variety of pathogens that tend to be specific to hosts; however Plasmopara obducens is the one that affects impatiens.  Some literature indicates that downy mildew favors about 50 – 72 degrees Fahrenheit nighttime temperatures.

HOSTS – Downy mildew affects all hybrids and varieties of Impatiens walleriana, also called Busy Lizzy.  New Guinea impatiens, Impatiens X hawkeri, is considered very tolerant.  Counties adjacent to Palm Beach County have reported less severe outbreaks in 2011 and early 2012.

Symptoms of downy mildew showing first on newer growth: leaf edges curling downward <UF Laura Sanagorski>

SYMPTOMS – Young plants and new growth are most susceptible and may show symptoms first.  Initially, leaves may look a little yellowish or speckled.  In fact, these symptoms look very similar to nutritional deficiencies.  You may see faint gray lines on the tops of leaves or notice leaf edges curling downward.  Sometimes the yellowing is not visible before leaf curling begins.

As the disease continues to progress, whitish downy looking growth will be visible on undersides of leaves. This whitish growth is spore-containing structures that have emerged from the lower leaf pores (stomata). Next, leaves and flowers will drop quickly, leaving mostly stems.

Continue reading

Algal Leaf Spot

Algal Leaf Spot <UF Laura Sanagorski>

An excessively rainy fall has  created the perfect opportunity for a number of our leaf spots and fungal diseases to make an appearance. This is Algal Leaf Spot, and this week I’ve seen it on Sausage Tree, Firebush, Cocoplum, and some other ornamentals.   It seems to be more prevalent than some previous years.

Algal Leaf Spot is predominantly an aesthetic issue, and it doesn’t often warrant treatment, although it can be managed with fungicides if necessary.  The spots are caused by an algae and start out a rusty color which gradually fades to grey.  We see Algal Leaf Spot in late summer and most of the fall, so it should be subsiding before too long.

Algal Leaf Spot: New rust-colored spots (above) fade to grey (below) <UF Laura Sanagorski>

Split Canopy on Date Palms

Split Canopy on Date Palm. UF Laura Sanagorski

I received a call about these Date Palms.  When we see older fronds drop like this, it is concerning, because older frond drop is a classic indicator of Lethal Yellowing (LY).  Fortunately, this isn’t LY.  The drop is pretty uniform around the trunks of the palms, so we can rule out wind damage (which has been very prevalent in eastern Palm Beach County recently).  Since the fruit are still attached, we can rule out LY.  If we saw palms that looked like these, and the dropped fronds were turning brown, paired with a sudden drop of the fruit, it would possible indicate LY.

Split Canopy on Date Palm. UF Laura Sanagorski

This look is referred to as “split canopy” and tends to occur when the Date Palm is fruiting.

We don’t have a full explanation of this, and it doesn’t happen to all fruiting Date Palms, but it may have to do with the weight of the fruit.

You can learn more about lethal yellowing through Dr. Nigel A. Harrison and Dr. Monica Elliott’s publication: Lethal Yellowing (LY) of Palm.

Wind Damage in the Landscape

I’ve gotten a lot of calls over the past few weeks about plants that are dying or burning up “suddenly” in the landscape.  The good news is that it is very likely wind damage.  If you remember, we had some incredibly windy days just over a month ago.   The persistent wind paired with salt spray off of the ocean caused salt and wind burning all over landscapes in Palm Beach County.  A lot of our landscape plants are just showing their stress now.  Most will recover quickly.  The telltale sign of the wind and salt damage we’re currently seeing is that the majority of the damage is on the portions of the plant exposed to the east.

Windburn on Jasmine. UF Laura Sanagorski

 The Jasmine, above, located on a property near the beach, is showing wind-burning on its eastern side.

UF Laura Sanagorski

The Beach Sunflower, above, is showing stress from the heavy winds a few weeks ago.  Beach Sunflower is a beach plant, and it is still susceptible to wind damage.

UF Laura Sanagorski

The Crown of Thorns, above, was partially defoliated by the winds.  

If you see signs like the ones above, don’t worry, its not a new disease- just a delayed response over some severe weather.  Keep plants healthy by ensuring that they aren’t over- or under- watered or fertilized, and minimize other stresses as much as possible, and they’ll be back to normal soon.