Bill Pramuk’s Blog

July 26, 2010

Trees and People
Bill Pramuk

A Problem with a Flowering Pear

Hi Bill,

I am concerned about my flowering pear tree. I just bought the home two weeks ago and a key selling point for me was the tree.  I didn’t notice the dead spots until I moved in.  On several branches all the leaves are dead and still hanging on.

Is this something that can be controlled or not?

Thank you,
RR

I had the opportunity to go see this tree, growing in a front yard, in a nice planned community in another city.  It is an ‘Aristocrat’  Callery pear (Pyrus calleryana ‘Aristocrat’) one of the most widely planted flowering shade trees in our region.  This specimen has a trunk diameter of 18 inches, which is quite large for this type of tree. With its strong looking trunk and symmetrical canopy, the tree fits nicely in the front yard and makes a handsome compliment to the home, except for the dead branches scattered through the canopy.

Looking both ways along the street I saw the same tree variety with the same problem in several other front yards.  It appears that all were planted about the same time when the development was built, about 25 years ago.

A close look at the tree and the dead branches revealed the  typical symptoms of fireblight:  brown dead leaves that remain on the tree, blackened stems and shriveled, dead branch tips. It is a bacterial disease that attacks many trees and plants in the rose family. Long term infection can kill pear trees.  The problem is made worse by persistent, warm, rainy spring weather and a supply of inoculum in neglected trees in a neighborhood.

The key question was:  Can the tree be treated and saved?  The answer was yes, since this tree was not too far gone.  But it will require a long term commitment to a care program, and it would help if the neighbors with the same problem made the same commitment.

The bacteria that cause the disease - Erwinia amylovora - are spread from cankers on infected limbs by various flying and crawling insects. Bees and splashing water can spread the bacteria from one flower to another.  The disease progresses from the flowers, or other openings in the plant downward, killing the stems and causing lesions on the bark.

Treatment for the disease is a management plan, not a cure.  Dead and dying stems must be pruned off about 8 to12 inches beyond any sign of stem discoloration, back to a healthy lateral branch, is possible.  Since the bacteria swim, the pruning should be done when the plants are dry to reduce the risk of spreading the disease.  Most references also recommend sanitizing pruning tools by keeping them wet with a disinfectant during the pruning process.  A handy one is a spray can of Lysol.  Interestingly, Dr. Robert Raabe, a plant pathologist at U.C. Berkeley  reported years ago that he deliberately attempted to infect plants with fireblight with “infected” tools and was unable to cause any infections.

Some references mention treatments for bark lesions on the trunk and limbs such as scraping away diseased tissue or painting them with various mixtures of chemicals.  I do not know of any professionals who currently uses those methods.

In addition to eradicative pruning to physically remove the bacteria from the tree, management must include a protective spray program.  One reference mentions that a dormant spray to control aphids can help reduce the spread of the disease.

The most commonly recommended and used protective spray is a weak mixture of a fixed copper compound sprayed at intervals throughout the the blooming period.  A tiny amount of copper inside an open flower is enough to stop the bacteria.

Unfortunately, ‘Aristocrat’ pear - one of the most widely planted varieties of Callery pear - is very susceptible to this disease.  There is no “immunization” or “induced resistance”, but a consistent management program can save trees that hold high value to their owners.

(Bill Pramuk is a registered consulting arborist.  Visit his website, www.billpramuk.com, email questions to info@billpramuk.com, or call him at 707-226-2884)

July 12, 2010

Trees and People

Bill Pramuk

Odd Trees

Hi Bill,
I was walking the new river trail in American Canyon last week and came upon these odd trees. I was wondering if you could tell me what they are, as well as what appears to be some kind of cactus growing with them.  Anything you tell me would be helpful.

MW

(Attached to that email were photographs of some very tall slender stems, perhaps 15 feet tall, with short branches  only on the upper half - the “trees” -  accompanied by short clusters of long, thick, fleshy, sharply pointed, blue-gray leaves - the “cactus”.)

Dear MW,

The “trees” are the flower spikes of the blue Agave (Agave americana),  commonly known as the “century plant”.  The “cactus” is the blue Agave plant itself, a succulent plant, but not a true cactus.

Locally, there are other varieties that have yellow and white striped leaves.  Once you know what they are and watch for them, you may notice that they are fairly common around Napa and Sonoma Counties.  Not native here, they were planted as sturdy  plants for summer-dry landscapes and, probably, as reminders of the homeland of the people who brought them here. There are lots of them growing wild along the shoulder of Milton Road in the Carneros area and many other locations around the region.

With a little digging I found that most of the 200-plus species of Agave are native to Mexico.  I recall seeing huge fields of the same or similar Agaves many years ago on a long bus ride from Mexico City to Puerto Vallarta.  Down there they are grown for the production of tequila. They have a long history, and pre-history, of cultivation and world travel.  Some were collected and taken back to Italy around 1561 where they thrived in the Mediterranean climate.

Being a fleshy, succulent type of plant that stores water in the leaves, Agaves are extremely resistant to heat and long periods of dryness. That characteristic not only allows them to naturalize in our summer-dry “Mediterranean” climate, but must  also help specimens survive when they are transported.

It is not likely that the local population of Agaves got started from seed.  There are localized wild colonies but no widely scattered, invasive seedlings. In nature, the flowers are pollinated only by a particular species of bat that is indigenous to the native region of the plant. To generate seed, nurseries that produce and hybridize Agaves as ornamental plants must utilize hand-pollinating techniques, while mass production is done by tissue culture (cloning).  On a smaller scale, Agaves are commonly reproduced by dividing the colonies and transplanting small plants.

Although I was not able to pin it down with specific references, I suspect that the old plantings of Agaves and prickly pear cactus (Opuntia ficus-indica), which are also pretty common around the region here in “Alta” California, and which travel well, resulted from transplanting by early settlers from Mexico.

The botanical name “Agave”, given by Linnaeus in the 1700‘s, is Greek for “noble”, probably referring to some of the species’ large, handsome, and bold structure.  The common name “century plant” refers to the long blooming cycle, supposedly once per century.  It’s not actually a century, but usually 10 years or more before the flower spike emerges on a young plant. The spikes start out looking like a giant asparagus spear, arising from the center of the rosette of leaves. They reach tree-like proportions, up to 40 feet tall, and persist for many years after the flowers have faded.

After the mature plant blooms one time, it dies, leaving little side shoots -”pups” - at the base, which then repeat the cycle.

(Bill Pramuk is a registered consulting arborist.  Visit his website: www.billpramuk.com, email questions to info@billpramuk.com, or call him at 707-226-2884)