Yard & Garden Line News December 1, 2002

Yard & Garden Line News
Volume 4 Number 18 December 1, 2002

Features this issue:
Why Do Ericaceous Plants Grow in Acidic Soil?
A Grab Bag of Seasonal Tips
Taking My Work Home with Me: Lily Anthracnose (Colletotrichum spp.)
Unwanted Guests During the Holidays: Insects and Real Christmas Trees
Editorial Notes

Why Do Ericaceous Plants Grow in Acidic Soil?
Michael Long, graduate student

Orchid Lights, a U introduction Photo credit: Michael Long

Foundation lime changes soil pH. Photo credit: Michael Long

Waterlogged azalea Photo credit: Michael Long

Iron chlorosis on azalea Photo credit: Michael Long

The Ericaceae (heath) family, which includes azaleas, rhododendrons, blueberries, cranberries and heaths, has long been recognized as very tolerant of acidic soil. Most members are slow-growing, small-sized plants that have low fertility requirements (aside from copper, sulfur and especially iron) and should not be over-fertilized. When ericaceous plants are grown on alkaline soil (or near foundations of homes where lime leaches into the soil raising the pH), iron deficiency can become apparent by a foliar condition called chlorosis in which the leaves take on a pale or yellow color, sometimes contrasted by dark green venation.

Soil pH is a logarithmic function of the concentration of hydrogen ions present in the soil. It is measured on a 1 to 14 scale where a pH less than 7 is acidic (e.g., vinegar), at 7 is neutral (e.g., pure water) and more than 7 alkaline (e.g., baking soda). The average gardener is already aware of the drastic effects pH has on the color of hydrangeas, for example. Dr. Carl Rosen explains more about pH in the August 2000 issue. http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Aug0100.html#soil

Globally, approximately 31 % of arable land is strongly acidic. In the United States, the southeast is considered to have very acidic soil. This area contains many of the finest azalea and rhododendron gardens.

Most plants grow in a pH range between 6 and 7, where most nutrients are available for plants. However, as pH decreases (falls below 7), the availability of iron increases and calcium and magnesium decrease, while increased levels of manganese, aluminum and hydrogen ions become more toxic to plants.

To deal with these nutrient problems at low pH, many acid-tolerant plants have mechanisms to ensure efficient nutrient acquisition while dealing with toxicities. Hence, these plants were able to evolve on soils that would otherwise be lethal to plants.

Iron When soil becomes more acidic (lower pH), iron is more easily solubilized and absorbed. Ericaceous plants may have high requirements for iron and/or inefficient acquisition mechanisms to obtain it, so the more acidic the soil, the more iron that the plant can potentially take up.

Calcium and Magnesium As the pH falls, so do levels of usable forms of calcium and magnesium; however, the magnesium requirement of rhododendrons is low. Some ericaceous plants have mechanisms to improve calcium absorption.

Toxicities At low pH, the uptake of manganese increases to the point that toxicity occurs in many plant families. The Ericaceae, however, have an apparently novel mechanism to deal with this toxicity. Excess manganese is sequestered in leaf compartments not involved in plant metabolic processes, while some necessary manganese is used by the plant. Too much manganese can lead to an iron deficiency. Hence, plants growing in acidic soils high in manganese may also exhibit iron chlorosis symptoms.

Unlike manganese, aluminum is not an essential nutrient and its toxic absorption at low pH is blocked in the Ericaceae. Toxicity associated with hydrogen ions themselves may occur but this topic is largely unexplored.

Other Factors:

Complex interactions occur between and among the aforementioned compounds and other compounds.

A special group of chemicals called phenolics are present in very high concentrations in some ericaceous plants and may serve to block a phytotoxic form of nitrogen. Phenolics may also help to stabilize plant cell walls when complexed with silicon, thereby trapping manganese and other toxic compounds more effectively. However, phenolics themselves may also be phytotoxic (toxic to plants).

Many ericaceous plants are able to lower the pH around their roots by acidification. Hence, iron and phosphorus absorption, for example, can be increased around roots. Some beneficial micorrhiza may not be able to survive at low pH and would not be available to acidic-growing plants.

(See Mycorrhizae--the Friendly Fungi, by Rhoda Burrows, Dept. of Plant Pathology in the Oct. 1, 1999 Yard & Garden Line News, http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Oct0199.html)

Ericaceous plants tend to have shallow, fibrous root systems without root hairs and tend to grow best in well-drained, acidic soil. When azaleas are grown in waterlogged conditions, leaf scorch and death are possible and may be confused with underwatering. When exposed to adverse soil types, new plants may not grow much out of their initial root ball.

Further information
Refer to the following references for more comprehensive information on soil acidity mechanisms and to the appendix of BU-1731-F Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants for recommendations on pH levels of some plants.

References
Carver, B.F. and Ownby, J.D. 1995. Acid soil tolerance in wheat. Advances in Agronomy. 54:117-173.
Korcak, R.F. 1988. Nutrition of blueberry and other calcifuges. In Horticultural Reviews. Ed. Janick, J. 10:163-227.

A Grab Bag of Seasonal Tips
Deborah Brown, Extension Horticulturist

Photo credit: Deb Brown

Arbor Lights Photo credit: Deb Brown

Paperwhites. Photo credit: Netherland Flower Bulb Info Center

* If you want a Christmas tree that's absolutely fresh, and has not been sprayed with "cosmetic" green vegetable dye, cut down your own at one of Minnesota's "U-pick" Christmas tree farms. It can be a fun and rewarding family excursion provided you dress for the weather – and don't get carried away and come home with a tree that's too tall for your living room. (They look smaller, growing in a field, than they will at home.)

* Though adding special preservative to the water in your tree stand won't hurt anything, it's really not necessary. Fresh cut trees take up a huge amount of moisture in the relative warmth of your home. The most important advice is to saw off two or three inches from the bottom of the trunk before plunging it into a large container of warm water, then check several times daily the first few days to make sure the container is full. The tree will slow down after a while, but you still should check at least twice a day. When there's no more water, the bottom of the trunk will seal over, leaving the tree unable to pull in additional moisture. Then it quickly becomes a fire hazzard.

* Christmas trees, along with swags, wreathes, and other holiday decorations made from real conifers may have spider mite eggs, so small they're invisible to the naked eye. Though these eggs may hatch because of the warmth indoors, they can't fly. Be careful that you don't inadvertently move the eggs or the mites to your houseplants by placing them too close together or touching one, then the other, as part of your regular houseplant care routine. Once the holiday greenery is gone, keep a close eye on your houseplants so you can detect any mite infestation that may have developed.

* To keep your poinsettia looking great for a long time, place it near a window so its leaves are exposed to bright light. It needn't be direct sunlight, but direct sun won't hurt any indoor plant this time of year. Of course you can put it wherever it looks best when you're expecting company. Just return it to a brighter location the next day. Check the soil frequently, and water thoroughly as soon as it no longer feels moist to the touch. If your poinsettia has a decorative foil or plastic covering over its pot, slice some holes in the bottom, then place it on a plate or tray to catch excess moisture that would have been trapped.

* Two must-see plant displays for the holidays: The traditional poinsettia show in Como Park Conservatory's sunken garden room (plus blooming plants scattered throughout the conservatory) and the outdoor Arbor Lights display at the Minnesota Landscape Arboretum. Instead of tiny twinkle lights on conifers, the Arboretum has used dramatic spotlights tucked among the branches of large shade trees to highlight them against the pitch black winter sky. Breathtaking!

* Both amaryllis bulbs and paperwhite narcissus bulbs make great holiday gifts because their flowers last well indoors. However, amaryllis bulbs may be made to bloom repeatedly for years, whereas paperwhites should be tossed out once they finish blooming the first time. Even if they were planted outdoors the following year – which is not recommended for forced bulbs because they rarely gather and store enough energy to bloom well a second time – paperwhite narcissus are not winter hardy here.

* Last, but not least, if you need some last minute gifts for gardening friends and relatives, please don't forget our Minnesota Gardening calendar. It's attractive and useful – and inexpensive to mail to folks who've moved away. The calendar is available at book stores, garden centers, and county extension offices. You can also order it directly from the University of Minnesota by calling (612) 624-4900 in the Metro area, or toll-free (800) 876-8636 in the rest of the state.

Taking My Work Home with Me: Lily Anthracnose (Colletotrichum spp.)
Janna Beckerman, Extension Plant Pathologist

Elliptica lesions, and spreading blight are symptoms that are found in other lily diseases. Photo credit: Janna Beckerman

Lilies are tremendously popular perennial plants. Their popularity stems, in part, to their beautiful, colorful flowers, fragrance and ease of care. The most popular of lilies are the Oriental and Asiatic varieties, followed by hybrid and species lilies. The versatility of lilies makes it surprising that they are so underutilized, considering that they have such diversity in heights, flower shapes, colors, and bloom times. Some people even consider them to be generally trouble-free-or are they?

Lilies, whether grown in the greenhouse, as a houseplant or outdoors, are subject to a number of serious diseases including tulip break virus, Pythium root rot and Botrytis. All of these diseases generally warrant disposal of the infected plant. Little is written about anthracnose of lily. So, when the disease appeared in my garden, I did what any good plant pathologist would do: Watch its progression.

This disease began as small, oval lesions appearing on the lower leaves. Examination with a hand lens did not reveal setae, the black, horsehair-like structures associated with Colletotrichum infections. However, incubation and microscopic examination revealed infection by Colletotrichum, including the development of setae. That said, several of my colleagues, when reviewing the photos alone, suggested Botrytis elliptica, a more common pathogen of lily with similar symptoms. In hindsite, I made a critical error in not researching the disease more thoroughly. Had I done that, I would have realized that Colletotrichum on lilies was not well described. This would have allowed me to do a more thorough investigation on this disease, including Koch's Postulates (http://www.sci.wsu.edu/bio/micro310koch.html).

The disease triangle models the interactions between the host, pathogen and environment and is a way to understand how these three factors result in disease development. The infection process was exacerbated by the extremely wet weather, which favors both Botrytis and Colletotrichum infection. Quickly, a little infection became an epidemic about the time the plants were flowering. And by the time flowering had finished, there wasn't much left of the plants!

Despite losing more than half of the foliage, the lilies still
flowered.

Plants were completely killed by mid-August. Photo credit: Janna Beckerman

What should I have done? Unfortunately, many people would suggest spray a fungicide. Daconil is one of the few fungicides labeled for home use*. And our wet weather would have necessitated more spraying (at least one time per week, but heavy rains would have necessitated even more frequent applications.) than I would be willing to undertake. Furthermore, spraying should have begun with the first few leaf spots, not after I lost half the foliage! Which leads us back to question: What should have been done? Or, perhaps a better question: Why did this happen?

The primary culprit to this epidemic is the weather. And, it goes without saying that we can't do much about the weather. But, if you look closely at the pictures at left, you'll notice that the plants are spaced closely-probably too closely together, preventing airflow, and increasing the amount of time the leaf is wet. This situation encourages infection of the plant and spread of subsequent spores produced by the offending fungus. Increasing plant spacing would be one strategy that should be implemented for good plant disease management.

Not obvious from the pictures is the sheer number of lilies that are in the garden-multiple bulbs of at least 13 varieties that I could find the plant tags! Planting too much of one type of plant sets the stage for epidemics like this to occur. Although different lilies gave different responses (the Oriental lilies were the least affected and the hybrid lilies the most severely affected), no lily cultivar demonstrated resistance of any kind. Diversifying the cutting garden to include other types of plants and prevent future epidemics would be a wise component to an integrated plant disease management plan. Unfortunately, the severity of the epidemic was such that many varieties were probably lost. They will be replaced with different perennials, not new varieties of lilies, to increase diversity and lower disease pressure.

Finally, one of the most important aspects of managing this disease is sanitation: All infected plant material was removed and disposed of. That said, the inoculum levels are so high, the stage is set for a new epidemic next year-assuming any of the lilies survived! For this reason, the fungicides have been purchased, and this section of the garden is now an experiment for next year. But isn't that what all gardens are?

A special thanks to Gary A Chastagner, Professor of Plant Pathology at Washington State University's Research and Extension Center in Puyallup, WA.

*Organic fungicides (Bordeaux mixture, Soap-Shield®) labeled for management of this disease include copper as an active ingredient. However, weather conditions would have required their reapplication after every rain.

Its time to store your chemicals!

Don't forget to properly store your garden chemicals! It doesn't matter if you are an organic gardener or a nozzlehead, be sure to store your chemicals in a place where they will not freeze. Make sure that packages are well labeled, and then put them in a place where children or pets cannot get them.

Please check out the new diagnostics web pages at
http://www.extension.umn.edu/projects/yardandgarden/diagnostics/

Unwanted Guests During the Holidays: Insects and Real Christmas Trees
Andy Birkey, Entomology Technician, Yard and Garden Clinic

Christmas tree lot. Photo credit: Jeff Hahn

Bringing a real Christmas tree into the home adds a natural touch to the holiday season, but sometimes too much nature arrives with the tree. Insects can become active and escape from a real Christmas tree. When they do occur, they are often too small or too few in numbers to be noticeable. They generally cause no harm in the home and should be considered nuisance pests. Some of these insects include:

· Adelgids attack the bark on pines, especially white pine. The adults are tiny with soft, cottony outer coverings that create a white, fluffy appearance on the stems of trees, which should be noticeable when purchasing a Christmas tree. Adelgids become active when the temperature reaches 50 degrees or when brought into the home. They are host specific and do not feed on houseplants.

· Aphids are small sucking insects that range from 1/16 to 1/8 inches in size. They may hatch from egg masses that were not pruned out. Since they are host specific, aphids that live on Christmas trees do not feed on houseplants.

· Spider mites often become active when the temperature rises, as it does in a home. They are very small, only about 0.5-1.0 mm long and may cluster together with some webbing present. They generally remain on the tree when active, but can become a pest on houseplants.

· Bark beetles feed under the bark of conifer trees. They can create a noticeable amount of 'sawdust' as they burrow out of the tree. Bark beetles that overwinter under the bark are usually 2-5 mm long and dark brown. They do not cause damage to furniture or household items.

· Spiders sometimes seek shelter under the bark of Christmas trees. They are rarely encountered but occasionally can be brought indoors on Christmas trees. Sometimes spider eggs are present on the tree and may hatch causing significant numbers. They are not dangerous to humans or pets.

Most retail Christmas tree outlets use mechanical shakers to remove dry and damaged needles. Shaking also dislodges many insects or spiders living on the tree. However, some may remain and be brought indoors. You also may not be aware of bark beetles and other insects that live inside the tree until after they emerge. Check over your tree once you bring it indoors and remove any insects that you find on the trunk, branches, or needles. If you miss some insects that emerge later, just physically remove them. A vacuum cleaner works great at eliminating these pests. Pesticides are NOT recommended and NEVER use flammable types of pesticides around conifer trees.

Get the low down on this month's insect pests at Insects http://www.extension.umn.edu/projects/yardandgarden/EntWeb/Ent.htm

Editorial Notes

Bartholdi Park Washington, DC Photo credit: Beth Jarvis

I spent some time in early November in Washinton, DC and Virginia. My slides aren't back yet so I can't show you shots of Lewis Ginter Botanical Garden in northern Richmond or any of the shots I took inside the new US Botanical Garden conservatory. The conservatory, which has opened within the last year, is very nice. In addition to Bartholdi Park (pictured), ground has been broken on a three acre national garden, directly adjacent to the conservatory. If a trip to DC isn't on your radar just yet, you can take several virtual tours of the conservatory and park.

In the Jan. issue, Charlie Rohwer will talk about the physiology of fragrance--why do flowers have a fragrance?

In a later issue, Doug Courneya, Regional Extension Educator, Olmstead County, will talk about buckthorn reduction. He says there's no such thing as "control" but we can reduce it.

Lee Frelich, of Forest Resources, gave an eye opening presentation on the dangers of exotic earthworms at the state Master Gardener conference in Sept. He's agreed write about it for the March 1 issue. Kathy Zuzek, from the Horticulture Research Center, will, hopefully, find the time to talk about the latest in her rose research.

By then it will be spring! Right now, it's painful to hear a weather person say there are 200 some days left of the snow season.

Please feel free to cut and paste any of the articles for use in your own newsletters. All we ask is that you give our authors credit.

Back issues Yard & Garden Line News are on the Yard & Garden Line home page at www.extension.umn.edu/yardandgarden/. Our home page has clickable links to most of the components of the Yard & Garden Line, such as Bell Museum of Natural History, INFO U and the Soil Testing Lab.

Deb Brown answers gardening questions on Minnesota Public Radio's (MPR) "Midmorning" program on the first Thursday of every month at 10 a.m. Katherine Lanpher hosts the program that is broadcast on KNOW 91.1 FM, and available state-wide on the MPR news radio stations.

If you have gardening questions, please call the Yard & Garden Line at (612) 624-4771.

If you would like to receive an e-mail reminder when the next issue of the Yard & Garden Line News is posted to the web, just send an e-mail to: listserv@lists.umn.edu (note: the second E in listserve is omitted), leave the subject line blank, then in the body of the message, type: sub yglnewslist or to unsubscribe, enter: unsub yglnewslist

Happy gardening!

Beth Jarvis
Yard & Garden Line Project Coordinator

Previous Page

University of Minnesota Extension Service Home Page
In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota county extension office or, outside of Minnesota, contact the Distribution Center at (612) 625-8173.

The University of Minnesota Extension Service is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation.