Yard & Garden Line News Volume 2 Number 3                                                              March 1, 2000

Features this issue: Apple Rootstocks: a Primer Weird Weather? Snow Molds in the Home Lawn Dormant Season Fungicide Application The Disease Triangle Snow Fleas Boxelder Bugs and Ladybird Beetles They Really Are Coming Out of the Woodwork: Powderpost Beetles Editorial Notes

Apple Rootstocks: a Primer
Doug S. Foulk, Extension Educator, Ramsey County

Apples. Photo credit:  U of MN Extension Service

When you purchase an apple tree, you are actually buying two genetically different plants grafted together. The ‘scion’ is the upper portion of the tree, the part that provides you with the type of fruit you want. ‘Haralson’ and ‘Honeycrisp’ are examples of scion cultivar names. The scion is what people normally pay attention to when buying an apple tree.

However, the scion you select is grafted to another genetic individual—the rootstock. You should pay as much attention to which rootstock you purchase as you do the scion. The rootstock you choose will contribute to mature tree size, anchorage, hardiness, adaptability to different soil types, the number of years required before fruit is produced, and ease of care.

Horticultural professionals recognize at least three rootstock categories: standard, semi-dwarf and dwarf. ‘Standard’ trees reach their full size potential, depending upon the vigor of the scion cultivar and the environment in which the tree is grown; mature size can be highly variable between different trees and locations. In most parts of Minnesota a standard rootstock results in an apple tree that grows to 16 or more feet tall.

A semi-dwarfing rootstock produces a tree between about 10 to 15 feet tall, again dependent upon the vigor of the scion cultivar and the environment in which the tree is grown. Trees grafted using semi-dwarfing rootstocks are often sold in nurseries as ‘dwarfing rootstocks.’ However, a true dwarfing rootstock generally restricts tree height to ten feet or less.

The apple tree you plant is comprised of a scion and rootstock grafted together. Note the swelling, which is easily visible at the graft union. Illus. credit: U of WI

Planting an apple tree grafted to a dwarfing rootstock has many advantages. Because tree size is so much smaller, a gardener can more easily fit one onto an urban or suburban lot. On a site with enough room for a full sized tree, using a dwarf tree instead allows one to grow up to four trees where only one standard tree would fit. You can then enjoy many different apple tastes and textures from your yard instead of just one.

Because a dwarf apple tree only grows six to ten feet tall, pruning, pest control and harvest are much easier. Few people enjoy working on ladders! And finally, a dwarf tree takes only two to three years to bear fruit in most cases. A standard apple tree requires five to seven years.

There are, of course, advantages to standard sized apple trees as well. A standard sized tree is generally longer lived than a dwarf tree and can likely be passed along to the next generation. Standard rootstocks also are generally more winter hardy, more tolerant to wetter and drier soils and are better anchored than are their dwarfing brethren. If these factors are of primary importance to you, and you have the space, then a standard sized apple tree may be your best choice.

So how do you know what rootstock you’re getting? In many cases you don’t. Some nurseries only mark their trees "standard" or "dwarf" and that’s a shame. Individual rootstocks each have their own set of advantages and disadvantages, and you should be able to put as much thought into the choice of a rootstock as you do the scion. In addition, a tree labeled by rootstock provides the consumer with a greater idea how to handle the tree and therefore a greater chance of success.

Fortunately, many nurseries, both local and mail order, are beginning to list the specific rootstock used in grafting. The following list of rootstocks should give you some idea what options are available and how they compare. Some are commonly available, while others will require some seeking out.

SEEDLING: A ‘seedling’ rootstock is actually grown from the seed of an apple, often McIntosh or another common, hardy cultivar. Although one doesn’t know exactly what one is getting with a seedling rootstock—every single seed is a genetically different individual—hardiness, anchorage and adaptability to different soil types are generally excellent.

MM.111: This rootstock, sometimes termed ‘semi-dwarfing,’ other times ‘semi-standard,’ produces a tree about 80 percent the height of a standard tree. In many areas of Minnesota, this can work out to roughly a 14-foot tree. MM.111 is an extremely hardy, well-anchored rootstock that can withstand drier soil conditions. An excellent choice especially for northern or western parts of the state.

Apple trees grafted to M.9, Bud.9 or the Polish series require staking for the life of the tree. The stake should be tall enough to support the tree’s full height. Illus. credit: Ministry of Ag & Food, Canada

M.7a: (also: EMLA 7) This semi-dwarfing rootstock produces a tree 10-12 feet in height. Trees planted on M.7a generally do not require staking, except on windy sites. M.7a would seem to be a good compromise between the standard and truly dwarfing rootstocks; however, it is unfortunately the least hardy of the lot and has a tendency to sucker profusely. There are better choices.

M.26: (also: EMLA 26) This dwarfing rootstock produces a tree 8-10 feet in height. Trees planted on M.26 generally require staking for the first few years of growth, or on windy sites, for the life of the tree. M.26 is reliably hardy, does not sucker as much as M.7a, but is especially susceptible to fire blight, a disease that can shorten the life of the tree if not carefully managed. Fruit is produced very early in the tree’s life, within two to three years from planting.

M.9: (also: EMLA 9) Producing a tree 6-8 feet tall, this rootstock requires staking for the life of the tree. Fruit is produced within two to three years. Although hardier than M.7a, this rootstock is as prone to suckering as its larger cousin and is as susceptible to fireblight as M26.

Bud.9: Short for Budagovsky 9, this rootstock from the former Soviet Union holds promise for use in the upper Midwest. Trees grafted to Bud.9 grow to 6-8 feet tall, produce fruit early and generate few suckers. Hardiness thus far has been excellent, although further testing is needed. Trees on Bud.9 require staking for the life of the tree.

You may find apple trees available on other hardy rootstocks as well, including Antonovka (100% standard size) and the "Polish series" of rootstocks, including P. 2 (35-40% s.s.) and P. 22 (25-30% s.s.). No matter which scion/rootstock combination you choose, knowing which rootstock you have and understanding its characteristics will improve your odds of enjoying a productive, healthy, well-placed tree for years to come.

Weird Weather?
Beth Jarvis, Yard & Garden Line Project Coordinator

New gooseberry. Photo credit: Nat'l Clonal Germplasm Repository

What to do? What to do??

Thanks to an unseasonably warm February, the gardening bug has already bitten some folk pretty hard. But, what can you do outdoors?

With "tropical" highs in the 40s and flirting with 50s, pruning apple trees should be at the top of your list. It's great weather to get out there and prune. You'll do a better job when you're not freezing and the branches are no longer brittle from cold. Hawthorn, mountain ash, cotoneaster, pear and crabapples should be pruned now, also as they too share apple's susceptibility to fireblight.

Black knot infections should be pruned out of plum and cherry trees now, too.

Since blueberries set fruit on wood less than five years old, removing the greyer,corkier stems will clear out older wood. It's easy to do now while the plants are leafless. As with most pruning, remove no more than 1/3 of the plant. For additional info, see: Blueberries for Home Landscapes

If your raspberry patch hasn't been cleaned out in a few years, go out and snip out dark brown canes. Clear-cutting the raspberry patch isn't a good idea because June-bearing raspberries grow vegetatively the first year and set fruit the second. Fall bearing set some fruit in the fall and produce a second crop the following summer. An easy way to tell if your canes have reached the end of the line and can be cut out is to check for side branches. Side branches all along the stem indicate a summer crop has been produced. Clip these canes off at the ground. If you only see branches on the top 12 inches of the stem, that's from the fall crop and the canes will still produce a summer crop. For more info, see Raspberries for the Home Garden.

Gooseberries and currants produce fruit one one, two and three year old wood. Now is an excellent time to prune out older growth, removing no more than 1/3 of the thickest stems. Click on Currants and Gooseberries in the Home Garden.for additional information.

Smokebush. Photo credit: Beth Jarvis

And of course, if your elms and oaks need to be pruned, do it NOW. Don't put it off for later. The beetles that spread Dutch elm and oak wilt become active when the weather's consistently as warm as 50.

Maples including boxelder, black walnut, butternut, birch, ironwood and blue beech produce heavy sap flows in spring and may "bleed" after spring pruning. This sap loss is largely unsightly and doesn't harm the trees. Prune after the leaves are fully expanded to prevent "bleeding" if you wish.

Shrubs that can be pruned now include: alpine currant, barberry, buffaloberry, burning bush, dogwood, honeysuckle, ninebark, peashrubs, purpleleafed sandcherry, smokebush and sumac. Pruning Trees and Shrubs contains diagrams and more complete plant lists.

If you have sandy soil and the frost has gone out of the ground, please water your evergreens. In much of the state, evergreens went into winter drought-stressed. Watering now will alleviate some stress. If you're not sure if the frost is out, try jabbing the ground with a pitchfork or shovel. Clay soils should warm up and dry out before trees need water.

Parrot tulips covered in snow. Photo credit: Beth Jarvis

Hold off on raking the lawn until the soil is dry and no longer feels spongy underfoot. Crabgrass preventer should be applied during first two weeks of May in the Twin Cities. (And a week earlier for folks 100 mi. south of the Cities and a week later for folks 100 mi. north.) If the weather continues warm, late April application may be appropriatefor the Twin Cities area. Unfortunately, we see some being applied as soon as early April.

And the spring bulbs??? The cooler the soil is kept, the more slowly they'll emerge. A March snowstorm, however, shouldn't be grounds to panic as snow is a wonderful insulator..

Test your diagnostic savvy monthly with Disease Watch at: http://www.extension.umn.edu/projects/yardandgarden/Plpa.htm
Snow Molds in the Home Lawn
Chad Behrendt, Extension Plant Pathologist

Snow mold on lawn. Photo credits: Plant Disease Clinic

Closeup.

Now that the snow has begun to melt and the temperatures have begun to rise, snow mold fungi may appear in the home lawn. These fungi become active at temperatures near freezing (32°F), and typically develop under a blanket of snow on unfrozen ground. Snow mold fungi may continue to infect home lawns after the snow melts, as long as conditions are cool and wet. As the temperature begins to rise and the lawn begins to dry, snow mold fungi become inactive. Therefore, there is no need for chemical controls. Simply rake the infected area to enhance drying, and wait for warmer, drier temperatures. Proper fall lawn care next year will help alleviate future problems.

There are two different types of snow mold, pink snow mold and gray snow mold, each caused by a different fungus. Snow mold fungi typically appear within the home lawn as circular straw-colored patches of grass. These patches usually appear matted and are often covered by white, pink, or gray fungal growth. Occasionally, very small mushrooms (fruiting bodies of the fungus) will appear emerging from infected turf.

Dormant Season Fungicide Application
Chad Behrendt, Extension Plant Pathologist

Black spot on rose. Photo credit: Chad Behrendt

As we move into March and out of dormancy, prepare to apply dormant season fungicides. Fungicides used during dormancy, mainly lime sulfur, should be applied before the buds begin to swell or leaves emerge. This time fluctuates from year to year, but is often near the end of March. A few of the common diseases controlled with lime sulfur include:
· black spot on rose
· black knot of cherry
· cane diseases of raspberry
· plum pockets

The Disease Triangle
Chad Behrendt, Extension Plant Pathologist

Disease triangle Illus. credit: Chad Behrendt

The disease triangle or plant disease triangle is a visual diagram used to explain the development of plant disease. In order for disease to occur a pathogen must interact with a host plant. If these two components do not come into contact, disease cannot develop. However, the development of plant disease is further regulated by environmental conditions, with ideal conditions favoring the development of disease and unfavorable conditions deterring disease. These three components make up the plant disease triangle:

Host: A host or host plant is the plant infected by the pathogen. There are different levels of susceptibility, which include immune, susceptible, and resistant.

Immune means that the host plant cannot be infected. Susceptible means that the host plant can be infected. Resistant means that the host may or may not be infected and that the host plant is able to prevent the pathogen from completely killing the plant. In other words, the plant may be infected, but at the time of infection the host plant produces inhibitory/defense compounds that kill or prevent the spread of the pathogen.

Pathogen: A pathogen is a living organism, which infects a susceptible host. The severity of disease is regulated by the virulence of the pathogen, with the level of virulence determining the aggressiveness of the pathogen.

Environment: Environmental conditions that favor the development of disease are those conditions that encourage the growth and virulence of the pathogen and discourage the growth and defense reactions by the host.

Since all plants can be considered hosts and a number of pathogens survive in the landscape, it is usually the environment that regulates the severity and incidence of disease. However, the severity and incidence of disease is also determined by the susceptibility or resistance of the host and the virulence of the pathogen. The two most important environmental factors regulating plant disease are temperature and moisture, with moisture levels being critical. In general, cool moist conditions are going to favor the development of disease, while hot, dry conditions will deter it.

Get the low down on this month's insect pests at Insects http://www.extension.umn.edu/projects/yardandgarden/Ent.htm
Snow Fleas
Jeffrey Hahn, Assistant Extension Entomologist

Snow flea Illus.: U of M Ext. Svc.

Small, dark, jumping insects found on snow in late winter and early spring are snow fleas, a type of springtail. Although both fleas and springtails are small and can jump, they are actually quite different insects. Springtails feed on decaying plant material and are extremely abundant in the soil and leaf litter. They do not bite people or transmit disease.

Snow fleas hibernate in the soil during winter. When the weather starts to warm up and the snow starts to melt, these springtails ‘wake up'. Because they have a higher tolerance for colder temperatures than most insects, they are able to move up through breaks in the snow, often occurring in large numbers. Despite their abundance, snow fleas are harmless and are just a curiosity.

Boxelder Bugs and Ladybird Beetles
Jeffrey Hahn, Assistant Extension Entomologist

Boxelder Bug

Japanese Ladybird Beetle Photo credits: Jeff Hahn

People are finding all kinds of insects in their homes that were previously sleeping away the winter. They most common have been boxelder bugs and multicolored Asian ladybird beetles which often occur in large numbers. This is directly attributable to the mild weather we have experienced recently. The insects that are being found now actually entered buildings last fall as they sought protected places to overwinter. They often ended up in wall voids, attics, and other unheated sites where they remain until spring.

However, warm winter days can fool these insects into thinking that spring has arrived. As they become active, many move into the interior of homes and become trapped. These insects are often found by windows and moving slowly. Despite the circumstantial evidence, these insects are not laying eggs indoors. Boxelder bugs and multicolored Asian ladybird beetles are harmless and short-lived indoors.

Unfortunately, once these insects get into walls and attics, there is not practical solution to prevent their entry into homes. The best control is to vacuum or in some other way physically remove them as they are seen. If insects are an ongoing problem during late winter and spring, the best time to control them is during late summer and fall. By sealing cracks and other spaces and the judicious use of insecticides, you can prevent many of these nuisance insects from entering your home.

They Really Are Coming Out of the Woodwork: Powderpost Beetles
John F. Kyhl, Entomology Technician

Adult powderpost beetle. Illust. credit: Ohio State University

Adult powderpost beetle. Illust. credit: U of MN Ext Svc.

Powderpost beetles are insects that damage wood in service. These beetles get their common name because they produce fine, powdered dust as they feed. The sawdust of true powderpost beetles has the consistency of talcum powder with no grittiness. Adults emerging from the wood leave small, circular exit holes (2-3 mm diameter). In many cases, the holes can become so numerous that the damage looks like it was created by a shotgun blast .

Adult powderpost beetles are small (1/32-1/4":), slender, reddish-brown to black in color . They are similar in appearance to some beetles that infest stored food (e.g., flour and grain beetles). Powderpost beetles infest hardwoods with large pores, such as oak, walnut, ash, mahogany, and hickory, but can also attack bamboo even though it is not wood. Softwoods like pine, spruce, and fir are not susceptible to powderpost beetles. Powderpost beetles can infest any wood product from flooring and furniture to carved figures, decorative wreathes, and picture frames.

The first step in controlling powderpost beetles is to determine whether the infestation is active. Vacuum all dust from the wood, and examine the area a week later. If the infestation is active, new holes and fresh sawdust should be visible. You may need to repeat this to be sure that you are not just seeing old dust that has been dislodged from vibrations. If there is no evidence of an active infestation, then no control is necessary.

The best control method for an active infestation depends upon the type and size of the wood product involved. Beetles in small pieces of wood can be killed by freezing at or below 0º F for four to seven days. You can also heating small wood products at 120º F for 2 hours to kill all stages of the beetle. Both methods must be used with caution, as damage to the wooden item may result. You can also apply paint or a wood preservative to help prevent powderpost beetles from laying eggs in the pores of the wood.

When this is not practical, consider using a residual insecticide. These products are only available to licensed applicators. Borates must be applied to unfinished wood, while other residual insecticides, such as synthetic pyrethroids, can be applied to finished wood. These insecticides do not affect powderpost beetles already in the wood; it only controls the beetles as they exit the wood and as they attempt to return to lay eggs.

Editorial Notes

Last month I mentioned my snowdrops were due to be blooming in late March. Well, they're a month early and the crocus are right behind. Scarlet elderberry, seemingly the first shrubs to break bud in spring, started to leaf out on campus on 2/28.

With the early start our garden season appears to be getting, here are some websites that you may find interesting.

The first one is the Sustainable Urban Landscape Information Series. This site contains everything from landscape design planning and lawncare to both herbaceous and woody plant selection databases. The URL is:http://www.sustland.umn.edu


For pesticide info, for both home owner and professionals, check out:

http://www.crc.agri.umn.edu/~mnhelps/

There's also some very interesting reading at Forest Products website. To get there from here, click on: http://www.cnr.umn.edu/FR/extension/

In April, we'll resume publishing two two issues per month. I'm hoping to have an article on potatoes for one of the April issues. Mary Blickenderfer, shoreline vegetation and landscaping specialist, will be sharing some ideas on landscaping around ponds in another issue.

I'm always looking for article ideas and have received some intriguing story suggestions. So, don't be shy!! Your story suggestions are most welcome!

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.

For plant and insect questions, visit http://www.extension.umn.edu/askmg. Thousands of questions have been answered, so try the search option in the black bar at the top left of the board for the fastest answer.

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
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Happy gardening!

Beth Jarvis
Yard & Garden Line Project Coordinator

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