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Extension > Yard and Garden News > April 2011

Saturday, April 30, 2011

Witches' Brooms on Landscape Shrubs

Michelle Grabowski, University of Minnesota Extension

Photo 1: Witches' brooms on willow.
M. Grabowski UMN Extension.

As spring slowly unfolds in Minnesota this year, many gardeners are anxiously watching the buds on their favorite landscape shrubs open up for the season. Some shrubs have one to several clusters of thin weak twigs arising from one point on a larger branch. These unusual growths are known as witches' brooms and they can form at the tip of the branch or lower down the stem. Many different problems can result in a witches' broom. Mites, fungi, aphids, salt or other damage to buds can all result in a proliferation of small branches, where only one should have emerged.

Another less known and little understood pathogen causes witches' brooms in Minnesota landscapes. Phytoplasmas are single celled organisms in a group known as fastidious bacteria. These tiny pathogens were unknown to scientists prior to 1967. They are very difficult and often impossible to grow in a laboratory and can only be seen with a powerful electron microscope.

As a result, little is known about phytoplasmas. Many of them do not even have proper scientific names (a genus and species) because they have not been properly described. Often they are categorized into groups of phytoplasmas that are believed to be related to one another. To truly identify a phytoplasma in a landscape shrub, DNA analysis needs to be done.

Phytoplasmas live in plant sap and interfere with photosynthesis, plant growth, development and seed production. Infected shrubs often have yellow, curled or distorted leaves.

Photo 2: Yellow leaves & dieback in a phytoplasma infected lilac.
M. Grabowski UMN Extension.

Branches are often stunted or unusually thin. Witches' brooms are very common in phytoplasma infected shrubs. The entire plant may be stunted and in very severe cases killed.

Phytoplasmas are carried from plant to plant by insects that feed on the sap. They can survive from year to year within the insect or the host plant. Once a shrub is infected, it cannot be cured. Some shrubs tolerate the infection; others decline for several years and then die. The diseases below are phytoplasma diseases that can occur in Minnesota.

Willow Witches' Broom

Witches' brooms in willows can be seen in willows growing alongside roads as well as in landscapes throughout the state. Black willow (Salix nigra) and pussy willow (S. discolor) are both susceptible to a phytoplasma in the aster yellows group. Willows infected with this phytoplasma have witches' brooms on one or more branches. These brooms often have stunted yellow leaves and may die during winter months. Exact identification of a phytoplasma requires a costly DNA analysis. Although the willows with witches' brooms in Minnesota have not undergone this analysis, phytoplasma is the most likely cause of the problem.

Photo 3: Brooms on phytoplasma infected lilac.
M.Grabowski UMN Extension.

Willows are considered tolerant of the disease and often continue to grow vigorously despite infection. In fact, it is believed that the willow is able to isolate the pathogen in the infected branch. The brooms themselves have a very high concentration of phytoplasmas, where as other branches in the same shrub are often completely free of the pathogen. Gardeners with infected willows should prune out any branches with witches' brooms.

Lilac Witches' Broom

Lilac witches' broom is caused by Candidatus Phytoplasma fraxini, a specific member of the aster yellows phytoplasma group that only infects lilac and ash trees. Over 20 species of Syringa are susceptible to lilac witches' broom. Common lilac (S. vulgaris) is tolerant and often shows no symptoms other than slower growth and shorter twigs. Japanese tree lilac (S. reticulata) and many hybrid lilacs are very susceptible to lilac witches' broom. These lilacs have yellow, distorted small leaves that often scorch brown on the edges by midsummer. Many tiny thin shoots form in clumps at the base of the plant. The shrub declines and is often killed a few years after the first witches brooms appear.

Dogwood Witches' Broom and Stunt

Witches' brooms caused by phytoplasmas have been identified on silky dogwood (Cornus amomum), gray dogwood (C. racemosa) and red osier dogwood (C. sericea). It is believed that the phytoplasma responsible for witches' brooms in dogwoods is a member of the aster yellows group. It is unknown how commonly this disease occurs in Minnesota.

Spring Pruning Tips for Woody Landscape Plants

Kathy Zuzek, UMN Extension Educator, Horticulture

Photo 1:Rodent damage girdled these stems.
K. Zuzek, UMN Extension.

Many shrubs are planted in landscapes for their wonderful floral display. One of the few shrubs that have bloomed to date during this slow-to-arrive spring is forsythia. With masses of yellow flowers, forsythias are wonderful plants in the spring landscape. But cultivars with appropriate flower bud hardiness for use in Minnesota are large shrubs with rapid growth rates. Their ability to quickly produce large quantities of upright and arching stems that tangle together to produce an unkempt-looking plant turns them into aesthetic liabilities in the landscape after blooming. Consistent pruning is the solution. Enjoy the floral display but remember that if you have reason to prune your forsythia, pruning should happen immediately after bloom.

Like many of our shrubs that bloom in early spring, forsythias produce flower buds on previous years' stems. To enjoy maximum bloom on these shrubs both this year and next year, it is important to prune these plants immediately after flowering stops this spring. This allows enough time for plants to produce stems during the 2011 growing season that will house next year's flower buds. A list of shrubs that bloom on previous years' wood can be found at

You may also need to prune shrubs this spring because of damage that occurred during winter thanks to rodents, rabbits, and low winter temperatures.

Mice and voles were busy under the snow line this winter stripping bark at the base of shrubs and young or thin-barked trees (Photo 1). Rabbits can create the same kind of injury by stripping bark above the snowline. If this feeding removed bark completely around stems and trunks, plants are effectively girdled and stems and trunks will die above the point of girdling.

Photo 2: Winter injury of rose canes.
K. Zuzek, UMN Extension.

Most deciduous shrubs have the ability to produce new shoots from their crowns or from roots. Prune off the girdled stems this spring and new shoots will replace the damaged stems over the next few years. On the other hand, the best course of action for a completely girdled tree is removal and replacement with a new tree. Many trees sucker below girdling damage. Many trees are also produced through grafting or budding onto rootstocks. Suckers from below the graft union that are trained into a new tree will give you a different (and probably undesirable) tree than the one you started with.

Rabbits will often clip entire stems off of deciduous shrubs as they feed during winter. If this browsing injury left you with irregularly shaped shrubs and stems of widely different heights, you may want to prune these plants to recreate their natural plant habit. Entire stems can be removed at the base to allow growth of new replacement stems from the crown or roots. Or stems can be pruned back to ¼" above a bud that will direct growth in a desirable direction as the bud expands and grows. Make this pruning cut at a 45° angle with the high point of the cut directly above the bud.

Winter injury should also be removed from woody plants this spring. Shrub roses are a prime example of this. Many repeat flowering shrub roses experience extensive cane mortality due to low winter temperatures. The injury is very evident in spring. Dead canes will be brown or black and there will be no actively expanding buds. Healthy canes can be green, red, or brown (depending on cane age and cultivar) and you will see buds along these canes actively expanding and growing in spring (Photo 2). Often low winter

Photo 3:Sap bleeding from pruning wound.
K. Zuzek, UMN Extension.

temperatures kill canes protruding above snow while the bases of canes covered by snow are not injured. Find an outwardly facing bud in the live portion of canes and prune canes back to ¼" above this bud using the 45° angle cut mentioned above. If canes are killed to the base, remove them completely and new canes will develop from the crown and roots of the plant.

As you prune woody plants in spring, you may see sap oozing from pruning wounds (Photo 3). This may continue for several days and happens to varying degrees on many trees and shrubs. It is most noticeable on trees because pruning wounds are larger and are at or above eye level. This oozing or "bleeding" occurs when increasing temperatures in spring activate enzymes that convert the starches that store energy in plants into sugars that can be easily transported throughout the plant. As starches are converted into sugar, water movement is initiated upwards in stem and root xylem tissue due to differences in sugar concentrations throughout the plant. Pressure increases in the plant and this pressure results in the "bleeding" from new pruning wounds. Bleeding can be unsightly but is not harmful to the plant. It actually signals active cell activity and a rapid pace of wound healing.

Photo 4: Oak wilt fungal spore mat.
J. O'Brien, USDA FS ,

Remember that oak trees should not be pruned in April, May, or June. Sap feeding beetles of the Nitidulidae family are the insects commonly responsible for long-distance movement of fungal spores that lead to new infection centers of oak wilt. The beetles are attracted to both fresh wounds (like a pruning cut) on oak trees and to the fruity smell of the fungal spore mats produced in April, May, and June on oaks infected with oak wilt (Photo 4). As they visit the mats, they pick up spores that can be transferred to a fresh wound on a healthy tree. As these spores infect the tree, oak wilt occurs.

What are your apples doing right now?

Karl Foord, UMN Extension Educator

What are your apples doing right now?
Apple trees have two kinds of buds; vegetative buds that only give rise to leaves and shoots, and mixed buds that give rise to flowers as well as shoots and leaves. The parts of the flower in their primordial state can be identified in the dormant mixed bud as seen in this cross section (Figure 1).

Figure 1.

Bud stages

Apple flower buds progress through a series of developmental stages. This begins with the Silver Tip stage where the bud has started to open but no green tissue is visible. The progression runs through to petal fall and the beginning stages of development of the apple fruit (Figures 2 - 10).

Figure 2 and 3.

Figure 4 and 5.

Figure 6 and 7.

Figure 8 and 9.

Figure 10.

Apple varieties can break bud at different times, and buds can be at different stages on the same tree. For example, the buds on my Haralson apple tree are almost all between ½ inch green and tight cluster. Only the leaves are visible but the tips of the leaves are red. My Honeycrisp has a few buds beyond the ½ inch green stage like the Haralson, but many of the buds are between silver tip and green tip.

Bud sensitivity to temperatures

As we move into May and watch our apple flowers develop, what temperatures should we recognize as damaging to apple flowers? The temperature at which flower buds are injured depends to a significant degree on their stage of development. Buds are most hardy during the winter when they are fully dormant. As they begin to swell and expand into blossoms, they become less resistant to freeze injury. The temperature sensitivities of the bud stages noted previously are listed in Table 1.

Table 1.

The month of May can bring some damaging low temperatures. On May 9 of last year a low temperature of 25oF was reached in many locations in Minnesota. The apple flowers of many trees were in full bloom and vulnerable to temperatures below 28°F. In some orchards the frost damage to the flowers was practically complete and the whole apple crop was lost except for some late flowers on Honeycrisp and a significant number of flowers on Sweet 16, a late flowering variety.

At the present developmental stage, apple flower buds in Minnesota could survive temperatures in the low 20's. Let us hope we have seen the last of the temperatures listed in table 1, for this year at least.

Photo credits:

Calendar: May 1, 2011

Echinacea purpurea 'Magnus'.
Julie Weisenhorn.
Most flowering annuals are available as sturdy seedlings at garden center and nurseries, but many will grow just as well or even better when you seed them directly into the garden. Zinnias, cosmos, bachelor's buttons, California poppies and marigolds are good examples of annuals that grow rapidly from seed. But if you want annuals for containers, buy well developed transplants that will look good the minute you pot them up.

Early May is a good time to plant grass seed, but for good results you need to rough up the soil first. Unfortunately, this exposes crabgrass and other weed seeds that will sprout right along with your new grass. To stop most weed seeds, apply a specially formulated version of pre-emergence herbicide right after seeding. The label must state clearly that it's meant for newly seeded lawns, otherwise it will kill desired grass seeds, too.

Attract butterflies to your yard by planting many good nectar-producing flowers. Include coneflowers, Russian sage, Joe-Pye weed, butterfly weed (Asclepias), beebalm, catmint, Mexican sunflower, and single or semi-double zinnias. Though butterfly larvae (caterpillars) may feed on your plants, don't use any insecticides in the garden. And don't put up "butterfly houses" unless you enjoy them as garden art... butterflies will never inhabit them!

The Yard and Garden News will be going back to a twice-monthly format for the summer! See you on the 15th!

Yard and Garden News Editor: Karl Foord
Technical Editor: Bridget Barton
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