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Extension > Yard and Garden News > July 2012

Monday, July 16, 2012

July 15, 2012

In this issue of the Yard and Garden News:

Aster Yellows common in Cone Flowers Throughout Minnesota

Michelle Grabowski, UMN Extension Educator

M. Grabowski, UMN Extension

Photo 1: Purple cone flowers on a plant infected with aster yellows

Many gardeners are noticing that their cone flowers (Echinacea spp.) are not so purple this year. In fact many flowers are partially to completely green and several have odd green growths sticking up from the center of the the blossom. These are symptoms of the disease aster yellows.

Aster yellows is caused by a tiny bacteria - like organism known as a phytoplasma. Phytoplasmas live with the phloem (nutrient conducting vascular system) of plants. The aster yellows group of phytoplasmas can infect over 60 families of plants including many common garden perennials, vegetables and weeds. Although many gardeners are noticing disease symptoms on coneflowers, infection of other plants such as cosmos, golden rod and carrot have also been reported.

Leaves of plants infected with aster yellows are often discolored yellow to red. The phytoplasma affects normal growth and development of the plant, resulting in clumps of weak shoots known as witches broom developing throughout the plant or on flower stalks. Flowers are often the most visibly affected part of the plant. Flowers often remain green and are distorted.

The aster yellows phytoplasmas are moved from plant to plant by aster leaf hoppers (Macrosteles spp.). These insects have sucking mouth parts and they inadvertently suck up phytoplasmas with the plant sap they are feeding on. From then, the phytoplasmas actually live and reproduce within the aster leaf hopper.

M. Grabowski, UMN Extension

Photo 2: Witches broom on the a goldenrod flower infected with aster yellows

Whenever the leaf hopper feeds, some phytoplasmas are released into the new plant, starting a new infection. Aster leaf hoppers do not overwinter in Minnesota. One possible explanation for the high numbers of aster yellows infected plants this year is that early warm weather allowed leaf hoppers to migrate into Minnesota very early in the growing season and infect garden plants at an early stage.

Unfortunately once a plant is infected with Aster Yellows, there is no way to cure it. Perennial plants may survive many years with the aster yellows phytoplasma and can serve as a source of the disease within the garden. The best way to deal with aster yellows is to remove infected plants. Plants infected with aster yellows can be thrown on the compost pile or buried because the phytoplasma will not survive once the plant dies. Unfortunately removal of infected plants is no guarantee that the disease will not return to the same garden. Infected leaf hoppers and infected perennial weeds can both be a source of new infections in the garden.

Welcome to Our New Turfgrass Extension Educator

Kathy Zuzek - Extension Educator, Horticulture

Extension would like to welcome Sam Bauer back to the University of Minnesota as a new Horticulture Extension Educator in EFANS. Sam will be starting July 16, 2012 and will be housed at the Regional Extension Office in Andover. Sam's primary responsibilities in horticulture are with consumer turfgrass. He will work closely with other Horticulture Extension Educators and with faculty in the Department of Horticultural Science. Sam received his B.S. and M.S. from the University of Minnesota in 2005 and 2011 respectively and is well versed on turfgrass issues facing the consumer. He brings with him tremendous international turfgrass experience and a solid understanding of how to connect with the public through social media. Sam can be reached at or (763) 767-3518.

Thursday, July 12, 2012

Variegated Cutworm Damage

Jeffrey Hahn, Asst. Extension Entomologist

Jeff Hahn

Photo 1: Variegated cutworm damage on hosta

A large flight of variegated cutworm moths moved through Minnesota as well as Wisconsin this spring. This was particularly noticeable when clusters of eggs were found on many buildings and other structures during May (see June 1, 2012 Yard and Garden News)

The result of this activity is now being felt in home gardens as many different herbaceous plants that are being damaged by their feeding. Unlike subterranean cutworms that many gardeners are familiar with, variegated cutworms are a type of climbing cutworm that will feed on the foliage of plants. They typically chew irregular holes between the veins on the leaves. They have also been known to bore into flower buds. Be careful not to confuse variegated cutworm feeding with slug damage which can look similar. Slug feeding usually results in more ragged, irregular holes but to be sure, you may have to catch the culprits in the act.

Variegated caterpillars are generally dark-colored, ranging from brownish to black. There are four to five yellowish diamond-shaped spots on the top of the body starting at the head. They may also have a dark-colored 'W' on top of its body near the posterior. Like other cutworms, variegated cutworms curl into a ball when they are disturbed. These cutworms are large when mature, growing to 1½ to two inches long.

Jeff Hahn

Photo 1: Variegated cutworm

The biology of variegated cutworms in Minnesota is not clearly understood. They apparently can overwinter in Minnesota either as pupae or larvae. However, most of them are probably carried up on the jet stream as adult moths and deposited into Minnesota during spring. They are reported to have two generations in the northern U.S. so we can expect to see them throughout the summer.

If you are experiencing problems with variegated cutworms, you have several options for managing them. You can try handpicking them. You might even be able to put out boards and trap some. If the problem is severe enough, you may resort to insecticides. Spinosad is a good option if you are looking for a low impact product. There are also a variety of residual insecticides to choose from, including permethrin, bifenthrin, cyfluthrin, and carbaryl.

A question that gardeners in northern Minnesota are asking is whether they will now start seeing this insect every year when they rarely or never saw it before. The good news is that the odds are in northern Minnesota's favor for not witnessing a repeat performance by variegated cutworms next year. We would have to experience the same perfect storm of weather conditions that allowed such a large number to migrate up to northern Minnesota and that is not likely.

Watch Out For Wasp Nests

Jeffrey Hahn, Asst. Extension Entomologist

Now is a good time to take a close look for wasp nests around your home. They are getting large enough to be noticed but have not reached their peak size yet. Wasps can nest in a variety of locations. Some species commonly nest under eaves of homes, the branches of trees and shrubs, and similar open, exposed areas. They also commonly nest in the ground, especially in old rodent burrows, as well as wall voids, attics, and other hidden sites. If you see any kind of persistent activity of wasps in a particular location, take a closer look to see if there is a nest involved.

Jeff Hahn

Photo 1: Wasp nests can be found in the most unexpected places

One wasp, the European paper wasp, is interesting because of its ability to construct small nests in many different, unusual sites. Just in the author's backyard, they have been found nesting in the tail pipe of a unused van and inside an unused bird feeder. Paper wasps typically nest on the underside of horizontal surfaces. However, European paper wasps have the ability to construct their nests at angles. This wasp is also somewhat unique because while other wasps do not reuse their nests, they frequently reuse them which can result in larger than normal sized nests (for a paper wasp).

If you do find a wasp nest, the particular control you use will depend on factors, such as where the nest is located, how close to human activity it is, and whether the nest is out in the open or hidden. Click here for more specific information on controlling wasp nests.

Quince Rust on Hawthorn Results in Strange Looking Fruit

Michelle Grabowski, UMN Extension

M.Grabowski, UMN Extension

Photo 1: Quince rust on a hawthorn fruit

Quince rust, caused by Gymnosporangium clavipes, can infect over 480 species of plants within the Rosaceae family. This includes serviceberry (Amelanchier spp.), hawthorn (Crataegus spp.), chokeberry (Aronia sp.), mountain ash (Sorbus spp.), apple and crabapple (Malus spp.). Quince rust frequently infects fruit and petioles, causing them to be swollen and deformed. At this time of year, the fungus produces short white cylindrical spore producing structures all over infected fruit. These spore producing structures open up to release bright orange powdery spores that give rust fungi their name. Infected fruit will die. Although odd looking, these infections are not harmful to the overall health of the plant.

Sunday, July 1, 2012

Flowering Plant Video Library - Prairie Plants II

Karl Foord - Extension Educator, Horticulture

Click on the link to see the video with host Dr. Mary Meyer, Professor of Horticulture

Purple Prairie Clover (Dalea purpurea)

Karl Foord

Photo 1: Purple Prairie Clover (Dalea purpurea)

False Indigo (Baptisia australis)

Karl Foord

Photo 2: False Indigo (Baptisia australis)

Prairie Dock (Silphium terebinthinaceum)

Karl Foord

Photo 3: Prairie Dock (Silphium terebinthinaceum)

Cup Plant (Silphium perfoliatum)

Karl Foord

Photo 4: Cup Plant (Silphium perfoliatum)

Harebell Campanula (Campanula rotundifolia)

Karl Foord

Photo 5: Harebell Campanula (Campanula rotundifolia)

Native Prairie Lily (Lilium philadelphicum)

Karl Foord

Photo 6: Native Prairie Lily (Lilium philadelphicum)

Golden Alexanders (Zizia aurea)

Karl Foord

Photo 7: Golden Alexanders (Zizia aurea)

July 1, 2012

In this issue of the Yard and Garden News:

Leafcurl ash aphids on ash

Leafcurl Ash Aphids on Ash

Jeffrey Hahn, Asst. Extension Entomologist

This June has seen a lot of activity by leafcurl ash aphids, a type of woolly aphid, not only in the Twin Cities but also in a number of other areas in Greater Minnesota. Like other aphids, leafcurl ash aphids use piercing - sucking mouthparts to feed on the sap in the leaves. This feeding causes leaves to become tightly curled, puckered, and distorted. To verify leafcurl ash aphids, unroll the leaves. The aphids are a light green and no more than 1/8 inch long. They produce a conspicuous white waxy material that covers the aphids as well as the leaves.

Kim Sullivan

Photo 1: Leafcurl ash aphid. Note the curled leaf and the white waxy material

These aphids also produce a lot of honeydew. Honeydew is a sticky waste material because the aphids are not able to digest all of the sugars in the sap. Any objects under a leafcurl ash aphid infested tree can get coated with this substance. Later you might find sooty mold, a black fungus developing on the honeydew. Fortunately, light infestations of sooty mold causes little damage to plants.

These aphids were even reported as nuisances when they would fall down on people in their yards (which is not conducive for graduation parties and other outdoor activities).

Leafcurl aphids feed on the new growth that expanded this spring. Colonies last until mid-summer. Winged forms are produced which migrate to the roots of ash where they remain for the rest of the year. Leafcurl ash aphids have always been in Minnesota but have been infrequently noticed or reported over the last 5 - 10 years.

Although the damaged leaves are conspicuous, when you look closely, just a small number of leaves within a tree are actually affected by leafcurl ash aphids. Even the leaves that are distorted can still photosynthesize so there is very little risk to the health of ash.

While insecticides, such as imidacloprid and dinotefuran are effective against these aphids, they are rarely warranted to protect trees and are not suggested. Even if you kill the aphids, the distorted leaves will remain for the rest of the season. While this can affect the trees' appearance that is of small consequence compared to other problems, especially the risk of emerald ash borer.

Tomato Leaf Spot Season Begins

Michelle Grabowski, UMN Extension Educator

M.Grabowski, UMN Extension

Photo 1: Leaf spot diseases starting on the lower leaves of a tomato plant

Warm wet weather throughout Minnesota has provided excellent conditions for tomato leaf spot pathogens to grow and spread. The first leaf spots of many tomato diseases have recently been observed.

Both fungal and bacterial leaf spot diseases are commonly found on tomatoes in Minnesota. Septoria Leaf Spot, Early Blight, and Bacterial Spot are all common problems on garden tomatoes. All of these diseases overwinter on infected plant debris in the soil. Rain or irrigation can splash spores or bacteria up onto the lower leaves of the plant to start new infections.

If you are growing tomatoes this year, take the time now to examine the lower leaves of your plants. Look for black to brown spots on the leaves closest to the ground. For help identifying which disease is affecting your plant visit the 'What's wrong with my plant?' online diagnostic tool.

Do not be disheartened if you do find disease on your plants. There are still a few things that can be done to prevent the spread of disease and end up with a good harvest of tomatoes.

  • On the next dry day, go to the garden and pinch off all leaves with leaf spots on them. Start with the worst infected leaves and stop when you have removed 1/3 of the plant's leaves (removing more than this hurts the plant).
  • Remove the infected leaves from the garden. Infected leaves can be composted if the compost gets hot or buried so that soil microorganisms will begin to break down the plant material that shelters the pathogen.
  • Stake plants to improve air movement around the leaves.
  • Avoid spraying leaves with water. Use drip irrigation or direct water at the base of the plant. If sprinkler irrigation is the only option, water early on a sunny day so leaves dry quickly.
  • Mulch the soil around the plant with an organic mulch like straw or woodchips or with plastic mulch to prevent spores from splashing up from last year's plant debris.
  • Remove any weeds that might be crowding the plant.
  • If you choose to spray a fungicide to protect leaves from Septoria Leaf Spot or Early Blight, sprays should begin now. Read all label instructions prior to applying the fungicide. Tomato MUST BE LISTED on the pesticide label and ALL INSTRUCTIONS must be followed or the tomatoes will not be safe to eat.
Remember that we grow tomatoes for the fruit not for the leaves. It is an acceptable practice to use the cultural control practices above to reduce leaf spot disease problems and tolerate some leaf spots. One study found that yield in tomatoes was not reduced until over 50% of the plant's leaf area was infected by Septoria Leaf Spot. That means plants will continue to produce the same amount of tomatoes until diseases reaches very severe levels.

Amazing Leaf-Cutter Bees

Karl Foord - Extension Educator, Horticulture

I was walking through the Horticulture Display Garden at the University of Minnesota St. Paul Campus. A bee landed on the ground and disappeared. I got my camera but couldn't find the hole it had disappeared into, so I waited. In a few minutes the bee returned with a bit of leaf rolled under its abdomen and again disappeared down the now somewhat visible hole.

This was no leisurely entrance and exit from the hole. The time from seeing the bee approach the hole to disappearance was less than one second. It took less time for the bee to show at the top of the hole and exit. The bee exited headfirst so it had had enough room in the hole to turn around.

The bees that nest in this way are aptly named Leafcutter bees and are in the genus Megachile (Photo 1).

Karl Foord

Photo 1: Leafcutter bee (Megachile spp.) on Sea Holly (Eryngium planum)

They are identified by the hair on the bottom of their abdomen which traps pollen. They do not have a pollen basket like honeybees.

Photo 2 shows a leafcutter bee next to a honeybee to show the size difference.

Karl Foord

Photo 2: Honeybee on left, Leafcutter bee on right

I documented this event with a video:

Leaf-cutter bee building nest

The bee took between 45 and 60 seconds to leave the nest retrieve a leaf piece and return to the nest. Entry and exit events have been patched together and slowed down by 50%.

Sometimes even at half speed the bee moves too quickly to appreciate what is happening, so I made a collage of still photos of nest entry (Photo 3) and nest exit (Photo 4).

Karl Foord

Photo 3: Leafcutter bee entering nest with leaf material

Karl Foord

Photo 4: Leafcutter bee exiting nest and leaf cuts for nest building

I also wanted to know where it was getting its nest material. I found that leaves of Long leafed Speedwell Veronica longifolia had the characteristic holes created by the leaf cutter (Photo 4). Note that not all the leaf cuts are the same; oval pieces are used to coat the sides of the nest, and circular pieces close up the nest cells.

Other leafcutter nest sites are created in gaps between stones or bark, hollow plant stems or other preexisting holes. These bees do no real harm to plants in collecting leaf material, and they are good pollinators. Keep an eye out for them in your garden. They are a treat to watch.

Hawthorn Mealybug: An Interesting Insect in the Landscape

Jeffrey Hahn, Asst. Extension Entomologist

Hawthorn mealybug, Phenacoccus dearnessi, has been found infesting several hawthorns in Minneapolis. This insect is globular and red, although it will appear to be white as it is covered with a white waxy material. In addition to hawthorn, it can also attack mountain ash (an infested mountain ash was found adjacent to the hawthorns), cotoneaster, juneberry (amelanchier), and other plants in the rose family.

This is not a common insect in Minnesota. In fact in Minnesota the best place to find mealybugs is on greenhouse and house plants and not landscapes. Even our neighbors in Wisconsin and Iowa have not seen the hawthorn mealybug (so far). It is, however, found in northeast Illinois.

Whitney Cranshaw, Colorado State University,

Photo 1: Hawthorn mealybug

This insect colonizes the bark of twigs and small branches using its piercing sucking mouthparts to feed on the sap. Hawthorn mealybugs also produce a lot of honeydew, a sugary waste material as a result of feeding on the sap. Honeydew is shiny, clear or whitish in appearance and sticky. Honeydew can also lead to sooty mold, a black fungus that colonizes the honeydew. Hawthorn mealybug has the potential to weaken branches and cause dieback, although that has not been noticed on infested trees here so far.

Hawthorn mealybugs appear to have one generation per year. They mature in the late spring. Eggs hatch and nymphs are active by early summer. After feeding on leaves briefly, the nymphs move to twigs and feed in protected sites.

Because of the white waxy material that is present and the habit of the nymphs to feed in protected places, direct insecticide control can be challenging. However, if management is necessary, an application of a systemic insecticide, like imidacloprid and dinotefuran should be effective.

Blow Flies and Flesh Flies

Jeffrey Hahn, Asst. Extension Entomologist

Some people have discovered the sudden appearance of medium sized flies in their homes. Blow flies are iridescent green, blue, or coppery colored flies while flesh flies have dark colored bodies with three black stripes on their thorax and a checkerboard pattern on their abdomen. Both types of flies lay their eggs on dead animals and decaying garbage. The larvae are smooth, cream-colored, legless maggots that are carrot-shaped with the narrowest end by the head. When fully grown, they are about 3/4 inch long.

Jeff Hahn

Photo 1: Blow flies on animal remains

When a dead animal becomes trapped inside a home, e.g. inside a ceiling or wall void, and dies, it is not uncommon for it to attract these flies which lay eggs on the corpse. Eventually they turn into adult flies which can emerge into the home. It is also possible to see the maggots inside a home. As mature maggots wander away from their food source to less crowded sites to pupate, they can inadvertently move through light fixtures or other spaces and fall into the living space of a home.

These flies are generally harmless to people and property, although because of their unsanitary habits they do have the potential to spread filth-related diseases such as diarrhea and dysentery. On the plus side, they are helping us out by removing and recycling organic material.

A blow fly or flesh fly infestation will persist until the carcass is consumed. The most effective method to control them is to remove the food source, i.e. the dead animal. Unfortunately, this is usually not practical as the animal is typically trapped in an inaccessible place. Be patient and eventually the dead animal will be removed naturally by the maggots. The flies and the maggots will go away on their own once the food source is consumed. This generally takes several weeks to happen.

Click here for more information on summer flies.

The Eyes Have It

Jeffrey Hahn, Asst. Extension Entomologist

Jeff Hahn

Photo 1: Eyed click beetle

During June, some people encountered eyed click beetles, Alaus oculatus, an interesting and conspicuous looking insect. An eyed click beetle is large, about 1 - 1 3/4 inch long. The wing covers are black mottled with small whitish patches. What are immediately noticeable are the two eyespots on its prothorax (the area behind its head) which are velvety black surrounded by a whitish ring. Eyed click beetles are associated with decaying logs and stumps and are found in open wooded areas. Watch for and enjoy the adults during spring. This insect is harmless and just a curiosity.

Black Rot on Crucifers

Michelle Grabowski, UMN Extension Educator

M. Grabowski, UMN Extension

Photo 1: Black Rot Lesion on a Broccoli Leaf

A bacterial disease of crucifers (cabbage, broccoli, cauliflower, radishes, Brussels sprouts and more) called black rot caused severe damage to many gardeners last year and has been found again in Minnesota this summer. Black rot is caused by the bacteria Xanthomonas campestris pv. campestris. This pathogen often enters a garden on contaminated seed. It can also survive on infected weeds like wild mustard and pepper weed or in plant debris from previous years infected crop.

Black rot can be recognized by the v shaped lesions that form on infected leaves. The tip of the V points towards the mid rib of the leaf. The center of the v is often dead brown tissue which is surrounded by a yellow halo. As the disease progresses, leaf veins turn black within the lesion. The infection can move into the plants vascular system and result in wilt and soft rot.

Black rot enters the leaf through natural openings on the leaf and through wounds. The sticky bacteria are easily spread on a gardener's hands or tools, on splashing water or by insects. The bacterial pathogen thrives in warm wet weather and gardeners may see plants wilt rapidly under these conditions.

M.Grabowski, UMN Extension

Photo 2: Black Rot on Radish Leaves

The best management of black rot is to prevent introduction of the bacteria into the garden in the first place. Look for seed and transplants that are certified disease free. Inspect transplants for signs of disease and reject any with v shaped leaf spots or wilted yellowing lower leaves. If disease shows up in one or two plants out of many, remove the infected plants completely. If infection is light (a few leaf spots) remove the infected leaves completely. Wash hands and tools with soap and water after touching infected plants.

Once disease is established in the garden, its spread can be slowed by several management practices. Avoid watering when dew is present and in early evening. Bacteria can spread in the dew and will thrive in moisture left on leaves after sunset. Rather water when the sun is bright and will dry leaves quickly. Avoid splashing water on leaves as much as possible. Do not work in plants when they are wet from rain, dew, or irrigation. Bacteria easily spread on hands and tools during this time. Copper sprays can be applied to slow the spread of the disease although they will not cure plants. Always completely read the label before applying a pesticide and follow all instructions.

At the end of the season, till under any plant residue as soon as the crop is harvested. Avoid planting any crucifer in that location for another two years.

Flowering Plant Video Library - Prairie Plants I

Karl Foord - Extension Educator, Horticulture

Click on the link to see the video with host Dr. Mary Meyer, Professor of Horticulture

Common Milkweed (Asclepias syriaca)

Karl Foord

Photo 1: Common Milkweed (Asclepias syriaca)

Foxglove Beardtongue (Penstemon digitalis)

Karl Foord

Photo 2: Foxglove Beardtongue (Penstemon digitalis)

Prairie Phlox (Phlox pilosa)

Karl Foord

Photo 3: Prairie Phlox (Phlox pilosa)

White Wild Indigo (Baptisia alba)

Karl Foord

Photo 4: White Wild Indigo (Baptisia alba)

Common Ox-eye (Heliopsis helianthoides)

Karl Foord

Photo 5: Common Ox-eye (Heliopsis helianthoides)

Butterfly Milkweed (Asclepias tuberosa)

Karl Foord

Photo 6: Butterfly Weed (Asclepias tuberosa)

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