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Extension > Yard and Garden News > May 2017

Tuesday, May 23, 2017

Powdery mildew covered shoots

M.Grabowski, UMN Extension Educator

Ninebark shoot covered in powdery mildew
M. Grabowski, UMN Extension
As the buds of trees and shrubs open and young shoots begin to grow, some will emerge already infected with powdery mildew. Powdery mildew, a common fungal disease of many garden plants, is easily recognized by powdery white growth on the surface of infected leaves, shoots, and other plant parts. Gardeners often describe infected plants as dusted by flour or having cobweb like growth on leaves.

Some powdery mildew fungi survive Minnesota’s harsh winter by colonizing young plant tissue within dormant buds. When the buds open in spring and the new shoot emerges, it will be covered by the characteristic white growth of powdery mildew. From that one severely infected shoot, spores are blown throughout the plant canopy starting new infections on healthy leaves from neighboring shoots.

Powdery mildew leaf spots that
originated from windblown spores
M. Grabowski, UMN Extension 
This type of winter survival is common on woody ornamental plants like ninebark, hawthorn, currant, and rose. Gardeners should carefully inspect young shoots of woody shrubs. If a powdery mildew infected shoot is found, it should be pruned out, and buried in the compost pile promptly.  Although spores can be blown in from other locations later in the season, removing a concentrated source from right within the plant canopy will help to delay the start of disease and slow the progression of symptoms.

Powdery mildew fungi steal nutrients from their host plants, but rarely cause significant damage to the health of the plant. By delaying the start of disease, gardeners can reduce this stress even further. 

Friday, May 19, 2017

Clover mites in homes

Jeffrey Hahn, Extension Entomologist

Clover mites are tiny arachnids, about the size of a pinhead, that are reddish or brownish in color. The first pair of their eight legs is particularly long and noticeable even at their size. People have been finding clover mites this spring on the outside of their homes, especially around windows, as well as indoors.

Clover mites.  Note the particularly long first pair of legs.
Photo: U of MN Extension
During summer, clover mites feed on grass and clover (they are not pests on these plants). They can occur in large numbers around buildings and have no problem getting inside, especially around windows, because of their small size. They love being in the sun and are most common on the south sides of homes. Fortunately, clover mites are not harmful to people or our property.

Physically remove small numbers of clover mites, e.g. with a vacuum or gently wipe them up with a damp cloth. Be careful to avoid crushing them as the can stain surfaces. Clover mites are a temporary problem that will go away on its own when the weather becomes warmer.

For more information, see Clover mites.

Clouds of chironomids (midges)

Jeffrey Hahn, Extension Entomologist

People in many parts of Minnesota have noticed this month large swarms of small, dark insects close to sources of water, especially lakes. These abundant insects are called chironomid midges. The larvae are usually aquatic and can feed on a variety of foods, such as algae and tiny bits of decaying plant matter. They are important food sources for fish and other aquatic animals.
Typical male chironomid midge adult.  Note the feathery
antennae.  Photo: J. Hahn, U of MN Extension

Different midge species emerge as adults at different times during the spring and summer. Emergence typically occurs in large numbers, especially during the evening. They are weak fliers and generally do not move far from the water. Midges vary in color from brown, gray, or green.

Chironomid midges are mosquito-like but unlike mosquitoes lack scales on their wings and a long proboscis (mouthparts). Their first pair of legs is longer than the others. Males usually have feather-like antennae.

Despite their similarity to mosquitoes, they do not bite people. But they can be annoying when they fly into people’s faces and land on surfaces in large numbers. Fortunately, these insects are short-lived; they will be active for only a few days and then go away on their own.
Swarm of midges in northern Minnesota.  Photo: Curtis Bronken

For more information see BugGuide.

Monday, May 1, 2017

Cedar apple rust is active

Orange, gelatinous spore producing structures form along the
branch of a juniper 'broom'
M. Grabowski, UMN Extension 
M. Grabowski,  UMN Extension Educator 

Gelatinous, orange fungi can now be found on junipers in Minnesota on rainy days. Cedar apple rust, cedar hawthorn rust, quince rust,and juniper broom rust are caused by a group of related fungi that spend half of their life on juniper trees and shrubs and the other half infecting members of the Rosaceae family, including crabapple, serviceberry, and hawthorn.  Despite their eye catching symptoms, these rust fungi do not seriously affect the health of either host plant.

These rust fungi overwinter as infections in woody branches of junipers. Cedar apple rust and hawthorn rust result in round woody galls. Juniper broom rust causes a cluster of small branches, or a broom, to form, and quince rust directly infects the branch.  In wet spring weather, these rust fungi come out of dormancy and produce gelatinous orange spore producing structures on rainy days. Spores released from these strange orange fungi are carried by wind and rain to infect nearby crabapple, serviceberry, or hawthorn trees. The orange fungal structures on juniper can dry out and rehydrate several times in the spring, releasing spores each time they are wet.
Orange fungi emerging from cedar apple rust galls.
M. Grabowski, UMN Extension 

Galls of hawthorn rust and cedar apple rust will die after releasing spores in the spring. Brooms and branch infections caused by juniper broom rust or quince rust may survive for many years, releasing new spores each spring.


Cedar apple rust gall during a dry period.
M. Grabowski, UMN Extension 
Although these rust fungi are eye catching, they cause little damage to either of their hosts. Junipers tolerate galls and branch infections and only suffer branch dieback when infection is unusually heavy. Crabapple, hawthorn, and serviceberry trees develop bright orange to red leaf spots, fruit infections, and rarely infections of green twigs. Leaf spots are limited to the leaves currently on the tree and will not spread to new leaves throughout the growing season. Galls, brooms, and branch infections can be pruned out and buried in the compost pile to reduce infection on nearby trees in the rosaceae family if desired. 

Selecting healthy plants

M. Grabowski, UMN Extension Educator


Salvia with the first stages of a leaf spot disease.
M. Grabowski, UMN Extension 
Starting out the growing season with healthy plants can be the key to a beautiful and productive garden. Plant pathogens can devastate a gardener’s dream of beautiful blooms or ripe red tomatoes. One key strategy in preventing plant disease problems is to purchase healthy transplants.

When visiting the garden center this spring, be a smart and selective consumer.

  • Read plant labels and choose plants that will thrive in the conditions of your garden. Think about the amount of sun the garden receives, soil type, and your ability to provide irrigation if Mother Nature does not.
  • Look for disease resistant varieties for common disease problems like powdery mildew or rust. 
  • Mix and match plants from different plant families. Many plant pathogens can only cause disease on one type of plant. Planting a diverse mix of plants reduces the pathogens ability to spread. 
  • Check how big the plant will get when fully grown. Space plants so that there is room for air flow between plants throughout the growing season. This will help leaves dry off quickly after rain and dew; which in turn will reduce problems with leaf spot disease. 


Inspect all plants prior to purchase. Reject any plants with symptoms that might indicate a plant disease problem.

  • Look at the inner and lower leaves where humidity tends to be highest.  Look for leaf spots, discoloration, or fungal spores. Be sure to examine both the upper and lower surface of the leaves.
  • Examine stems and branches. Stems of herbaceous plants like flowers and vegetables should be green and firm. Reject plants with sunken, soft, or discolored areas of the stem. Woody branches should be a uniform color and texture. Cracks, discolored bark, and oozing of sap are symptoms of infection. 
  • Pull the plant out of the pot and look at the roots. A healthy plant will have firm white to cream colored roots with many fibrous root hairs. Transplants with few roots, gray to brown soft sunken areas on roots, or that are lacking fibrous root hairs are suffering from root rot disease. 
  • Don’t buy plants from a group where some plants appear to have a disease problem. In a nursery setting, plants are tightly spaced and share many cultural practices like watering, fertilizing, and pruning. As a result it is easy for a pathogen to spread from one pot to another. If many plants in a group have a disease, it is likely that the few healthy looking plants are also infected but are not yet showing symptoms. 
  • Poor growth in this flat of vinca transplants indicates a problem.
    M. Grabowski, UMN Extension 
  • Don’t try to ‘save’ a sick plant. Plant pathogens can be brought into a garden on infected transplants and spread to other plants in the garden.  Many plant pathogens, once introduced, can persist in garden soil and plant debris, causing disease for years to come.  


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