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Category: Diseases

Keep an eye out for powdery mildew in your landscape…

By Christian Webb

As summer starts winding down, you may begin to notice some of your bedding plants developing a white powdery coat on the upper surface of their leaves. No, this is not your plants putting on a winter coat for the cool autumn nights. It’s actually caused by the infection of a fungus on the surface of your plants leaves called powdery mildew.

Powdery mildew can be commonly found developing on many types of landscape plants during the late summer. On most plants, this disease will develop on the lower leaves and move up through the canopy. The typically symptoms of the infection will appear as a white to grey dusty growth on the upper surface of the leaves. In severe cases, leave may appear distorted and the cause defoliation.

Pictured to the left: Powdery mildew on Ox-eye Daisy (Heliopsis helianthoides)

To develop, the fungus requires high relative humidity and temperatures around 70⁰F. Many woody and herbaceous ornamentals may be affected by powdery mildew. As a general rule, the powdery mildew on one host will only affect hosts in that genus. For example, the fungi that you will find on oaks trees will only affect oaks and the powdery mildew on grapes will only affect grapes.

If you are looking to control powdery mildew in your landscape, there are a few ways to protect your plants. For annuals, increasing the spacing between plants will increase airflow through the canopy create more unfavorable conditions for powdery mildew develop. Along with altering the plant canopy, consider planting resistant hosts. Replanting the same susceptible hosts will likely result in a reinfection.

Pictured above on left: Powdery mildew on Zinnia (Zinnia elegans). Right: A microscopic image of the powdery mildew spores on the surface of a Zinnia leaf.

There are labeled fungicides for the control of powdery mildew. But you consider apply any product, be sure that your infection is caused by powdery mildew. Downy mildews looks similar to powdery mildew, but are caused by a different type of organism and may not be controlled by products labeled for powdery mildew. Powdery mildew is typically found on the upper surface of the leaf, while downy mildew is associated with the underside of the leaf. For recommended products for the control of powdery mildew, please consult the following factsheet: Link.

For further assistant in identifying powdery mildew, contact your local extension office.

To find your local extension office, the list of Kansas extension offices can be found here.

Further discussion of powdery mildew on woody ornamentals can be found at this Link on pgs. 48-50




Powdery Mildew Resistant Peony Varieties

By Kenneth Dodson

Powdery Mildew is a common fungal disease that can appear on a wide range of plants. This year has seen a high level of powdery mildew on peonies, leaving bushes looking snow-covered or dusty. The best way to avoid dusty looking peony bushes is to use resistant plants. A look through the peony demonstration planting at the K-State Gardens showed a wide range of disease susceptibility to powdery mildew.

Pictured above: Powdery mildew on Peony (Paeonia spp.) 

Here are a few peony varieties that showed good resistance to powdery mildew. In terms of Herbaceous Peonies: Red Charm, Lillian Wild, and Pink Luau were the best options, followed by Madame de Verneville, and Pink Hawaiian Coral. If you’re looking for Tree Peony varieties, there are several that show strong resistance to powdery mildew: Rockii, Joseph Rock, Kamata Nishiki, Shima Daijin, Hakuo Jishi, Kinkaku, and Kokamon all looked good. Lastly, the Itoh or intersectional peonies Pea Green, Yellow Charm, and Morning Lilac exhibited good disease control against powdery mildew.

Just fyi, the major distinction between the three growing types is the in the way they overwinter. Tree peonies will have a woody stem that stays above ground, while herbaceous peony stems will die back to

the ground every year. Itoh varieties have herbaceous stems as well, and will die above ground, but Itoh peonies typically have the same flowering types as tree peonies.

Ash Leaf Spot on Green Ash Trees in Kansas

By: Christian Webb

You may notice brown spots beginning to develop on the lower foliage of green ash trees. Symptoms of Ash leaf spot include irregularly shaped brown spots on the leaf that merge to form large patches of brown spots. In severe cases, the tree may defoliate four to six weeks earlier than normal causing the tree to appear bare long before other trees lose their leaves.

Pictured above on the left: A branch of lower leaves on an Ash tree affected by leaf spot. Right: Brown leaf spots with white speckling on the surface of the upper leaf caused by Ash leaf spot

The fungi that cause Ash leaf spot (Mycosphaerella spp.) survive in the leaf litter underneath the ash tree. In the early summer, the fungi in the leaf litter will begin producing spores capable of infecting leaves. Initial infections will appear on the lower leaves, and then quickly spread throughout the tree. By late fall, the fungus will produce structures able to survive the harsh conditions of winter and be ready to infect new leaves during the following summer.

The severity of the disease will depend on the conditions present during the period of leaf infection. Typically, the disease will become apparent in the late summer and early fall, which may not cause significant cosmetic damage. But in summers with abnormally wet conditions, the disease may develop earlier than normal and cause premature defoliation. Multiple years of these conditions may lead to weak trees with reduced vigor prone to branch dieback.

Pictured above: A microscopic image of fungal structure emerging from the surface of the leaf.

There are few recommended methods to protect ash plantings from Ash leaf spot. Since the disease will survive in the residue around the trees, raking up fallen leaves will reduce the amount of disease carried over to the next growing season. No ash trees resistant to leaf spot have been reported although they may vary in susceptibility.


More information on Ash leaf spot may be found at the following Link on page 46-47. https://www.fs.fed.us/rm/pubs/rmrs_gtr335.pdf

Turf disease update

(Megan Kennelly, KSU Plant Pathology)

Around this time every year we all look forward to early September. Often Labor Day weekend marks the end of “summer diseases” and the beginning of fall repair and renovation to our cool-season grasses.

Moisture and humidity continue to trigger turf diseases. Here are a few recent photos:

Brown patch

Brown patch in tall fescue, with mycelium present on a wet, humid morning. The first photo shows the characteristic lesions. As I write this on Friday Aug 16 there is a stretch of hot days and very warm nights ahead, so this disease may continue. But September is not far away…


Dollar spot

Dollar spot in an untreated bentgrass area:

The photo above is a worse-case scenario, with a susceptible cultivar untreated. If you are having breakthroughs, review your fungicide regime and maybe your N regime, as low N can make this one worse (but too much is a problem, too!). Check your calibration and coverage. If you’ve never read it, read the dollar spot section starting on page 14 here:


Unlike brown patch, dollar spot loves the more moderate temps  of September, so this one does not fade away after Labor Day. So don’t let your guard down yet.

Continued root and crown woes

Another issue I’ve been seeing lately is continued problems with saturated roots, and there were more heavy rains this week. The wet soils deprive the roots of oxygen plus they hold heat, so turf does not have any “chill out time” overnight. Organic matter in the profile makes it worse. On top of the physiological issues, those wet soils can trigger Pythium root rot and sometimes anthracnose crown rot comes in on top of that.

For more details, you can check out the report this week from our great neighbor Dr. Miller here:


Or here:



Localized dry spot

Finally, another issue I’d like to mention is hydrophobic soils. Despite all the rain, hydrophobic soils/localized dry spot can still occur.

Here is what localized dry spot can look like, and it can come on fast.

Keep an eye on your soil profile so you can stop this damage before it starts. When you get damage to the extent shown in those two photos above it is a long road to recovery.

To check for localized dry spot, pull up some cores and use the “droplet test” by putting drops of water on the plug. If the drops just sit there, not wicking in, you have a problem. The soil is water-repellent.

Sometimes there is a defined hydrophobic layer with normal soil above and/or below, so check at different depths, all the way down the rootzone:

You might also notice water beading up on the surface and not wicking in.

You don’t want this to sneak up on you. Keep an eye on your soil profile, especially locations with a history of problems. Aerification and use of wetting agents can help get moisture where it needs to go.

Declining Oaks

By Judy O’Mara

Declining Oaks...are not that easy to sort out. There is an assumption that if an oak tree is dying it must be due to a disease. That is usually NOT the case. Most of the poor growth that we see on oak trees in Kansas is generally due to a combination of site factors and environmental extremes. 2018 and 2019 provide back to back examples of weather extremes. Add in site factors like tree planting depth (too deep), construction damage, heavy soils or compacted soils that drain poorly, sandy soils that don’t hold on to water very well, and trees can develop root systems that are not as vigorous as they should be.  The result is a situation where trees are prone to declining under weather extremes – and Kansas is all about extreme weather.

A look at some weather data for Johnson County (Olathe, KS) provided by the K-State Weather Mesonet shows that the 20yr average annual precipitation is 40.04” for that location. At a glance it is easy to see that some years run wetter than average and some years are drier than average. I picked the Johnson County, KS location to highlight because we frequently get oak tree samples in the K-State Plant Disease Diagnostic Lab from the Kansas City area with questions about disease.

2018 was a dry year due to a winter drought and we saw trees decline across the state, including quite a few oaks from the Kansas City area. There was below average precipitation between November 2017 and September 2018. This weather snapshot also shows extreme weather during 2019, particularly during April and May. Saturated soils resulting in root damage and tree decline has been a common problem this year. Young trees and shrubs are particularly at risk due to smaller root systems. In many cases, damage wasn’t immediately apparent until June or July when higher temperatures put a greater demand on the root system. So bottom line, extreme weather patterns frequently play a role in the health of our landscapes.

That said (most oak problems are not due to disease), we have picked up two different oak diseases in the Kansas City area this summer that have the potential to cause tree decline, Hypoxylon canker and oak wilt.

Hypoxylon canker, also known as Biscogniauxia canker (Biscogniauxia atropunctata; B. mediterranea) is a disease common on oak trees throughout Kansas. It is considered to be a stress disease, usually associated with drought stress. Trees showing symptoms in 2019 were likely triggered by the dry growing conditions of 2018.

The initial symptoms of Hypoxylon canker are a tan or silver fungal mat on the trunk of the tree. This light colored surface layer quickly wears off and exposes a black crusty mass that looks like asphalt. Leaves turn yellow and shed and infected trees generally die within the first year. By the second year the dead oaks slough off the remainder of their bark. The best protection against Hypoxylon canker is to avoid stress. This starts with planting into well-drained sites and providing deep, regular watering during prolonged periods of dry weather. Also take care to avoid damage due to construction activities.

Oak wilt (Bretziella fagacearum) has been picked up in a couple of red oak trees in the Kansas city area (Johnson County, KS) this summer. It is an occasional problem on red oaks and pin oaks in northeast Kansas and in counties along the Missouri state line.

Oak wilt causes a partial leaf scorch (brown at the leaf tip and green at the base). Beneath the bark tissue, the disease produces dark brown streaks in the sapwood (see photos). Oak wilt on red oaks tends to start on a major limb or in a section of a tree and then progress until the tree dies, usually within a season.


The disease can be spread by sap feeding insects so it is important to avoid pruning oaks between April-June. Oak wilt can also spread to nearby red oak trees through roots that have fused.

There is no cure for the disease. Infected trees should be removed and destroyed. The wood should not be kept for firewood. If there are red oaks nearby, the root grafts between the trees should be severed.  Healthy red oaks nearby may benefit from a fungicide injection.

The best time to test for oak wilt is when trees are freshly wilted, generally in the spring.

More information on oak wilt can be found at the K-State Horticulture Information Center. Link is listed below:


Common Oak Diseases in KS

  • Hypoxylon Canker
  • Bot Canker

Less common oak diseases in KS

  • Tubakia leaf spot
  • Bur oak blight
  • Oak wilt

Odd but interesting oak problems in KS

  • Oak galls
  • Obscure scale, Lecanium scale
  • Oak tatters

A little bit of everything in the landscape.

By: Judy O’Mara

If you drive around town you will notice that most ornamental pear trees have orange leaf spots. It can look pretty dramatic. This is a fungal disease called pear rust (Gymnosporangium globosum). It is part of the complicated group of cedar rust diseases that are common in Kansas which cycle back and forth between cedar trees and their alternate host (in this case pear trees). By the time you see the symptoms it is too late to manage the disease. Preventive fungicides can be applied in the spring during the months of April and May.

A nice fact sheet on this disease can be found at: https://hnr.k-state.edu/extension/info-center/common-pest-problems/common-pest-problem-new/Pear%20Rust.pdf

I look forward to the annual exploding London plane trees (and sycamore) on the K-State campus. Sometime during early to mid-July the outer bark slips off the tree…explosively. Bark pieces can wind up 20 feet away. The first time I saw this interesting event (30 yrs ago), I was worried that the tree was going to die. I’ve since learned that this is just a normal growth habit of the tree.

My colleague and I were walking through a landscape this week and she saw this long, tan linear symptom on some hosta leaves. We both knew immediately what it was caused by…Hosta Foliar Nematodes. The nematodes in the tan narrow lesions are limited by the veins, so symptoms are long and blocky. This disease requires really wet conditions to thrive and spread.

Here are some interesting facts about foliar nematodes. They are microscopic roundworms and can survive on leaf tissue or in the soil. They can complete their life cycle in 2-4 weeks. The nematode has the ability to survive in a dormant state as a fourth stage juvenile for up to two years. Spread of foliar nematodes is by infected leaves touching other wet leaves. The disease is unattractive but rarely kills the plant. Sanitation of infected plants will help to clean up the problem

Missouri has a nice article on Hosta Foliar Nematodes: www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/nematodes/hosta-leaf-nematodes.aspx

Rust on Turfgrass

(By Jared Hoyle, KSU Turfgrass Research and Extension)

Last week I came home after the KSU Turfgrass Annual Field Day that was held in Olathe, KS at the Olathe Horticulture Research and Extension Center and before I even made it into the house I knew I had rust in our back yard.  The shoes in the garage were a dead give away. The shoes that my wife were wearing in the back yard were orange. This disease can occur just about anywhere, when the leaves of turf are wet and when plants are stressed they are more susceptible.  We see it on perennial ryegrass but can occur on other species.

Don’t worry too much as the turf typically can grow out of it.

For more information on rust check out the KSU publication below, some past blog posts from Dr. Megan Kennelly and myself, as well as some information from Richard Jauron at Iowa State University.






Hydrangea Decline + Root Rot on Coreopsis

Wet then Hot….Hydrangea shrubs crash and burn

By: Judy O’Mara

At first glance, the declining hydrangea makes you think that they were wiped out by a disease. A quick check of the very useful K-State website (http://mesonet.k-state.edu/weather/historical) for historical weather data for Morris County (where hydrangea plants were grown) shows that it got 27.83″ of rain in the first half of the year (January 1 -July 7, 2019). Of that amount, almost 23″ (22.95”) of rain was received during April 29 –July 7th with several periods of 4-6″ over that ten week period. Basically that is a lot of rain.

The impact here is that lots of rain can keep soils wet and soggy, which in turn damages the root system. Plants with damaged roots may not have expressed symptoms during May or June because temperatures during that period were fairly moderate. More recently temps jumped into the 90s and damaged root system were unable to support the tops of the plant. Affected Hydrangeas crashed and burned. Mulch is generally a useful tool to help suppress weeds and keeps soils moist. Too much mulch over heavy clay soils can aggravate the problem by keeping soils too wet.  It can be very hard to regrow damaged root systems so, recovery potential for these plants isn’t very good.

Rhizoctonia Root Rot on Coreopsis

Many plants with poor growth this summer have been suffering due to saturated soils and high temperatures. However, sometimes plant decline is due to a disease. This Coreopsis bed sustained damage from a fungal disease called Rhizoctonia crown and root rot. The disease can survive in the soil for long periods and is triggered by high temperatures and moist soils. Long-term the best management option is to start over with new Coreopsis plants in a new location.

Turf disease update

Megan Kennelly, KSU Plant Pathology

Moisture and humidity are causing continued disease activity in both ornamentals and turf.

Below is some dollar spot activity in untreated controls in a fungicide study. As you can sort of tell from the photo, the disease has gotten to the point where the infection centers are not just off color but are also sunken/pitted.

If you are struggling with dollar spot control, consider your fertility regime. Low N can make the turf more susceptible, but high N comes with its own issues. Check your nozzles and spray equipment to ensure good coverage. Be sure to rotate among different modes of action. For more details check the dollar spot section starting on page 14 of THIS DOCUMENT.


We are getting some heat and humidity, and brown patch is becoming more active. Typically we think about brown patch as affecting tall fescue, perennial ryegrass, and our putting greens. However, this disease can affect bluegrass, too. This photo shows a sample from a Kentucky bluegrass fairway. You can see the fungal mycelium. With all the moisture, foliar Pythium also came to mind as a possibility, but in the microscope it was clearly the brown patch pathogen.

In the photo below, you can also discern that this is a heavy clay soil – see how shiny and glossy it looks. And, there is a thatch layer starting to build up, which can lead to its own set of problems. This site already has an aerification plan in place for fall.


The final disease I want to mention is Pythium root rot in putting greens.

Saturated soils alone can cause major decline in root systems. Unfortunately those same conditions can trigger Pythium root rot. The photos below illustrate typical symptoms I’ve seen recently, including several already this week:

When you pull up a turf core, the root system does not cling together well, and the whole core kind of lacks structure. If you wash them off, they are brown and mushy.

Canopy is thinned out and yellowing:

In the microscope the roots are dark, sloughing outer tissues, lacking root hairs, and have Pythium oospores present. (The physiological decline from wet soils alone can also cause browning, lack of root hairs, etc)

Here are a couple of resources about this disease:




Hosta…one of my favorite plants.

By: Judy O’Mara

Hosta is a standard go to plant for every shade garden. They are very hardy and have few problems and you can plunk them in the ground and come back 20 years later and they will still be going strong. They really do have few problems…and yet, they aren’t problem free. Here are two things that I’ve seen on hosta this spring.

Anthracnose on hosta will show up as brown scattered spots. The centers sometimes drop out creating a shot hole appearance. Like all fungal diseases, it has been favored by the wet conditions this spring.

The impact of anthracnose on the plant is cosmetic. The disease is fairly easily cleaned up by picking off the infected leaves.

Aside from rainy weather, prolonged leaf wetness from watering late in the day can also promote conditions that favor the disease. Switching to morning irrigation will allow the leaves to dry out quickly, which in turn reduces disease activity.

A bit more serious is Hosta virus X (aka HVX). This disease is sap transmitted. If you divide an infected plant and then use the same tools to divide a healthy plant, the virus can be spread to the new plant. Virus diseases tend to weaken the plant, so infected plants may end up smaller and less vigorous than healthy plants nearby. The best way to avoid the problem is to inspect hosta plants when purchasing them. If you discover a plant with Hosta virus X in your yard dig it up and discard it.

Hosta virus X is tricky to identify based on symptoms because it can look similar to normal hosta variegation. Confirmation of the disease is done in a lab with a serological test (ELISA). However… one symptom that I have seen consistently associated with Hosta virus X is called ‘ink vein bleeding’. This is where the veins of the leaf have the opposite color and appear to be bleeding like an ink blot.

                   Healthy Hosta                                       Hosta with HVX

Iowa State University has a great publication on Hosta Diseases and Pests.


Spring Leaf Drop – Sycamore Anthracnose

by Judy O’Mara

I walk past a big beautiful Sycamore tree on my way into work every day. I recently noticed leaves dropping to the ground. A close look at the defoliated leaves showed the characteristic symptoms for a fungal disease called sycamore anthracnose – brown lesions along the veins and on the petiole. Once the water conducting tissue in the veins is damaged, the leaf gets knocked off the tree. Also visible in the tree were also small, blighted shoots.

Sycamore anthracnose is a spring disease, meaning it is favored by cool (60-73F), wet conditions. With a prolonged spring weather pattern, the disease can sometimes look dramatic. In Kansas, chemical control of this disease isn’t necessary. With the move into warmer summer weather, sycamore anthracnose ceases activity and the tree pushes out a new flush of growth.