Plant and Insect Diagnostic Clinic Update - May 20, 2016

The following are highlights and updates about samples and questions recently received in the Plant and Insect Diagnostic Clinic. Visit the PIDC's Facebook page to ask questions and for updates and more pictures.


Ticks.  The past few weeks we have received numerous tick samples.  Most have been blacklegged ticks (a.k.a. deer tick) and American dog ticks.  Be sure to check yourself and children when returning indoors from wooded, brushy or grassy areas and promptly remove any ticks.


It is the time of year for honey bee swarms.  These gatherings of honey bees can look intimidating, but there is nothing to fear.  They often settle for the night or during cold weather, but will move on as soon as it is warm and sunny.  See the photo below.


It is also time to check conifers for sawflies.  European pine sawflies were spotted defoliating this week.  The damage can be unsightly, but one larvae are large treatment is not necessary.  Trees should look better as new growth is added.

Plant Diseases

Broadleaf Trees


We have received inquiries about how to collect a sample from bur oak trees that may be suffering from Bur Oak Blight. In spring and early summer, when symptoms on new leaves may be on the way but not yet evident,  we rely on inspecting the petioles of brown leaves attached from last fall. See photos below.


Once you have found 3 to 5 branches with symptoms, collect them and send to us following guidelines found on our Clinic website.  For more information on Bur Oak Blight visit our updated article on the Clinic website.  


Are you concerned that your oak tree may be suffering from oak wilt? If you are considering submitting a sample, remember that this pathogen is hard to diagnose, and we need a very specific sample. Check out our updated oak wilt guidelines on the Clinic website. A printable version is available.  

Coniferous Trees


Many of our recent samples have been from spruce trees. A majority of the spruce samples have been diagnosed as stigmina needle cast. We have also received several fir samples that were diagnosed as phyllosticta needle blight. Both of these diseases are good indications that the affected trees are experiencing some type of stress.

Perennials and Annuals


We diagnosed black root rot in flowering annuals. This disease is caused by the fungal pathogen Thielaviopsis basicola. This pathogen is capable of infecting begonia, cyclamen, geranium, gerbera, kalanchoe, pansy, petunia, poinsettia, primula, snapdragon, sweet pea, and verbena, among others. When purchasing plants, inspect the roots to make sure the roots are thick and that no discoloration is present. Once introduced into your flower beds, this pathogen structure (see picture at the end of the document) can survive in the soil for many years.


Powdery mildew is becoming evident on susceptible peony varieties in shady locations.  See the picture on our Facebook page



We received turfgrass samples where management practices were leadings to stress and disorders on lawn. It is a good time of the year to consider fine tuning your care of Kentucky blue grass lawn.  Learn more in ISU publication PM 1063


Powdery mildew is also making its appearance in lawns, especially in shady locations.  See an overall view of powdery mildew and a close-up look on the Clinic's Facebook page.

Fruit  (Small and Tree Fruit)

Peach leaf curl was confirmed on samples this week.  See the article elsewhere in this issue.

Fire blight in apples and apple scab on apples and crabapples have been spotted.



A few instances of chemical injury were observed last week. Remember that the Plant and Insect Diagnostic Clinic does not test for chemical residue and our assessments are based on visual symptoms and spray records.  Other laboratories do offer residue testing for a fee.


Drift is not the only way herbicide can cause injury in vegetables.  Carryover can also be problematic. For more information on chemical injury see Horticulture & Home Pest from June 26, 2015.

Swarm of honey bees on a tree
Honey bee swarms have left an established colony and are resting on their way to a new location.
European pine sawfly feeding on old needles
European pine sawfly larvae feed in clusters on the old needles.
Brown oak leaf still attached to the tree
To check for bur oak blight in spring, collect brown leaves that are still attached from last fall.
Brown oak leaf still attached to the tree
Bur oak blight can be diagnosed from pustules on the petioles of last year's leaves still on the tree.
Microscopic image of Thielaviopsis basicola chlamydospores on a plant root
Thielaviopsis basicola chlamydospores on plant root suffering from black rot.


Lina Rodriguez Salamanca Extension Plant Pathologist and Diagnostician

Dr. Lina Rodriguez-Salamanca is a diagnostician and extension plant pathologist with the Iowa State University Plant and Insect Diagnostic Clinic  (, a member of the National Plant Diagnostic Network (NPDN, ...

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