Plant and Insect Diagnostic Clinic Update - July 1, 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.


Japanese beetles are emerging statewide and feeding on the wide variety of foliage, flowers and fruit for which they are known.  Favored host plants were listed in the Horticulture & Home Pest News, July 14, 2010.   To the list of "most favored host plants" we can today add Fine Line® buckthorn (Rhamnus frangula ‘Ron Williams’ PP14791) a ferny-leafed, small shrub with non-edible berries that resists deer but attracts Japanese beetles.


Gardeners in eastern Iowa have been reporting low numbers of Japanese beetles.  Reports are that they don't have the "outbreak" levels of five years ago but they do have more than a year ago.  Japanese beetle populations crashed after the winter of 2013-14, and it appears the populations have started to recover.   Callers from western Iowa are describing large populations ("hordes") in areas where Japanese beetle is still a recent arrival.


Control of Japanese beetles is described in an earlier newsletter (May 9, 2014).  In addition to traditional, synthetic insecticides there are two organic insecticides that can be used when beetles are in high numbers.   NEEM insecticide (azadirachtin) is a plant-based feeding deterrent, not a control, that would be appropriate for use on food crops and plants in high traffic areas.  "Surround" (kaolin clay) is a sprayable powder that coats the plant with a dusty film that discourages feeding, at least until the next rain.  Recommendations for control of Japanese beetles in field crops can be found in the ISU Integrated Crop Management News from July 13, 2014.


Lightningbugs!  Friends and colleagues around the state who live out in the country say this is one of the best years for fireflies in a long time.  Populations of these well-known, dusk-flying beetles are reported to be higher in many parts of the state (though there is no census of lightningbugs for making an informed, statistical comparison).  Learn more about fireflies at the Plant & Insect Diagnostic Clinic website.  And for a visual treat, check out the video and close-up photos at



Apple and pear scab, both fungal diseases caused by related fungal pathogens, continue to cause defoliation of susceptible crabapple trees (photo below).  At this point fungicide sprays will not help.  Rake and destroy fallen leaves to help reduce the severity of infection next year.  The best line of defense against this disease is to plant scab-resistant cultivars and to prune and train trees in the spring to allow good air circulation (see "Fruit Cultivars for Iowa” and "Pruning and Training Fruit Trees" for more details. Read more about scab diseases in the Iowa State University Extension and Outreach Yard & Garden news release from June 8, 2016, and on the Plant & Insect Diagnostic Clinic website.  

Broadleaf Trees.  The frequency of oak samples has increased in the Clinic in the last month. Before sending a sample, observe if the symptoms appear to be at the bottom or the top of the tree. In most cases, if the top of the tree is the most affected (scorching, bronzing and browning of the leaves) this may be an indication of oak wilt.


A very specific sample is needed to increase our chances of recovering the pathogen that causes oak wilt. The sample must be refrigerated to keep the pathogen alive. Please see our checklist of how to collect and submit samples for oak wilt detection.


On the other hand, if the symptoms are mainly in the lower canopy of the tree this may be an indication of bur oak blight (on bur oaks) or anthracnose (in other types of oaks). See our website for details on the type of sample we need if you suspect your tree is suffering from BOB.

Turf.  Samples we received from lawns in the past two weeks have confirmed two fungal diseases: pink thread and anthracnose. What these two diseases have in common is the effect of fertility and irrigation. Proper fertilization based on soil test results and proper mowing height and frequency can help keep your lawn healthy. Avoid applying nitrogen at high rates under dry weather or high-temperature conditions. If you choose to irrigate your lawn, water in the morning hours and deeply but as infrequently as possible. See the resources below for more information.


Map of distribution of Japanese beetles in Iowa as of July 1, 2016
Reported distribution of Japanese beetles in Iowa as of July 1, 2016.
Pear with pear scab symptoms. It is mottled with brown spots on the fruit and leaves
Pear scab symptoms on fruit and leaves. Photo by Lina Rodriguez Salamanca, Plant & Insect Diagnostic Clinic



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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