Plant and Insect Diagnostic Clinic Update - May 8, 2015

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.




This week in the clinic we received potted plants (geranium) with nutritional problems derived from inadequate soil pH, that affect the absorption of macro and micronutrients. The pH in the soil determines how available macro and micronutrient are available for the plant roots to uptake. Learn more from University of Georgia Extension Publication B1256. Plants grown in suboptimal conditions can be weakened and may become prone to diseases.


We also received our first sample of powdery mildew for this year.  The sample was a mint plant from a home gardener.  See photo below.  The pathogens that cause powdery mildew are fungi that infect live plant tissue but rarely kill the host plant. Since powdery mildew is favored by warm and humid conditions watch watering frequency, increase plant spacing and increase air flow. You can also prune away the infected tissue in the fall to decrease the amount of the pathogen that may overwinter in your garden.  In very humid years, powdery mildew is hard to manage.


Now is the time to help prevent oak wilt by not wounding or pruning oak trees.  Avoid pruning oaks from April through July to reduce attraction of sap beetles that carry the oak wilt fungus. Learn more at the PIDC website.




Adults of the hawthorn leafminer, pictured below were spotted on hawthorn trees on the ISU campus.  The larvae of these sawflies feed inside the leaves of hawthorn trees and cause brown, dead leaf tips later in the year.  Although unsightly it does not harm the tree.  More on the PIDC website.  


Ticks have been numerous in samples the past week.  The two most common species received in the Clinic have been the American dog tick and the blacklegged tick (aka deer tick).  Remember to check yourself for ticks when returning indoors and to remove ticks promptly by grasping the tick with tweezers and pulling straight out of the skin.  For more information on ticks please see ISU E&O Pamphlet PM 2036, Iowa Ticks and Tick-Borne Diseases.


Last year’s 17-year cicadas are long gone but the damage the females caused is still present on Iowa trees.  Below are photos showing last year’s egg-laying scars.  Not all twigs and stems will grow over the egg-laying wounds and survive.  Twig die-back can be expected on some plants but impact to woodland and landscape trees is usually minimal.


Watch for more periodical cicadas again this year, only this time in southwestern Iowa, Nebraska, Kansas, Missouri, Oklahoma, a tiny part of Arkansas and Texas.  Learn more about periodical cicada Brood IV at



Powdery mildew on mint leaves and stem.Powdery mildew on mint leaves and stem.


Hawthorn leafminers adults are a type of wasp called sawflies.  The larvae will live inside the leaf tip causing it to turn brown by mid-summer.Hawthorn leafminers adults are a type of wasp called sawflies. The larvae will live inside the leaf tip causing it to turn brown by mid-summer.



Trees can grow over periodical cicada egg-laying scars.

Trees can grow over periodical cicada egg-laying scars.

Periodical cicada egg-laying scars from the previous year.

Periodical cicada egg-laying scars from the previous year.


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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