Plant and Insect Diagnostic Clinic Update - July 24, 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.




A quick snapshot of the diagnosis in your clinic in past two weeks include Oak wilt, Dutch elm disease, cucumber anthracnose and SNEED in conifers.


Oak wilt.  July is the time of the year when infection risk is low, which means infection can occur, but the chances decrease. Please review the symptoms of oak wilt and the special type of sample we need in the Clinic if you are considering testing on the Clinic website.  Samples must be refrigerated to improve our chances of recovering the pathogen.  Resources:  ISUUSDA Forest Service


We continue to receive oak samples with symptoms and signs of Bur oak blight. This disease causes premature browning of leaves and can get worse over time. Bur oak blight (B.O.B.) is caused by the fungus Tubakia iowensis. Typical symptoms include browning of leaf veins, wedge-shaped lesions on leaves, and early defoliation. For more information about B.O.B. please visit the Horticulture and Home Pest News from February 9, 2011, or the USDA Forest Service.


Dutch elm disease. We received our second case of Dutch elm disease (DED) this year. DED is caused by the pathogen Ophiostoma spp. and it infects the tree's vascular (water conducting) system and causes the tree to lose leaves, wilt, and eventually die. Trees with confirmed DED should be removed to avoid infection in other elms in the area, as beetles will move this pathogen from diseased to healthy trees. More information is available on the Clinic website.


We received a sample of cucumber anthracnose, a disease caused by the fungus Colletotrichum orbiculare.  The characteristic symptom is small, yellow, water-soaked lesions on the leaves that over time enlarge and turn brown. For more information visit Purdue University Extension website.


We received a Black Hills Spruce suffering from sudden needle drop or SNEED. This disease is not as commonly encountered in the Clinic as are other needle pathogens. See the photo below.  Sudden needle drop disease is caused by the fungus Setomelanomma holmii. From more information see the Horticulture & Home Pest News from February 6, 2008.   




Magnolia scales are large white scales that are easily seen on the undersides of magnolia branches.  These soft scales produce large amounts of honeydew so the ground and leaves will often be covered with this shiny, sticky substance and often a black sooty mold that feeds on the honeydew.  See the photo below and the August 28, 2008 Horticulture & Home Pest Newsletter.  Contact insecticides including horticultural oil can be used for control in early Spring or in September when crawlers are active.  Systemic insecticide such as imidacloprid can be applied as a soil drench in the spring when spraying the plant is not practical.  


Strawberry root weevil is an accidental invader that wanders into the house from the yard. These hard-shelled, pear-shaped weevils are as harmless as they are cute!  They cannot bite or sting and they do not feed on people, plants, property or pets.  The practical control is to vacuum them up as they appear and discard.  More.


One favorite we don't see nearly enough is the variegated fritillary butterfly or its caterpillar.  Our thanks to L. Terry for sending the photo below.  The variegated fritillary caterpillar can grow to 2 inches before forming a chrysalis and transforming to the 3-inch wing span butterfly.   The caterpillar feeds on a variety of plants, but is not a crop or garden pest (unless it eats your pansies).  Host plants include: violet, pansy, passion-vine, lamb's ear, mayapple, sedum, purslane, "and others."



Symptoms of SNEED on Black Hills spruce.Symptoms of SNEED on Black Hills spruce.


Magnolia scales found on the underside of magnolia branches are the largest scale insect in Iowa.Magnolia scales found on the underside of magnolia branches are the largest scale insect in Iowa.


Strawberry root weevils have a distinctive "light-bulb shape" and shiny black color.Strawberry root weevils have a distinctive "light-bulb shape" and shiny black color.


The variegated fritillary caterpillar is shiny orange, black and white with branched spines along the sides.

The variegated fritillary caterpillar is shiny orange, black and white with branched spines along the sides.


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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