Plant and Insect Diagnostic Clinic Update - August 21, 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.




Tomato Mystery.  The following interesting description reached our Clinic.  Can you guess the problem?  Garden tomatoes were not being productive and the plants looked stunted. Symptoms included yellow leaves (no spots) and with a shriveled look. 


Beginner gardeners and even experienced ones may not be aware that black walnut trees close to a garden can affect the growth and productivity of many plants, but especially tomatoes.  Walnut trees (including the roots) produce chemical compounds (juglone) that can be toxic to tomatoes and other plants and inhibit their growth.  For a review of juglone toxicity to vegetables, trees, shrubs and perennials, see the Iowa State University Extension & Outreach Garden Column from July 11, 2005


Oak Botryosphaeria Twig Blight is commonly observed this time of the year. The symptoms are clusters of brown, wilted leaves on the tips of branches and twig dieback extending only 4 to 6 inches inward from the tips. Photo below. Botryosphaeria Twig Blight should not be confused with oak wilt. To compare the symptoms of the two visit pages 10 for twig blight and page 12 for oak wilt on the resource “How To Recognize Common Diseases of Oaks in the Midwest” from the USDA Forest Service.


We have received rose samples that were suffering from cankers.  Several fungal pathogens can be responsible for the symptoms on the stem. Common control measures include pruning affected tissue to reduce further infection and planting disease-free plants. More information on fungal cankers see the University of Illinois Report # 626, "Rose Cane Cankers." 


Apple bitter rot is being observed by growers. This disease was reported for the first time in Iowa in 2007 and it is here to stay.  The picture below shows early lesion development.


“Mummified fruit and dead wood should be removed to reduce the source of inoculum. No apple variety is completely immune to disease; however, some varieties like Fuji, Golden Delicious, and Empire are more susceptible”. For more information on apple bitter rot visit Michigan State University IPM website, "Bitter Rot."    





We have received many samples of spruce with spruce bud scale this summer.  This soft scale looks like a small brown pea and can be found nestled in branch nodes (looking like a bud).  Populations are usually not large enough to warrant treatment.


This is the time of year when calls about wasps become common.  Colonies that started as a single queen in the spring have grown throughout the summer and populations are now large and noticeable.  The most problematic wasps are the yellowjackets.  They are cavity nesters and often nest underground and will aggressively defend the colony.  A new multi-state Extension pamphlet on wasp and bee control is available online.  


Galls caused by tiny wasps and midges are a common sight on oak leaves.  Many are very beautiful and they do not harm the health of the tree.  Occasionally galls can be so numerous that they can curl and deform leaves.  There are no practical controls for insect galls as the wasp larva is well protected from insecticide inside the plant tissue that has formed around it.  Galls cannot be cured after they have formed and prevention is not practical.



Rose with cankered stem.

Rose with cankered stem. Fungal fruiting bodies on cankered rose stem.

Fungal fruiting bodies on cankered rose stem. Apple bitter rot symptoms on empire apple

Apple bitter rot symptoms on empire apple Oak spangles galls look like small flattened saucers on the underside of the leaf.

Oak spangles galls look like small flattened saucers on the underside of the leaf. Severe gall infestations may curl or deform leaves.

Severe gall infestations may curl or deform leaves.


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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