Plant and Insect Diagnostic Clinic Update - October 21, 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.
Broadleaf Trees
Oak powdery mildew has been diagnosed in a few recent samples. The fungus responsible for powdery mildew (Erysiphe spp.) appears as a gray, powdery blotches on the leaves.  The fungus overwinters on dead leaves and thrives under wet weather conditions. To reduce the source of infection in spring, remove and destroy infected leaves from under the tree.
Fruit (small and tree fruit)
Cherry leaf spot, caused by the fungal pathogen Blumeriella sp. was confirmed last week in the Clinic. Sanitation practices and increased aeration (proper pruning and training) can help reduce the impact of this disease. Removing infected tissue, symptomatic and fallen leaves, can contribute to reducing the numbers of the pathogen. Avoid overhead irrigation to prevent splash dispersal of the pathogen from symptomatic to healthy tissue. Water early in the day and avoid locations with long periods of shade. All these practices aim to decrease moisture and leaf wetness needed for the pathogen to infect and cause the foliar symptoms observed. 
Perennials and Annuals
Geranium bacterial leaf spot in geranium is caused by Pseudomonas sp. To manage bacterial disease of geranium, focus on sanitation and clean propagative material. Do not overwinter or take cuttings from infected plants.
We confirmed bacterial soft rot in iris in recent samples. Rhizomes were discolored, rotting, and isolations from rhizomes yielded Erwinia spp the causal agent of bacterial soft rot. The infection by soft rot usually begins where there has been some sort of physical damage or open wounds. This pathogen is commonly associated with the iris borer since the borer creates wounds that allow the bacterium and other pathogens to infect healthy tissue.
Avoid planting plant material showing any discoloration or soft tissues. Remove the infected plant tissue from the affected area and avoid replanting iris in this area. Unfortunately, no fungicide or bactericide are recommended for managing this disease. However, monitoring and managing iris borer can help reduce the soft rot development and severity.


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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