Plant and Insect Diagnostic Clinic Update - April 28, 2010

The following are highlights and updates about samples and questions recently received in the Clinic:


Powdery mildew on Kentucky Bluegrass caused by the fungus Erisiphe graminis. Fungus spore production and infection of leaves are favored by cool humid cloudy periods.  Selective pruning of trees or ornamental shrubs that will allow good penetration of sunlight and increase air circulation in turf canopy will help reduce the severity of the disease in home lawns.  Disease is known to be severe in turf heavily fertilized with nitrogen.

Pythium seedling blight and damping-off of spinach.  Water-soaked, greasy lesions may form on hypocotyls and roots after emergence when infected with Pythium spp. causing small seedlings to collapse and wither. Seed treatment with broad-spectrum fungicides is effective means of reducing disease. For organic gardeners microbial biopesticides may be considered.    

Bur Oak Blight (BOB) is a serious disease of Bur Oak trees caused by a new species of Tubakia fungus.   This time of the year plant samples are dead leaves that fail to drop in the fall. Petioles of these leaves are carrying numerous dark fungus structures, each full of hundred of spores ready to infect current year plant material.  Unfortunately newly infected leaves will turn brown in the fall and drop or remain in the tree where fungus survives winter and produce more fungal spores for next year.  Research is under way at Iowa State University to be able to understand better and find ways to control this rapidly growing threat to Bur Oaks in Iowa. 


Several samples of Pine tortoise scale on pine have been submitted to the clinic.  Pine tortoise scales are a softshell scale that produce honeydew (this is basically scale insect excrement) that is rich in plant sugars. A mold called sooty mold grows and feeds on these plant sugars and causes a black coloration on the needles and branches. The sooty mold does not directly harm the pine, but it can block sunlight and kill needles. The adult scales are about 1/4 inch, rounded and tan to reddish in color.  In Iowa late June is when crawlers are active and treatments should be applied.


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