Plant and Insect Diagnostic Clinic Update - March 3, 2010

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


Larger Yellow Ants and Carpenter Ants.  Recent pre-spring warmer temperatures are causing household ants to stir and become active, and noticed again, often for the first time since late last year.  Larger yellow ants that wandered into the house to spend the winter have remained hidden in wall voids or foundation cracks.  Sunshine and warmer temperatures have given them a dose of "spring fever" and they are out walking about.  The only necessary action is to vacuum or sweep up and discard the ants.  More information is on our Insect Notes website.

Carpenter ants that are nesting in walls, window sills and other structural elements are also getting sufficiently warmed on some days for at least a few to wander out into the open.  Treatment is much harder to accomplish in late winter when activity is limited to a few wanderers.  See our Insect Notes for more information.


Stem and stolon canker have been observed on mint growing in greenhouse conditions. This disease is caused by the fungus Rhizoctonia solani. Roots, runners and rhizomes can present progressively rotting areas. To manage the disease it is important to remove and discard infected plant material, improve drainage and avoid leaving the soil too wet for long periods of time. In addition, disease can be managed with fungicide applications.

Powdery mildew was observed on a Garden Phlox plant sample. Powdery mildew is common on many flowers and other garden plants. Symptoms or powdery mildew are visible white powdery fungal mycelium on top of leaves. Garden phlox cultivars vary in their susceptibility to powdery mildew, however under the right environmental conditions for infection even the most resistant cultivar can get the disease. Avoid shaded conditions and overhead irrigation that provide ideal conditions for the fungus. If the plants are in pots, move the pots to an area where they receive more light and air circulation.

Downy mildew was observed on salvia “Vista Red” seedlings grown on greenhouse. The fungal pathogen was Peronospora spp. Symptoms of downy mildew are soft, fluffy gray, brown fungal sporulation developing on the underside of leaves. Cool and humid conditions are ideal for this fungus to infect plants. To manage the disease it is important to remove and discard infected plant material and avoid long hours of wetness duration. Preventive fungicides will help stop the spread of the disease.


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