Plant and Insect Diagnostic Clinic Update - July 28, 2010

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


Cicada killer wasps continue to strike terror into the hearts of homeowners.  These docile critters need not be feared, but many still seek control advice.  You have 3 choices:  coexist in peace; "ping" them from the air with a tennis racket; treat the nest entrances at night with insecticide dust.  See our Insect information Note.

Squash bugs are a common sap-feeding pest on squash and pumpkins.  Recommendations for control always start with the admonishment to treat while the nymphs are small for best results.  See the photo below for how small the first instar nymphs are when they emerge from the eggs (lower left corner of the photo).

Ailanthus webworm moth is a small moth too attractive to ignore.  See the photo below.  The caterpillars of this moth feed on the tree of heaven (Ailanthus sp.) but cause no appreciable damage to the foliage.  For more about this insect see BugGuide.  Tree of heaven is a small tree or shrub that often becomes invasive.  See the Invasive Plants of the Midwest Fact Sheet.


Raspberry anthracnose caused by the fungus Elsinoe veneta.  Symptoms of the disease start as small purple circular or elliptical spots on canes. As disease progresses, the spots enlarge and the centers become sunken and gray with margins appearing slightly raised and purple.  Fungus can attack leaves, petioles, pedicels, flower buds and fruit. To manage the disease avoids overhead irrigation to prevent spread of the disease. Improve air circulation within plants to reduce hours of wetness duration required by the fungus to cause infections.  Avoid excessive application of fertilizer especially nitrogen. After harvest infected plant material should be removed and destroyed to reduce or eliminate fungal inoculum.  

Fusarium stem rot of pepper. Plants infected early in the production season have lesions at the base of the stem, while those infected later in the season are more likely to have lesions at upper  nodes. Removal and disposal of infected plants will reduce the amount of inoculum. No fungicide has been registered for control of Fusarium stem rot of pepper. Sanitation is the best management practice to control this disease in the greenhouse and in the field. High relative humidity is also favorable for the disease so increasing the air movement through the crop will help reduce infections. Fusarium spores survive in the soil, so using clean/sterile soil or seed treatment will also help.    

Bacterial blight of Lilac caused by the bacterium Pseudomonas syringae pv. syringae. Symptoms include black spots on leaves and stems of the plant. Bacterial infections will sometimes infect shoots causing them to turn black and bend backward similar to Fire Blight disease on pears. Management is based mainly on sanitation removing and discarding infected plant parts below the point of infection.  Stresses caused by the lack of nutrients and/or water can predispose the plants to an infection.    

Euonymus leaf spot caused by the fungus Exosporium concentricum. Symptoms are irregular spots with dark brown to purple margins on leaves and stems (see picture below). Dark color fungal structures containing spores can be seen in the center or the spots.  Remove and discard (burn or buried) infected plant parts to reduce the amount of fungal inoculum.  A fungicide spray (chlorothalonil, thiophanate methyl or cooper based fungicides) in early May and June will help reduce infections.   

Anthracnose and black rot of grape.  Two common diseases of grape in Iowa. Anthracnose will cause purple spots first on leaves, stems and fruit. In a closer look the spots will have concentric dark brown to orange concentric ring that are masses of fungal spores. This spores can spread easily with water to cause more infections. Black rot disease symptoms are irregular brown spots on leaves presenting tiny black spot that are the fungal structures also containing hundred of spores.  Lesion can also be observed on stems and fruit forming small round to oval sunken spots. Infected fruit becomes rotten, shrivel, become black, hard and remains in the plant.  Management is based mainly on sanitation and preventive fungicide sprays.

Early Blight and Septoria Leaf spot of tomato, both diseases can occur together.  The sample received showed characteristic concentric rings of early blight disease some spots presenting also brown small fungal structures containing hundreds of filiform Septoria lycopersici spores.

First instar squash bug nymphs, just emerged from eggs (lower left).  Photo by Mark Crawford.

First instar squash bug nymphs, just emerged from eggs (lower left). Photo by Mark Crawford.

Ailanthus webworm moth is interesting, attractive and rarely noticed

Ailanthus webworm moth is interesting, attractive and rarely noticed.

Euonymus leaf spot.


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