Plant and Insect Diagnostic Clinic Update - July 14, 2010

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

 
INSECTS 


The buzzing of cicada's in the trees and the presence of empty cicada shells on tree trunks and other plants indicate that the cicada killer wasp will also be making its presence known. Cicada killer wasps are the largest wasp we have in Iowa and the large size leads to a mistaken assumption that they have a nasty sting. They are not aggressive and I have never heard of a person being stung. It is actually the males that are most often seen, flying a few feet off the ground around their territories, and as with all wasps and bees, males cannot sting. The females who are capable of stinging are busy digging a burrow in the ground, collecting paralyzed cicadas for their young, burying the cicadas in the burrow and laying an egg on them. Burrows have an entrance about the size of a quarter and there is usually a pile of dirt next to it. For more information on these interesting wasps please see our IIIN or Prof. Chuck Holiday's Cicada Killer page (Lafayette College, PA).


The wet weather the past 3 springs and summers has led to large populations of earwigs. If they are accidentally getting indoors you can seal cracks and gaps, reduce moisture by raking back mulch and other ground cover, or apply a barrier treatment (approx 2 ft. up the foundation and 3-5 ft. out into the yard) of insecticide labeled for earwigs around the entire house.   For more information on earwigs please see this Yard and Garden Column


Now is the time to notice the grapevine beetle, a large member of the Junebug family. It resembles a light tan Junebug, but is quite large at 1 inch in length.  There are 6 small black dots on the wing covers. The adults feed on grape foliage but are not a significant pest. Adult beetles found on grapevines need only be handpicked and discarded. The larvae are huge white grubs that live in well-rotted stumps and logs.  Grubs like the one shown below are found occasionally in old stumps, logs and landscape timbers.


DISEASES


Fire Blight of pear and  apple is a bacterial disease caused by the bacterium Erwinia amylovora.  Symptoms appear in the spring as discolor leaves that latter turn dark brown on apple or shots on pear that bent backward and appear scorched by fire.  Symptoms on different apple/pear variety are similar, but might differ in severity.  Management of the disease in home gardens might be limited to removal of infected plant parts when symptoms are first seen and in dry weather . It is important to remove and discard overwintering cankers early in the spring when the plants are still dormant to greatly reduce the amount of inoculums for the next season.  Application of a copper based bactericide at green tip will help prevent infections by any remaining bacteria in the tree.  


Sooty mold is a black crust-like growth over needles of pines, arborvitae and other trees that grows on honeydew from sap feeding insects such as aphids and scale insects.


Bacterial wilt on squash is caused by the bacterium Erwinia tracheiphila. Plants can be severely affected by the disease since vascular plugging by bacteria results in wilting of the runners or the whole plant.  In some cases, summer squash may remain vigorous and continue to produce. Control of bacterial wilt is directed  primarily to control of the cucumber beetle vectors and the prompt removal and discarding of wilted plants to reduce the amount of  bacterial inoculum.


Anthracnose on tomato  (see picture below) caused by the fungus Colletotrichum coccodes most likely.  Anthracnose is primarily a disease of ripe tomatoes however small fruit might have a latent infection.  Susceptibility  and subsequent losses increase as fruit matures.  Symptoms first appear as small slightly depressed circular lesions. A lesion enlarge to about 1 -2 cm and become more sunken with concentric ring markings formed by hundreds of fungal spores (Picture below).  Management practices in home gardens include avoiding overhead irrigation and removal of infected plant material to avoid the spread of the disease.



Empty annul cicada shell.



Cicada killer wasp.



C-shaped larva from rotting wood.



Anthracnose on tomato.

Authors: 

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