I have wort-like growths on the undersides of my grape leaves. What is it and does it need to be controlled?

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Question: 

I have wort-like growths on the undersides of my grape leaves. What is it and does it need to be controlled?

Answer: 

The wort-like growths on the grape leaves were probably caused by the grape phylloxera.  The grape phylloxera is a small, aphid-like insect.  The grape phylloxera has a complex life cycle that affects the leaves and roots of the grapevine. 

The foliar form survives the winter as eggs under the bark of the grapevine.  The eggs hatch in spring and the young insects (nymphs) move to the new leaves.  They feed on the upper surfaces of the leaves, causing galls to form on the lower leaf surfaces.  Mature phylloxera females lay eggs within the galls.  Subsequent generations of nymphs emerge, crawl to the shoot tips, settle on new leaves, and form new galls.  Fortunately, leaf-infesting phylloxera do not cause serious harm to healthy grapevines.  Control measures are not necessary. 

Root-infesting phylloxera overwinter as nymphs on the roots.  They mature in spring and produce eggs that hatch into subsequent generations of nymphs, which start new galls on roots.  Winged adults develop in late summer or early fall and emerge from the soil to lay eggs on the grapevine stems.  The root form of the grape phylloxera will destroy heavily infested plants.  Fortunately, the grape varieties grown in Iowa are not damaged by the root form. 
 

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