I visited a nursery this past Thursday morning. Problem: Unidentified damage on hydrangea liners - severe leaf distortion and plant stunting, resembling herbicide injury, but this hypothesis made no sense given grower expertise and practices and the spatial relationship of affected plants. The picture above illustrates the difference between normal and affected hydrangeas. One mistake I made: I did not take out my hand lens. Left samples at OSU-Plant Pathology by Thursday afternoon.
Diagnosis: Received e-mailed results from Nancy Taylor of the C. Wayne Ellett Plant and Pest Diagnostic (PPDC) by Friday afternoon - Broad mites (Polyphagotarsonemus latus). Contacted grower Friday afternoon.
Management: As per OSU’s BugDoc, Dave Shetlar: Applications of abamectin miticide (Avid).
This is an example of the OSU team working at its best, with the PPDC the key center of diagnosis and confirmation that is essential to helping Ohio’s green industry. Additional help in this case came from Dave Shetlar, with background information and an image of the broad mite eggs below from OSU’s Beth Scheckelhoff and Luis Canas.
Broad mites are tiny mites about 1/150 an inch in size, about half the size of two-spotted spider mites, and thus needing a good hand lens or dissecting scope for viewing. They have a broad host range (polyphagous) that includes many annual and perennial flowers as well as shrub species such as hydrangeas, clethra and others. Survival of mites through winters is mostly in protected greenhouse and nursery settings. Uncontrolled, these mites can make plants unsalable.
Broad mites find their way into dense foliage and buds and on undersides of leaves, so they require excellent coverage with miticides such as Avid that control this type of mite. Obviously, labels of the product actually used in each case should be carefully reviewed to determine safe and effective use and for any information relative to plant sensitivity.
The family of broad mites is Tarsonemidae (the thread-footed mites) and includes cyclamen mites. This is a different mite family from Tetranychidae, the spider mites. Broad mites have a rapid turnover of generations, especially during hot weather, so once diagnosed use of labeled miticides post-haste is recommended.
Damage often includes a bronzing of the undersides of the foliage, along with stunting and downward and inward cupping of foliage. This type of damage is often mistaken for 2.4-D or other growth-regulator herbicides or some type of disease, so rapid and accurate diagnosis is important. Broad mite eggs are translucent, elliptical, and have white tufts or “dots with wax-like domes”, and are distinctive enough for identification.