University of Maryland Extension

What Crops Up on Impatiens?

Garden impatiens and New Guinea impatiens are significant crops for the greenhouse industry and are used heavily in home and commercial landscapes.  With the recent occurrence of Impatiens downy mildew on garden impatiens, many growers decided in 2013 to reduce or eliminate production of garden impatiens and increase other types of impatiens as well as look for alternative crops to market in their place. Both crops of impatiens require close scouting in order to detect pest problems early and keep them from escalating out of control.

Downy Mildew

Maryland had its first reported case of downy mildew on garden impatiens in early June 2012. All varieties of garden impatiens (seed or cutting propagated) are susceptible to downy mildew. New Guinea impatiens (Impatiens x hawkeri) are reported to be tolerant of the disease. SunPatiens, a hybrid impatiens developed by Sakata Seeds, is reported to be resistant. The impatiens downy mildew pathogen (Plasmopara obducens) does not infect any hosts other than impatiens. Symptoms of this downy mildew can be very subtle – slight chlorosis (yellowing) or mottling of leaves, leaf edges curling downward, and plant stunting are early symptoms, followed by leaf drop. Look for the white to grayish white fuzzy growth on the undersides of the yellow or curled leaves as the key sign of downy mildew infection. If you are in doubt, send a sample to the UMD Plant Diagnostic Lab for diagnosis. See the Downy Mildew on Impatiens Pest Alert for details on management of this disease.


Impatiens downy mildew

INSV on New Guinea impatiens

Tospovirus

New Guinea and garden impatiens can become infected with a tospovirus, Impatiens Necrotic Spot Virus (INSV), which is vectored by western flower thrips. Symptoms include dark black or purple lesion on the stems and leaf veins and dark ringspots or blotches on leaves. Infected plants are stunted, and young leaves may be small and misshapen. Inspect plants regularly for virus symptoms. There are ELISA test kits available for INSV or you can send plant samples to Agdia, Inc. for testing. It is important to keep weeds in and around the growing area under control to reduce the source of thrips and possible infection. Destroy plants showing virus symptoms. Keep thrips’ populations under control. The tolerance threshold for thrips is very low if the virus has been detected in the greenhouse.

                                                  
                               INSV on garden impatiens was confirmed by testing.

Western Flower Thrips Vectors Tospovirus

Western flower thrips (WFT) tend to hide out in tight spots such as the flower buds of plants which make them difficult to detect early and control easily. A tap test is a good monitoring method that dislodges thrips from the plant so you can determine if they are present and in what numbers. Thrips feed on buds before they open causing them to be distorted.  Look for silver streaking on flowers and foliage.


Western flower thrips on gloxinia

Western flower thrips on sticky card

Management and Control: Make sure you examine incoming plants for INSV symptoms and thrips – keeping the virus and the vector out of the greenhouse is the best management strategy. There is no cure for infected plants – they must be discarded. Use sticky cards and tap tests to monitor for thrips in the greenhouse. Control for thrips include Overture, Pylon (greenhouse only), Mesurol, Pedestal, and Conserve.


Broad Mite And Cyclamen Mite

Cooler weather helps these mites flourish. As greenhouses in Maryland heat up in June and July, damage usually trails off at a time when many of the bedding plants have been sold into the marketplace so the problem may be going out the door.

Broad mites and cyclamen mites are extremely small which makes it difficult to detect the mites before severe feeding damage is evident. The mites hide in dark, moist areas within tender buds or deep within the flower further hindering detection. Cyclamen mites are shiny and elliptical in shape with four pairs of legs. Females are translucent yellow-to-orange whereas males are light brown with a claw on each back leg. Broad mites are slightly smaller than cyclamen mites and are colorless-to-pale brown with a white stripe down the center of their backs.


Broad mite damage

Cyclamen mite damage

One way to distinguish cyclamen mites from broad mites is by the egg stage. Cyclamen mite eggs tend to be laid in dark, moist areas. Eggs are smooth, elliptical and about 1/2 the size of the adult female. Broad mite eggs are elliptical but are covered by small whitish bumps that look like rows of diamonds. Broad mite eggs tend to be laid so they are more exposed on the underside of the leaf or stem surface than cyclamen mite eggs. Cyclamen mites prefer to feed in buds and young leaves. Leaves curl inward and develop a puckered appearance. Pit-like depressions can also form. Leaves may become brittle or appear streaked. Flowers can become shriveled and discolored. Sometimes, flower buds may not open at all. Cyclamen mites have a broad host range and can feed on New Guinea impatiens, cyclamen, dahlia, gloxinia, ivy, snapdragons, vinca, chrysanthemum, geranium, fuchsia, begonia, and petunia.

Broad mites inject a toxin from their saliva as they feed. Leaves become twisted, hardened and distorted with bronzed lower surfaces. Young terminal buds can be killed. Leaves frequently turn downward. Broad mites, like cyclamen mite, have a wide host range and can feed on ageratum, begonia, cyclamen, dahlia, gerbera, gloxinia, hibiscus, ivy, jasmine, impatiens, New Guinea impatiens, lantana, marigold, snapdragon, verbena, and zinnia. Many of you are growing vegetable transplants and broad mites can infest vegetable transplants such as beans, peppers and tomatoes.

Check your plants. Both broad and cyclamen mites are too small to be seen without the aid of a microscope. Regular inspection of crops for their feeding is needed. These mites tend to avoid light and are found in the crown of host plants. If characteristic symptoms are seen, send samples to an entomologist at your local Extension office to confirm identification.

Control options include: Bifenazate (Floramite), Fenpyroximate (Akari), Chlorfenapyr (Pylon), Pyridaben (Sanmite), and Spiromesifen (Judo).


Botrytis

Botrytis is a common disease in production greenhouses. When weather is wet and cool, this pathogen flourishes. It often develops on dropped flower petals especially from plants overhead in hanging baskets. Maintaining good air circulation and keeping plants on the dry side help reduce the incidence and spread of Botrytis. Good sanitation is also important as a management tool. See the Botrytis section in the in the Total Crop Management for Greenhouse Production manual for chemical control options.

                     
                      Botyrtis on New Guinea impatiens where there had been
                      cold damage


Cold Injury

New Guinea impatiens are susceptible to cold injury which causes dark, necrotic areas on foliage. Botrytis can also show up on New Guinea Impatiens at areas where foliage had cold injury.

Also look for...
aphids, mealybugs and slugs which feed on a variety of greenhouse crops.

Authors: Karen Rane, Plant Diagnostic Lab, University of Maryland, College Park, and Stanton Gill, Commerical Horticulture at the Central Maryland Research and Education Center in Ellicott City, University of Maryland Extension

Maintained by the IET Department of the College of Agriculture and Natural Resources. © 2020. Web Accessibility

University programs, activities, and facilities are available to all without regard to race, color, sex, gender identity or expression, sexual orientation, marital status, age, national origin, political affiliation, physical or mental disability, religion, protected veteran status, genetic information, personal appearance, or any other legally protected class. If you need a reasonable accommodation to participate in any event or activity, please contact your local University of Maryland Extension Office.