Southeastern Ornamental Horticulture Production and Integrated Pest Management

News alerts and tips from Southeastern universities.

New Downy Mildew Affecting Impatiens!!!

Jean Williams-Woodward, UGA Extension Plant Pathologist
 
A relatively new downy mildew disease is infecting impatiens (Impatiens walleriana), a shade-tolerant, flowering annual plant that is a staple in landscapes. Downy mildew on impatiens caused by Plasmopara obducens was first seen in 2004 and it has shown up sporadically in the US since then, primarily in the northeastern US. However, this disease is currently devastating nursery crops and landscape plantings in central and southern Florida. Several factors may be the cause of the outbreak, including the movement of infected plants within the ornamental trade and the mild winter in Florida that has kept impatiens in the landscape longer than usual, which has provided an overwintering site for the pathogen. Infected plants often show downward curling leaves, small new growth, reduced flowering and a leaf stippling pattern that resembles spider mite feeding injury. This downy mildew is a prolific spore-producer. Sporangia can be seen on the leaf underside in cooler weather. Eventually, infected plants defoliate and may die. Oospores (survival spores) have been seen within the stems of infected plants in New England states that could allow the pathogen to survive within a landscape bed.

 

In response to this disease outbreak, it may be advisable for growers and landscapers to limit impatiens orders for spring sales and use begonias or New Guinea impatiens as replacement plants as these are not susceptible to infection. If plugs are coming from Florida growers, supplies of impatiens may be limited. We don’t know how the disease will progress during our hot, summer months; however, past experience with this disease in a few states suggests that the pathogen stops producing spores and infecting plants during hot weather. One of the main concerns for growers is that although they may be able to reduce infection and symptom development within their operation through preventive fungicide applications, once these plants go out into the landscape, the disease may cause severe losses for their customers in the spring and fall as fungicide use is often not continued in landscapes. Impatiens downy mildew infection can be reduced through fungicide drenches (28-day interval) or sprays (7-day interval) of Subdue MAXX (mefenoxam), Adorn (fluopicolide), Vital (potassium phosphite), Protect T/O (mancozeb), Pageant (pyraclostrobin + boscalid), Disarm (fluoxastrobin), Segway (cyazofamid), Stature SC (dimethomorph), FenStop (fenamidone) and Heritage (azoxystrobin) plus Capsil as a surfactant. Aliette (fosetyl-Al) has not provided control of this disease in several trials. Fungicide resistance development is a real concern for downy mildew diseases, so rotate fungicides making no more than two consecutive applications of the same fungicide or a fungicide with the same mode of action (with the same FRAC code).

 

Since most downy mildew diseases are blown northward from southern regions during storms, it may be only a matter of time before this disease shows up in Georgia and other southeastern states. If you have questions, please contact me at jwoodwar@uga.edu.