An Update on Impatiens Downy Mildew

Impatiens downy mildew, one of the most notorious diseases to affect ornamental greenhouse production, has been in the country since the late 1800s. However, it was only seen in the greenhouse for the first time in the spring of 2004. During that year, it was reported in eight states, ranging from California to West Virginia. Reports after 2004 were sporadic in the U.S., until 2011.

In 2011, impatiens downy mildew (IDM) was rampant in not only the U.S., but almost every continent. The disease, caused by the pathogen Plasmopara obducens, decimated the impatiens market. One of the most popular bedding plants across the globe, impatiens quickly became a rare commodity. There is an exponential opportunity for growth in the market, as seen below.


The Biology

Downy mildews, including IDM, are caused by oomycetes, which thrive in cool, moist conditions between 59°F and 73°F. Zoospores, the primary spores that cause infection are produced within sporangia, which make up the white, downy coating on the undersides of the leaves. The sporangia are produced in response to light, temperature and/or humidity changes, and then disperse through water or air.

The period between infection and visible sporulation varies from five to 14 days. In dry, warm conditions, sporulation may not occur, or is reduced, and symptoms may be more subtle. This can sometimes result in inadvertent movement of infected plants, allowing the pathogen to continue spreading.

 The Symptoms

IDM can be mistaken for other issues, including foliar spray injury, nutritional imbalance, chilling or even spider mite infestations. Symptoms can be delayed with IDM, or masked by high fertility levels, so it is imperative to monitor crops closely to catch the disease before it spreads. Leaf discoloration or spotting, often within the veins, are the most common initial symptoms, as well as:

  • Pale foliage with yellow, tan, or red blotchy areas
  • Distorted, wilting, or downward curling of the leaves
  • White or light gray/purple fuzz on the undersides of leaves
  • Small emerging leaves
  • Flower buds fail to form
  • Stunting of plant growth/malformation of leaves and flower buds

Advanced infections result in severe defoliation and plant death.

The Solutions

Recent innovations in plant protection products and genetics provide the tools needed to successfully produce impatiens in the greenhouse and grow them in the landscape. Practicing good resistance management is critical to preserve plant protection chemistries and crop genetics.

Implementing a rotation program that uses a “systemic sandwich” approach is recommended. With this approach, systemic fungicides are applied as a drench at transplant or at the beginning of production and again prior to shipping. Other fungicides with translaminar activity should be applied as sprays in between. The use of systemic products as a drench prior to shipping provides extended protection in the landscape for the consumer, maximizing garden performance. The sprays applied between the drenches are part of a resistance management strategy to take the pressure off the control provided by the drench.

Segovis fungicide is a recent chemistry with a unique mode of action that can be applied as a drench for long-lasting control of IDM. This product is ideal for rotation programs as its unique active ingredient in FRAC Group 49 can help diversify treatments. Mural and Micora fungicides can be applied as sprays between to offer translaminar activity and continued control of downy mildews, as well as other diseases.

New genetics are also available that are more resilient to IDM than ever before. A combination of reliable plant protection products and impatiens varieties can help you successfully grow impatiens once again and take back the shade.