Editor’s Note: This article was written with the assistance of Ball Horticultural Company’s “Grower Guidelines” on Impatiens Downy Mildew and plant pathologist Dr. Colleen Warfield. For the most recent version of grower and retail guidelines, visit www.ballseed.com.
Impatiens downy mildew is a destructive foliar disease of Impatiens walleriana that is capable of causing complete defoliation or plant collapse, especially in landscape plantings under moist conditions and cool nights.
While there have been sporadic reports of impatiens downy mildew in U.S. greenhouses since 2004, it was not until the summer of 2011 that regional outbreaks of this disease were seen for the first time in landscape beds and container plantings in North America.
In early January 2012, outbreaks of impatiens downy mildew were observed in landscape beds and greenhouses in south Florida. It is unclear whether this was a continuation of the 2011 outbreaks or a new cycle of disease for 2012. The spread of this disease continued throughout the spring, and by October 2012, impatiens downy mildew had been confirmed in landscape beds and/or greenhouses in 32 states and Washington, D.C.
Your Plants May Be At Risk If …
Young plant and finish growers are at in increased risk for this disease if:
1. They are located in a region where production of I. walleriana coincides with plantings of I. walleriana growing in the landscape.
2. The source of incoming liners and plugs is in a region where infected impatiens are currently growing or have been reported in the landscape.
3. They are growing in a region where infected impatiens were confirmed in the landscape in 2011 or 2012.
All cultivars of I. walleriana (common garden impatiens) and interspecific hybrids with an I. walleriana parent are susceptible, including Fusion, Fiesta and Patchwork.
A few wild species of impatiens are also susceptible; however, there are no other bedding plant species that are known hosts.
Both vegetatively propagated and seed-raised I. walleriana are susceptible but there is no evidence of seedborne transmission of P. obducens.
New Guinea impatiens (Impatiens hawkeri), including Fanfare, Divine, Celebration, Celebrette and Sunpatiens, have high resistance to this disease.
How It Spreads
Sporangia (sac-like structures filled with zoospores) produced on the underside of infected leaves are easily dislodged and can be spread short distances by water splash, and longer distances by air currents.
There are two main routes for entry into a greenhouse facility:
1. Infected plant material (plugs, cuttings, liners)
2. Wind-dispersed, aerial spores from infected plants growing elsewhere. These may travel on the order of hundreds of miles.
Caution: Infected plants not yet showing symptoms may result in the inadvertent movement of the pathogen.
Symptoms Of The Disease
Young plants and immature plant tissues are especially susceptible to infection. Symptoms are often first observed on terminal growth. Seedling cotyledons are highly susceptible to infection.
Early symptoms include:
- Light-green yellowing or stippling of leaves
- Subtle gray markings on upper leaf surface
- Downward curling of infected leaves
- White downy-like fungal growth on the undersides of leaves
Advanced symptoms include:
- Stunting in both plant height and leaf size when infected at an early stage of development
- Leaf and flower drop resulting in bare, leafless stems
- Infected stems become soft and plants collapse under continued wet and cool conditions
Cultural Control Tips
- Minimize greenhouse humidity and leaf wetness (less than four to five hours)
- Frequently scout crop, with particular attention to early leaf symptoms
- Remove symptomatic plants and any fallen leaves immediately
- Bag plant(s) and seal before carrying out of greenhouse; do not compost
- If sporulation is visible, remove adjacent plants within 3 feet
- Segregate impatiens from different sources
- Grow seed-raised and vegetatively propagated impatiens in separate greenhouses to minimize risk of contamination
Chemical Control Tips
- Preventive application is critical. Control is nearly impossible once sporulation has occurred in a growing facility.
- Make first fungicide application at transplant (Adorn+Protect or Adorn+Subdue MAXX tank mix recommended).
- Under low disease pressure or low risk, reapply at 7-day intervals
- Under high disease pressure or high risk, seven-day intervals with foliar applications may not be sufficient due to limited residual activity.
- Apply final application within one week prior to shipping.
- Drenches of Adorn or Subdue MAXX exhibited the longest residual efficacy of all fungicides in a limited number of research trials. However, each product must be tank-mixed with another fungicide from a different class (FRAC code) to reduce the risk of fungicide resistance.