article written by Robert LaHoff

Here it is quite simply… one of the most popular annuals grown, marketed and sold in the Northeast may be difficult to obtain this coming spring. “Impatiens avoidance behavior will largely be seen for the first time in 2013, says Cornell University Plant Pathologist Margery Daughtrey” (greenhouse Grower). Impatiens have and are being attacked by Impatiens Downy Mildew (IDM), Plasmopara obducens, a destructive foliar disease that has aggressively taken on Impatiens walleriana, your garden variety Impatiens. Since 2011, in North America, there have been substantial regional outbreaks of this disease and it is growing exponentially. Capable of causing complete defoliation and or plant collapse in your landscapes, this disease poses a serious threat. Europe has been dealing with this problem before us and there is no “practical” spray regimen to control or cure this problem for the home gardener. Reacting to the symptoms of this disease is too little too late as your plants can be infected 5-14 days earlier, its latent period, before physical signs appear. Scary stuff to see first hand, last year in New Jersey, as it wiped out entire crops of Impatiens with ease.

Going forward, growers can be as clean as humanly possible growing their Impatiens crop, but there is no guarantee that they will reach the end user without infection. Both vegetative propagated and seed-raised Impatiens walleriana are susceptible. There is no evidence, yet, of seedborne transmission of Plasmopara obducens. This pathogen moves primarily by being wind-dispersed and by aerial sporangia (spore containing sacs) thus, infecting plant material plugs, liners, cuttings and finished plants. Further more, 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 (Ball Horticulture). Primary symptoms of (IDM) include light green markings or stippling on the leaves, leaves curling down at the margins, white downy-like fungal growth on the underside of the leaves, stunting and leaf and flower drop. Late season defoliation has also been called “Green Stick Syndrome”, seeing only the naked stems of the plant. This was apparent, in central New Jersey, last summer to me. I also saw first hand Impatien plants that simply looked stunted refusing to grow.

Adding to this problem are OOSPORES! An oospore is a thick-walled sexual spore that develops from a fertilized oospore in some algae and fungi (Wikipedia). More simply put, it is an overwintering or survival spore. Oospores in soil cause new infections for the following season. Infected plants from the previous year, that collapsed, release these spores into your soil and contaminate it for the following year and beyond. It is unclear how long these oospores can survive. However, a similar downy mildew oospore with sunflowers, Plasmopara halstedii, was reported to survive 8-10 years in the soil. Harsh winters may or may not help, as oospores have been known to survive 5 degree Fahrenheit temperatures.

So what can be done to minimize the risk? Don’t allow infected plants to remain in planting beds. Remove, but don’t compost any parts of the plant; including roots, leaves and stems. Avoid planting Impatiens walleriana… maybe?

There is a bright side to this problem however. New Guinea Impatiens, Impatiens hawkeri and SunPatiens (a cross between New Guinea and wild Impatiens) are non-host plants, meaning they are not susceptible. Not typically sold in flats the same way Impatiens walleriana are, perhaps they will be marketed this way going forward? Other annuals commonly found for sale in the spring and susceptible to downy-mildew diseases are: coleus, alyssum, snapdragons and salvia. Rest assured, these annuals are not susceptible to the same downy mildew (IDM), as that is a different species.

Impatiens Downy Mildew is an aggressive disease that is having a tremendous impact on the green industry as we know it. Growers and retailers are struggling with this issue and like it or not, supplies will be diminished going forward until a solution is found. Research is being done to breed resistant varieties. Since 2003-2004, Europe has been conducting studies but as of yet, has had little luck finding those with natural resistance. Preventively, growers have found that this is a disease that can be managed! However, once their Impatiens leaves their controlled environments there are too many variables that can affect the outcome. This is an environmental issue with potential long distance spread of aerial spores and oospores contaminating landscape beds.

Finally, you may want to consider Begonia’s, Cyclamen, Euphorbia’s, Lobelia, New Guinea Impatiens, Nicotiana and Torenia as flowering shade tolerant substitutions. Coleus, Dichondra and Plectranthus would all be good foliage options to consider. Good luck, be patient, stay informed and…. now you know.