Downy mildew on impatienshas been recently diagnosedin Michigan. Growers visit-ing from the UnitedKingdom last November

mentioned that they had experienced asevere outbreak of downy mildew on hybridimpatiens earlier in the year. The disease hadtaken the growers by surprise, as they werenot familiar with downy mildew on impa-tiens. Snapdragon, rose, alyssum, pansy, andsalvia are susceptible to downy mildew andoften suffer from the disease. Other cropssuch as cineraria, gerbera, lisianthus, ranun-culus, larkspur, anemone, garden balsam andimpatiens may also become infected by adowny mildew pathogen, although the dis-ease problems are less frequent.

The downy mildew that affects Impatiensspp. is caused by a fungal-like microscopicorganism called Plasmopara obducens .Although downy mildew diseases look thesame, they are quite different microscopical-ly with each preferring a particular plant.Snapdragons, roses and impatiens each havea unique downy mildew pathogen. The

downy mildew on snapdragons cannotblight roses, nor can the downy mildew onimpatiens spread to any plant outside of theimpatiens group. Plants in the impatiensgroup include bedding plant impatiens, NewGuinea impatiens, vegetatively-propagatedimpatiens and garden balsam.

Downy mildew is a pathogen of Impatiensspp. throughout the world and has beenreported in the United States, Canada, Asia,Europe and India. This disease has not beena persistent problem for the United Statesand has been reported at only sporadic inter-vals (i.e., 1942, 1953, 1955, 1959, 1960, 1973and 1982) in various locations, includingMontana, Minnesota, Wisconsin, Mississippi,Missouri, Indiana, West Virginia, theNortheast and now Michigan.

The disease spores may be spread withina greenhouse by air currents and/or splash-ing water. Spread from one greenhouse toanother may occur via movement of infect-ed plants. U.S. growers must be vigilant andbe prepared to take quick action. Giventhese recent problems and the confirmationof the disease in Michigan, growers in the ➧

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IT’S BACK

Sporulating Leaf. (Photos courtesy of M.K. Hausbeck)

Downy MildewSymptoms

• Yellowish or pale green foliage

• Downward curling of the leaves

• Distortion of the leaves

• White to light gray fuzz on the

undersides of the leaves

• Emerging leaves that are small

and/or discolored (yellow or

pale green)

• Flower buds may fail to form

• Plants may be stunted

By M.K. Hausbeck

Downy mildew on impatiens in the United Kingdom…a problem for the United States?

United States should review the downymildew basics because this can be an explo-sive, destructive and costly pest if it is notmanaged.

Apparently, it can be eradicated given thegaps in the disease reports. However, giventhe widespread losses growers in the UnitedKingdom suffered just a few months ago —their plant losses mounted before they couldhalt the disease — we should assume thatlosses from this disease have the potential tobe large. Early detection and a rapidresponse can avert the devastation that someU.K. growers experienced as a result of thisdowny mildew.

WHAT TO DOKnow the symptoms. Downy mildew dis-

torts plants, blights the foliage and can bevery destructive if not detected early.Disease symptoms may be localized withinthe plant without causing severe injury.Leaf spots are “local infections” and aremild symptoms because the downy mildewis l imited within the plant. If downymildew advances and invades the internal

or vascular system of the plant, the diseasebecomes “systemic” and is more destruc-tive. When the infection is systemic, theinternal mechanism (vascular system) of theplant is affected. Plants become stunted andyellowed with distorted leaves. Whenyoung plants and seedlings are infected,they generally do not survive.

The most obvious sign of downy mildewis the white, grayish fuzz that develops onthe underside of the leaves. This can gounnoticed because growers are not accus-tomed to turning over leaves to look forplant problems. Perhaps other than an espe-cially severe case of Botrytis, no other dis-ease causes such obvious gray fuzz especial-ly on the underside of the leaf. This diseaseshould not be confused with powderymildew, which affects some floriculture cropsand has white fuzzy growth.

Scout for the disease. Unlike other dis-eases that are readily seen from the top ofthe leaf, downy mildew is most obvious onthe undersides of leaves. Growers may notbecome aware of downy mildew infectionuntil the disease is well established and dif-

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The top leaf is healthy, while the bottom one is infected.

ficult to control. A small amount of downymildew can become established over severaldays or weeks on the undersides of leavesand remain unnoticed until the environ-ment is favorable for rapid production ofspores. Once spores are produced in great

quantities, the disease spreads rapidly andbecomes evident to growers. Growers expe-riencing losses due to downy mildew onsusceptible crops often report that the dis-ease seemed to appear overnight. Oncedowny mildew is established at a low level,

wet weather can prompt an explosion ofdisease. Fungicide sprays are best startedprior to a disease outbreak.

When receiving impatiens, scout themimmediately by examining fully-expandedleaves, paying special attention to theundersides of leaves. Scout a minimum ofone out of every 30 plants. Since plants maybe infected with downy mildew but notshow white or grayish fuzz immediately,they should be scouted weekly. If diseasedplants are discovered, they should be dis-carded immediately.

Dispose of diseased plants. Downymildew has a unique, thick-walled survivalstructure that allows it to persist in soil,growing media or diseased plants for years.When disposing of diseased plants, place theentire plant, including the growing mediaand pot, in a bag and promptly seal it. Do notcarry diseased plants through the greenhousefor disposal because spores on the infectedplants will be released and may infect nearbyhealthy plants of the same plant type.Disposing of diseased plants removes asource of spores that would otherwise ➧

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Keeping the Fungicides Working: A Sample Program

Downy mildew has the bad habit of changing and becoming resistant to systemic

fungicides. Since Subdue MAXX is used so often and is therefore especially at risk, it

should only be used once in a production program and must only be applied as a drench.

An example of a fungicide program for downy mildew:

•Subdue MAXX drench at planting only.

•Weekly foliar sprays could include the following:

Spray 1. Mancozeb

Spray 2. Stature DM 50WP+ Mancozeb

Spray 3. Mancozeb

Spray 4. Aliette + Mancozeb

Spray 5. Mancozeb

Spray 6. Strobilurin (examples: Compass 50WDG or Heritage 50WG) + Mancozeb

Spray 7. Repeat the program beginning with Spray 1

allow the downy mildew tospread. Do not place diseasedplants in a cull or compost pile.They should be removed from thegreenhouse vicinity and destroyedor disposed of in a landfill.

Healthy-appearing plants thatare adjacent to the diseasedplants should also be discarded.

All remaining impatiens any-where on the premises should betreated with fungicides.

PREVENTIONMaintain a dry environment.

Downy mildew can be explosivewhen the weather is wet andhumid. A film of water on the

plant’s surface for more than sixhours allows the downy mildewpathogen to germinate andinfect. Keep the relative humidi-ty below 85 percent to preventcondensation from forming.Water plants when they can dryquickly.

Apply effective fungicides pre-ventively. Fungicides should beapplied preventively before dis-ease develops. Protectant fungi-cides act as a barrier to thedowny mildew pathogen.Mancozeb (Protect T/O, ClearyChemical) is the preferred pro-tectant fungicide for downymildew. Mancozeb is notabsorbed by the plant so thespray program must beginbefore downy mildew becomesestablished. At Michigan StateUniversity, downy mildew stud-ies with snapdragons and veg-etable crops have shown thatweekly sprays of mancozebeffectively suppress downymildew if applications are madeearly and repeated frequently.

Systemic fungicides can beespecially helpful in managingdowny mildew because theseproducts are absorbed by theplant and can help fight newly-

established infections. Systemicfungicides include mefenoxam(Subdue Maxx, Syngenta) as asoil drench and dimethomorph(Stature DM 50WP, SePRO) as afoliar spray. Aliette 80WDG(Bayer Environmental Science) isapplied as a foliar spray and is aunique fungicide because ithelps prompt the plant todefend itself. In research trialson snapdragon, mefenoxamapplied to the soil performedwell against downy mildew.Stature WP, a relatively newproduct, was also effective. Forresearch, other non-registeredand registered fungicides wereused, as some provided goodcontrol and others did not.

M.K. Hausbeck is professor andextension specialist in theDepartment of Plant Pathology atMichigan State University, EastLansing, Mich. She can be reachedby E-mail at hausbec1@msu.edu.

GPN

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Aliette WDG Bayer Environmental Science

Alude Systemic Fungicide Cleary Chemical Corp.

Camelot Whitmire Micro-Gen

Cygnus The Scotts Co.

Compass 0 Olympic Horticultural Products

Dithane Valent USA

Fore Valent USA

Heritage Fungicide Syngenta Professional Products

Manzate 75WD Griffin

Milstop BioWorks

Pentathlon Griffin

Plantshield HC BioWorks

Protect T/O Cleary Chemical Corp.

Quell Sipcam Agro

Stature SePRO Corp.

Subdue MAXX Syngenta Professional Products

Triact 70 Olympic Horticultural Products

ZeroTol BioSafe Systems

Zyban The Scotts Co.

Figure 1. Partial Listing of chemicals available for control of downy mildew.

LearnMoreFor more informationrelated to this article, go to www.gpnmag.com/LM.CFM/gp030405