Downy Mildew in Ornamental Crops

Although the disease “downy mildew” sounds similar to the more familiar disease called “powdery mildew,” the downy mildews are caused by fungal relatives called Oomycetes.  These fungal-like organisms, troublesome in during cool, moist weather, attack tender, above-ground plant parts, dispersing rapidly in films of water.  In susceptible ornamental plants, downy mildews are most often caused by species of Peronospora, although species of Plasmopara, Pseudoperonospora, Sclerospora, and Bremia also cause this disease. 

In New Jersey, downy mildew is especially common on roses.  Indeed, the Rutgers Plant Diagnostic Laboratory recently reported the incidence of downy mildew on Knock Out® Roses. (See recent posts to this blog for more information.)  Although all roses are susceptible to downy mildew, disease severity varies by cultivar.

Also in the news is downy mildew of impatiens, caused by the oomycete Plasmopara obducens.  Garden impatiens (Impatiens walleriana) is very susceptible, as are balsam impatiens (I. balsamina), the jewelweeds (I. pallida, I. capensis), and interspecific hybrids with I. walleriana.  First observed in 2004, downy mildew was confirmed in greenhouse and landscape impatiens in 32 states in 2012; in New Jersey, the disease was especially troublesome in Monmouth and Ocean Counties last June.  Note that this particular species of Plasmopara is not an issue for New Guinea impatiens or other flower or vegetable crops.

Symptoms

Symptoms of downy mildew most often appear on the upper leaf surface as a patchy yellow, purple, or brown discoloration bound by leaf veins.  The disease may be mild to severe depending on the host species or cultivar.  Affected leaves may defoliate prematurely, and stunting may also occur in some species, such as snapdragon, and in seedlings and bulb crops.  To see why the disease is called “downy mildew,” turn the leaf over.  Evident on the lower leaf surface are the characteristic downy tufts of white to purple/grey fungal growth.  This growth consists of spores (called sporangia or conidia) that are easily dislodged and disseminated by wind and splashing water.  Symptoms of downy mildew may be sometimes confused with those caused by foliar nematodes.

Disease development

Downy mildews develop on susceptible crops during cool (58 to 72ºF), fairly humid (> 95% RH) conditions when spores from previously infected tissue are splashed or blown by air currents to colonize new sites.  As infection progresses, symptoms develop, and the downy tufts of spores associated with leaf lesions may appear on the lower leaf surface.  Fungal development ceases for most downy mildews when weather becomes dry and warmer than 80ºF for 24 hours.  In the absence of susceptible plant tissue, downy mildew pathogens in cold climates survive in plant debris, soil, or weeds.

Watch closely for symptoms in susceptible crops during periods of cool, humid weather.  When you see foliage that is discolored, turn the leaves over to check for mites or downy mildew sporulation.  Symptoms of this disease can be confused with other fungal (such as Botrytis blight and powdery mildew) or bacterial diseases, foliar nematode, or some nutritional deficiencies.  To distinguish these diseases, look for signs (fungal growth, bacterial streaming, or the bodies of nematodes).

Disease management

Preventive controls for downy mildew are critical.  To manage this disease, first “manage the moisture”:  space plants to ensure good air circulation and rapid drying of foliage after irrigation.  Avoid overhead irrigation when the weather is generally cool.  In greenhouses, reduce relative humidity to less than 85% by, again, properly spacing plants, and use horizontal air flow fans to improve air circulation throughout the house.  Fill the house with warmer, drier air by venting and heating two or three times per hour at dawn and at dusk.

Scout crops regularly for evidence of disease.  Practice good sanitation techniques.  Discard all diseased plants as well as plant debris that may harbor spores, and control weeds that may be another source of downy mildew inoculum.

Protect susceptible crops during cool, wet weather with preventive fungicides.  Active ingredients labeled for control of this disease on one or more ornamental crops include azoxystrobin, copper (ammonium complex, basic copper sulfate, cuprous oxide, hydroxide, octanoate, oxychloride, pentahydrate, salts), cyazofamid, dimethomorph, fenamidone, fludioxonil (use caution), fluopicolide, fluoxastrobin, fosetyl-Al, imazalil, kresoxim-methyl, mancozeb, mandipropamid, mefenoxam, phosphorous acids, polyoxin D, pyraclostrobin, trifloxystrobin, and the combination products boscalid + pyraclostrobin, chlorothalonil + thiophanate-methyl, copper hydroxide + copper oxychloride, copper hydroxide + mancozeb, and triadimefon + trifloxystrobin.  Biorational products labeled for downy mildew management include oils (essential, neem, soybean), disinfestants (Consan Triple Action, Zerotol), and potassium bicarbonate.  Biological controls include strains of Bacillus subtilis and Streptomyces lydicus.  Consult label for hosts, timing, and rates.

Note that for some downy mildew diseases, such as downy mildew of impatiens, chemical control in growing facilities is difficult once the fungus sporulates.  Preventive action is key.  In addition, homeowners with affected impatiens have few options; consider alternatives for planting beds this year.

Warning:  To reduce the possibility of the development of fungal resistance to some of the newer systemic fungicides with single or few modes of action, rotate these chemicals with protectants such as mancozeb.  Avoid the sole use of any fungicide for extended periods of time when other reliable products are available, and refer to label for timing, host plants, and rates.