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Diseases of Landscape Plants

PURDUEEXTENSIO

www.btny.purdue.edu

Downy MildewJanna Beckerman

Department of Botany and Plant Pathology, Purdue University

BP-68-

Figure 1. The different strains ofdowny mildew can be very host

specific, or it can infect manymembers of the daisy family, includin

this Rudbeckia.

Figure 2. The large lesions caused bdowny mildew can easily be mistakefor other kinds of damage.

Downy mildews are a group o closely relatedpathogens that can cause major damage in the nursery,greenhouse, and landscape in the orm o lea spots,blights, and distortions. Some common downy mildewhosts include alyssum(Alyssum spp.), aster(Aster spp.),butterfly bush (Buddleia spp.), coreopsis(Coreopsisspp.), cranes bill or hardy geranium (Geranium spp.),

Geum (Geum spp.), kale (Brassica oleracae), deadnettles (Lamium spp.), pansy (Viola x wittrockiana),phlox (Phlox spp.), rose (Rosa spp.), rosemary(Rosemarinus spp.), sage (Salvia spp.), snapdragon(Antirrhinum), veronica (Veronica spp.),viburnum(Viburnum spp.), and violets (Viola spp.).

Tis publication examines downy mildews in thehome landscape and provides strategies or managingthe disease.

Downy Mildews

Although similar in name, do not conuse downymildews with powdery mildews. Powdery mildews aretrue ungal pathogens that produce white, flour-likecolonies usually on upper leaves. Downy mildews,on the other hand, are a completely different kingdomo organisms, more closely related to algae than toungi. Downy mildews produce grayish, uzzy lookingspores and mycelium on the lower lea suraces.

Te distinction between powdery mildews anddowny mildews is important, because the ungicideseffective against one are not usually effective againstthe other although, as with every rule, exceptionsdo exist.

Symptoms Vary by HostAlthough there are many different downy

mildew pathogens, most downy mildews that inectornamentals belong to either the genus Peronosporaor Plasmopara. Tese pathogens canhave a very wide or a very narrow range o hosts. For example, the downy mildew thatinects roses (Peronospora sparsa) only inects roses. But the downy mildew that inects mint(Peronospora lamii)can inect several members o the mint amily, including dead nettlesand salvia, and possibly basil and coleus. Plasmopara halstedii is a different downy mildewpathogen that inects members o the daisy amily, most notably Rudbeckia(Figure 1).

Due to the wide host range o this pathogen, symptoms vary significantly rom host tohost (and even between cultivars), making it difficult make generalizations about downy

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Figure 3. Downy mildew symptoms can vary widely, evenamong different cultivars like these roses.

PhotobyJannaBeckerman

mildew symptoms. Tis variability o lesion color and size,and the severity o inection ofen makes the disease difficultto diagnose. On upper lea suraces, lea veins ofen delimitgrayish brown spots. Tese spots then develop into angularlesions that can be easily misdiagnosed as chemical damage,oliar nematode damage, or bacterial lea spot (Figure 2). Asthe lesions coalesce and large amounts o tissue are damaged,

the leaves drop.On roses, symptoms vary by cultivar (Figure 3). However,

one thing most cultivars have in common is that there aresigns o whitish to grayish, downy ungal growth on theundersides o leaves and beneath areas o upper leadiscoloration (Figure 4).

Disease Cycle and Management

Understanding the disease cycle is critical to managingdowny mildew and minimizing reliance on chemicals (Figure5). Te ungus overwinters in or on plant parts as mycelium oroospores (thick-walled, gumball-like structures that orm the

resting stage o the pathogen).emperature and humidity play key roles in the pathogens

development.During cool (50-75oF), wet conditions withhigh relative humidity (85 percent or higher), downymildew outbreaks develop when germinating oospores ormsporangiophores, which resemble a bunch o grapes emergingrom the plant stomate. Each grape is a sporangium. Andeach sporangium is filled with dozens o zoospores that swimto susceptible plants and inect them even when just a film oree water is available.

Prolonged periods o lea wetness promote sporegermination and the diseases spread, so keeping plants dryminimizes the spread o this disease. Increasing air circulationaround the plant by thinning and pruning it, reduces humidityand minimizes inection. Te disease cycle, rom the initialinection to the production o additional spores and secondaryinection, is usually seven to ten days, but can be as short asour days under warm and humid conditions again, thisvaries depending on the species o downy mildew.

Clean Up Infected Areas

Sanitation, however difficult, is essential or properlymanaging the disease. Sanitation minimizes the amount oinoculum that overwinters as mycelium or oospores in or on

plant parts. Under severe inection, removing the inected plantmay be the only realistic management option.

Prevent Water Buildup on Leaves

Real ungi do not produce spores that swim, so theycan only spread by being blown by the wind, or moved byequipment, insects, or other means. On the other hand, downymildews and other water molds produce zoospores that swimin ree water on plant suraces. Tey can keep swimming untilthey find tissue to inect on the already inected plant or ona neighboring plant.

Tat means managing water is the key to managing thesewater molds. Lea wetness early in the day is critical todowny mildew development. Spore release and zoosporeswimming typically cease by late morning, or early afernoon whenever the dew or lea moisture dries up. Tereore, anyaction that reduces the amount o lea moisture early in theday (such as watering in the late afernoon or changing romoverhead irrigation to a soaker hose) will reduce the spreado this disease. Lower lea suraces cannot dry out wherethere is dense canopy growth and tight plant spacing, so suchconditions encourage downy mildew spore production. Asimple management tool, then, is to space plants properly.

Use Fungicides for Prevention

Te role o water and this disease is obvious; downy mildewepidemics generally cease when its hot and dry. However,keeping plants dry may prove impossible, or nearly so. Whenthats the case, proper ungicide use comes into play to preventadditional infection from occurring.

able 1 lists ungicides labeled or managing downy mildewsMost o these ungicides are not available to homeowners so proessional applicators will need to apply these products.

Figure 4. Downy mildew causes whitish to grayish downyfungal growth on the undersides of leaves.

PhotobyJannaBeckerman

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PURDUEEXTENSIODiseases of Landscape Plants Downy Mildew BP-68-W

Figure 5.The downy mildew disease cycle.

Illustra

tionprovidedbyC.B.

Kenaga,E.B.

Williams,andR.J.G

reen;PlantDiseaseSyllabus.

Remember, successful downy mildew management relies onpreventative fungicide applications do not wait to see downymildew symptoms before you begin spraying, as that will be

too late!

Make fungicide applications every seven to ten days, beingsure to rotate or tank mix fungicides with different chemicalclasses to prevent fungicide resistance from developing. Tisdisease is much easier to prevent than to eradicate, so beginany spray program early and keep to a regular schedule.

References

Agrios, G. 2005. Plant Pathology. 427-433.

Dankers, H., Kimbrough, J. W., and Momol, M. . 2004. Firstreport of Plasmopara halstediion perennial black-eyed susan

in North Florida. Online. Plant Health Progress doi:10.1094/PHP-2004-0119-01-HN.

Rossman, A.Y. Systematic Mycology and MicrobiologyLaboratory, ARS, USDA. 14 December 2004. InvasiveFungi. Peronospora radii Downy mildew of margueritedaisy. Retrieved April 8, 2008, from http://nt.ars-grin.gov/sbmlweb/fungi/index.cfm.

Reference to products in this publication is not intended to be

an endorsement to the exclusion of others that may be similar.Persons using such products assume responsibility for their use

in accordance with current directions of the manufacturer.

It is the policy of the Purdue University Cooperative Extension Service that all persons have equal opportunity and access to its educational programs, services, activities, and facilitieswithout regard to race, religion, color, sex, age, national origin or ancestry, marital status, parental status, sexual orientation, disability or status as a veteran.

Purdue University is an Afrmative Action institution. This material may be available in alternative formats.

PURDUEAGRICULTURE REV 4/09

Order or download materials at the Purdue Extension

Education Store www.extension.purdue.edu/new

For other publications in the Diseases of LandscapePlants series, visit the Purdue Extension Education Store,

www.extension.purdue.edu.

Table 1. Fungicides Labeled for Downy Mildew Control1

Group Code2 Common Name Trade Name

11strobililurins: azoxystrobin, trifloxystrobin, pyraclostrobin,kresoxim,-methyl Heritage, Compass, Insignia, Cygnus

11+7 pyraclostrobin + boscalid Pageant

21cyazofamid Segwayfenamidone3 Fenstar

40 dimethomorph Stature DM

M (33)fosetyl-al Aliettephosporous acid Alude, Biophos, Vital, Agri-Fos

M

Multisite: mancozeb Fore, DithaneMulitsite: peroxide ZerotolMultisite: copper Phyton 27, Kocide, Camelot, Liquid Copper

1Fungicides set in bold are available for home use.2The FRAC code is l isted in parentheses under the EPA Group code when the codes differ. Neither system includes biofungicides.3Different chemistry but same mode of action.