Downy mildew and purple blotch are here in Michigan onion fields

Mary Hausbeck<hausbec1@msu.edu>, Michigan State University Extension, Department of Plant, Soil & Microbial Sciences - July 23, 2014

Control downy mildew and purple blotch now to preserve your onion harvest.

Downy mildew

Downy mildew on onions has been confirmed in Michigan. This disease occurs every three to four years in Michigan. It is an especially devastating disease because it can multiply rapidly and spread through a growing region if left untreated.

Downy mildew is caused by the pathogen Peronospora destructor and can defoliate the crop prematurely. It first infects older leaves, occurring as pale, elongated patches that may have a grayish-violet fuzzy growth appear early in the morning during moist periods. Infected leaves become pale green then yellow and can fold over and collapse. Lesions can be violet-purple in color; affected leaves become pale green then yellow and can fold over and collapse. Symptoms of the disease are best recognized when dew is present in the morning.

Premature death of onion leaves reduces bulb size. Infection can occur systemically, with stored bulbs becoming soft, wrinkled, watery and amber in color. Asymptomatic bulbs can sprout prematurely and form light green foliage. Bulbs can become infected by secondary bacterial pathogens, resulting in storage rot.

The downy mildew pathogen initiates infection during cool temperatures, less than 72 degrees Fahrenheit, and wet conditions. Multiple infection cycles can occur in a season. Spores are produced at night and are easily blown long distances in moist air. They can germinate on onion tissue in one and a half to seven hours when temperatures are 50 to 54 F. High daytime temperatures and short or interrupted periods of humidity at night can prevent sporulation.

Overwintering spores, called oospores, can form in dying plant tissue and can be found in volunteer onions, onion cull piles, and in stored infected bulbs. Oospores have thick walls and a built-in food supply so they can withstand unfavorable winter temperatures and survive in the soil for up to five years.

Grayish-violet sporulation on leaf of onions infected with downy mildew.

Purple blotch

Purple blotch, caused by the fungus Alternaria porri, is a commonly occurring foliar disease of onions in Michigan. It first appears as small, water-soaked lesions that quickly develop white centers. As they age, the lesions turn brown to purple, surrounded by a zone of yellow. Lesions can coalesce, girdle the leaf, and cause tip dieback. Occasionally, bulbs are infected through the neck or wounds on the scales.

Spores of A. porri can form repeatedly on lesions with cycles of low and high relative humidity. When free water is available, spores can germinate in 45-60 minutes at 82-97 F. Spores can form after 15 hours of relative humidity greater than or equal to 90 percent and can be spread by wind, rainfall and irrigation. Fungal growth is favored by temperatures of 43-93 F with an optimum temperature of 77 F. Old and young leaves injured by onion thrips are more susceptible to infection.

Symptoms can appear one to four days after infection, and new spores can appear by the fifth day. Purple blotch can defoliate the onion crop prematurely, compromise bulb quality and can result in storage rot caused by secondary bacterial pathogens. The purple blotch pathogen can overwinter as fungal threads (mycelium) in onion debris.

Purple blotch lesions on onion foliage.

Fungicide programs

When selecting fungicides, alternate among products with different modes of action (FRAC codes). The table below lists products labeled for use on downy mildew and purple blotch of onions in Michigan. Michigan State University Extension says to remember that the pesticide label is the legal document on pesticide use. Read the label, as they change often, and follow all instructions closely.

Products labeled for control of downy mildew (DM) and purple blotch (PB).

Product	Active ingredient	Disease	FRAC code
^DMAliette WDG	Fosetyl-al	DM,PB	33
^PB*Bravo products, Echo products, Equus products	Chlorothalonil	DM,PB	M5
^PBCabrio	Pyraclostrobin	DM,PB	11
Catamaran	Potassium phosphate/chlorothalonil	DM,PB	33/M5
*Coppers: Badge SC, Champ products, Copper Count N, Kocide products, Nu-Cop 3L, Cuprofix Ultra Disperss	Copper compounds	DM,PB	M1
^DM,PBDithane products, Manzate products, Penncozeb 75DF	Mancozeb	DM,PB	M3
Folicur 3.6F	Tebuconazole	PB	3
Fontelis	Penthiopyrad	PB	7
^DMForum	Dimethomorph	DM,PB	40
^PBInspire Super	Difenoconazole/cyprodinil	PB	3/9
^PBIprodione, Rovral 4F, Meteor	Iprodione	PB	2
ManKocide	Mancozeb/copper hydroxide	PB	M3/M1
Phostrol, Prophyt	Phosphorous acid salts	DM	33
^PBPristine	Pyraclostrobin/boscalid	DM,PB	11/7
Propimax EC, Tilt	Propiconazole	PB	3
^PBQuadris	Azoxystrobin	DM,PB	11
^PBQuadris Opti	Azoxystrobin/chlorothalonil	DM,PB	11/3
^PBQuilt, Quilt Excel	Azoxystrobin/propiconazole	PB	11/3
Reason	Fenamidone	DM,PB	11
^DMRevus	Mandipropamid	DM	40
^DMRidomil Gold Bravo	Mefenoxam/chlorothalonil	DM	4/M5
^DMRidomil Gold Copper	Mefenoxam/copper hydroxide	DM	4/M1
^DMRidomil Gold MZ	Mefenoxam/mancozeb	DM	4/M3
Scala	Pyrimethanil	PB	9
^PBSwitch 62.5WG	Cyprodinil/fludioxonil	PB	9/12
^DMTanos	Famoxadone/cymoxanil	DM,PB	11/27
Trilogy	Neem oil	DM,PB
^DMZampro	Ametoctradin/dimethomorph	DM	45/40

* Not all of these products are labeled for both downy mildew and purple blotch; check the label.
^DMEspecially recommended for downy mildew control.
^PBEspecially recommended for purple blotch control.

Dr. Hausbeck's work is funded in part by MSU's AgBioResearch.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).