Downy Mildew
Downy Mildew is an especially devastating disease because it spreads
rapidly and is not readily controlled. Downy Mildew of onions is caused
by the pathogen, Peronospora destructor, and it first infects older
leaves, occuring as pale, elongated patched that may have a grayish-violet
fuzzy growth.
Symptoms of the disease are best recognized when dew is present in
the morning. Infected leaves become pale green then yellow and can
fold over and collapse. Premature death of onion leaves reduces bulb
size. The downy mildew pathogen initiates infection during cool temperatures
( < 72 degrees F) 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 50-64 degrees 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 infected bulbs
in storage. 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.
Research studies have shown that weekly appplications of mancozeb
(Manzate 75 DF) protect against downy mildew when spray coverage is
good and sprays are begun before disease appears. Some growers choose
to include Ridomil Gold MZ 68WP in alteration with Manzate 75DF although
this program is more costly. This year’s test included a new
product, Pristine 38WG, in rotation with Ridomil Gold MZ 68WP for
downy mildew control. The program with Pristine 38WG and Ridomil MZ
68WP alteration was effective for downy mildew. It is likely that
Pristine 38WG alternated with Manzate 75DF would also be effective
but needs to be tested. In addition to downy mildew, Pristine 38 WG
has activity against leaf blights including purple blotch and Stemphylium.
In a field test for purple blotch and Stemphylium leaf blight,
Pristine 38WG alternated with Braveo Weather Stik 6SC was a stand-out
treatment.
Purple Blotch and Stemphylium Leaf Blight
Purple blotch 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. Ocassionally, bulbs are infected through the
neck or wounds on the scales. Spores of Alternaria 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 degrees F. Spores can form after 15 hours
of high relative humidity (greater than 90 percent) and can be spread
by wind, rainfall and irrigation.
Fungal growth is favored by temperatures of 43 to 93 degrees F, with
an optimal temperature of 77 degree F. Old and young leaves injured
by onion thrips are more susceptible to infection, and new spores
can appear by the fifth day. The pathogen can overwinter in onion
debris.
Stemphylium leaf blight is an occasional foliar problem in
Michigan. Symptoms begin as small, light yellow to brown, water-soaked
lesions that develop into elongated spots that turn dark olive brown
to black with spore development. Coalescing spots can blight leaves
but rarely affect the bulb. The pathogen normally invades dead and
dying tissue. Disease development is favored by long warm periods
with leave wetness. Fungicides effective against purple blotch are
also effective against Stemphylium leaf blight.
