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ISSUE 13  JULY 29, 1999


SUNFLOWER RUST ALERT

    Based on available information, we have had favorable weather for the development of sunflower
rust. It is possible that rust will increase rapidly in fields where it has already been observed. Answers
to a few questions follow.

    Spore production is best at 68-95F.
    Spore germination is best at 60-80F. At 77F, over 90% of spores germinate if the leaf stays
wet for only 3 hours; but at 68F, the leaf must stay wet 8 hours for 90% of the spores to germinate.
Thus, spore germination and infection can occur rapidly on warm nights with only minimal amounts
of dew. Back in the drought years of the late 1980s I observed a field with severe rust. Most of the
rust was along the main veins of the leaf where the dew remained a little longer, since there wasn’t
much dew in those years. Post infection development is best at 63-82F.

    Rust cycles in only 8 days when night/day temperatures are 75F/85F. It cycles in 14 days when
night/day temperatures are 55F/65F. In addition, pustules are larger at high temperatures, so that
up to 80% more spores are produced. This may help explain why rust levels were low in the early
1990s, when July and August temperatures were below average.

    When does rust quit? Cooler nights in late August should slow rust down. When temperatures
drop below 50F for an extended period, the rust pustules stop producing the brown summer spores
and produce black winter spores. Once production of winter spores begins, production of summer
spores ceases. At that point, rust infection for the season has ended, since the winter spores do not
infect until next season. Sometimes this change to winter spore production may begin as early as late
August, but this varies from year to year. The change is readily observed by the changing color of
the pustules.

    When is action required? If rust severity in the top four leaves averages at least 3% before ray
petal wilt, a fungicidal application is likely to provide an economic return. Folicur has emergency
section 18 registration for use on sunflower (confection and oilseed) in North Dakota, but not in
South Dakota or Minnesota. This is because in recent years rust has been a problem primarily in
north central and northeastern North Dakota. It is a serious problem in the high plains of western
Kansas and Nebraska and eastern Colorado.

    Folicur can be applied at 4 fl oz/A up to 50 days before harvest, which is usually about ray petal
wilt. Two applications at 14 day intervals are allowed, but two applications are not likely to be
needed in North Dakota unless rust severities exceed 3% at early bloom or pre-bloom.

    How do I estimate rust severity? NDSU Extension Circular PP-998, Sunflower Rust, contains
drawings depicting various rust severities.

    When will Folicur be registered for sunflower? All of the field work for minor use (IR-4)
registration was completed in 1998 and the laboratory analysis of residues was completed in 1999.
IR-4 expects to submit a label request to the Environmental Protection Agency in 2000.

 

WHITE MOLD IN DRY BEANS

    Questions are starting about using rescue treatments in dry beans where white mold is already
present. In general, Benlate and Topsin M are more effective at preventing infection than they are
at stopping established infections. If the crop is still flowering profusely, an application of fungicide
may help to prevent new infections as the blossoms die and become food sources for the white
mold fungus to infect. The question is whether there will be enough yield response from the late
application of a fungicide to provide an economic return.

    Keep in mind that the best time to apply a fungicide is 4-10 days after the onset of bloom (see
Timing of Applications in Crop and Pest Report No. 10). I suspect that by the time white mold
shows up in the crop, it has been considerably more than 10 days after the onset of bloom. If onset
of bloom was more than 12 days ago, or if there are pods up to 3 inches long, the economic return
will be reduced and profitability may be questionable.

Art Lamey
Extension Plant Pathologist
alamey@ndsuext.nodak.edu

 

SMALL GRAIN SURVEY

    During the past week several problems became more apparent in wheat. In the southwest and
west central districts, Amy Dukart found symptoms of wheat streak mosaic virus in some fields
in Adams, Hettinger, Golden Valley, Stark, Dunn, and McKenzie counties. Percent of plants infected
in affected fields ranged from only 4% to as high as 90%. This virus disease, characterized by pale
yellow streaks and yellow mosaic symtpoms on the leaves, is carried by wheat curl mites. These tiny
mites overwinter only on living plant tissue, usually winter wheat crops or winter wheat volunteers,
but also in some perennial grasses. The abundance of symptoms in this area, though generally not
severe, indicates a possible lack of control of volunteers and grassy weeds prior to planting of the
spring grain. Less herbicide inputs this year may have contributed, plus the mite and virus flourish
under hot weather, which we have had recently. The white heads associated with wheat stem
maggot
damage also were observed frequently by Amy, with incidences ranging from 1 - 13%.

    In the northcentral and northwest districts, Brittany Sund consisently found tan spot on the
flag leaves, but at low severity levels. Grain aphids were common in the fields she surveyed, and
several wheat fields showed symptoms of barley yellow dwarf virus, which is transmitted by
these aphids. Brittany also surveyed one field that had a high incidence of root rot.

    In the southcentral and central districts, Jerry Schneider found wheat leaf rust in 27 out of 35
fields surveyed, with incidences from 4 to 100% and severities on the flag leaves from 0 to 100%.
What was left of the flag leaves in these fields often was infected with tan spot or Septoria or barley
yellow dwarf virus. Jerry found head scab in 10 out of the 35 fields surveyed, but the % of tillers
showing scab symptoms in these fields was very low, from 2 to 10%, and the severity was low.
Jerry also found barley yellow dwarf in 17% of the fields he surveyed this past week, with severities
high in some fields.

    In the southeast, Jerry Ries found wheat leaf rust in all fields surveyed, plus Septoria was common
on flag leaves. Fusarium head scab was also found in all fields, but incidences within fields ranged from
5 to 20% and severities within wheat heads was low, giving average predicted field loss from this disease
around 1-2% in this area. In the northeast, Jerry found leaf rust in each wheat field surveyed, with some
flag leaf severities as high as 30%. Septoria also was common, and scab was found in 63% of the fields
surveyed, but incidences and severities were very low in these affected fields.

 

NORTHERN CORN LEAF SPOT

    Two corn samples with northern corn leaf spot were observed this past week. One sample was
from Richland county and one from Foster county. The leaf spots were oval, about 1/4 to inch in
length, and were light tan in the center, with a darker border. The tips of the leaves were severely
infected and tattered or torn looking. This spotting was caused by a fungus, Helminthosporium
carbonum
. This disease is common on corn in the US, but is considered of minor importance and
of little threat to hybrid corn. This corn leaf disease is favored by moderate temperatures and high
relative humidities.

Marcia McMullen
Extension Plant Pathologist
mmcmulle@ndsuext.nodak.edu

 

PLANT DIAGNOSTIC LAB REPORT

    This past week has been a smorgasbord of plant problems in the lab. The following is a summary
of the some of the samples and the diagnoses from the 51 samples processed this week.

Crop

 

Canola Growth regulator herbicide injury
Potato Late blight
Poor tuber production due to wet soils
Wet soils and old seed--reduced tuber set
Sugarbeet Aphanomyces root rot
Rhizoctonia mature root rot
Dry beans Bacterial brown spot
Halo blight
Hail injury
Soybean Growth regulator herbicide injury
Pythium root rot
Rhizoctonia root rot
Iron chlorosis
Sunflower Growth regulator herbicide injury
Lightning injury
Durum BYDV
Wheat BYDV
Melanism, or False black chaff
Common root rot
Barley BYDV
Raspberry Spider mite feeding injury
Spruce Cytospora canker
Juniper Winter desiccation
Maple Iron chlorosis
Elm Chemical injury
Dutch Elm Disease (DED)
Crabapple Apple scab
Plant ID Perennial sowthistle (Sonchus arvensis)

    One notable sample is the wheat that exhibited symptoms of melanism. The sample was the fairly
newly released variety, McKenzie. Melanism is also called False black chaff, brown necrosis, or pseudo
black chaff and produces purple to black streaks on the glumes. It differs from black chaff in that there
are generally not symptoms on the awns or on the peduncle, as there are with black chaff (a bacterial
disease). One diagnostic feature of melanism that is usually present is a dark collar or streak above the
joint. For more information on black chaff and false black chaff, see NDSU Extension Bulletin PP-749,
available from the Plant Diagnostic Lab or the NDSU Extension Distribution Center (701.231.7882).
This sample showed symptoms typical of melanism, but it also exhibited signs of anthocyanin production
on the stems indicative of a response to stress. It will be interesting to see how widespread false black
chaff is on this variety.

Cheryl Ruby
Plant Diagnostician
diaglab@ndsuext.nodak.edu


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