ISSUE 14 AUGUST 5, 1999
CHECK SWATHED CANOLA FOR DISEASE
Right after swathing is a good time to check for disease in a canola
crop. Inspect eight different
locations in the field, pulling up five stems at each location, for a total of 40 stems inspected. Check
for the presence of both Sclerotinia stem rot and blackleg, and keep a record of any field that had high
levels of either disease. Sclerotinia can be identified by bleached or white stems that are hollow or that
have shredded tissues. Occasionally the stems will contain one or more hard black bodies, the sclerotia,
but these are not always present on canola. Blackleg can be identified by a dark gray to black lesion at
the base of the stem or by dark gray to black lesions surrounding the point of attachment of one or more
lower leaves. The dark gray to black lesions should have numerous tiny black bumps or fruiting bodies
present. By cutting into the stem, one can determine if the lesions have penetrated the tissues deeply,
which is indicative of the damaging high virulence strain of blackleg. The low virulence strain does not
penetrate the tissues deeply, and usually causes little damage. Occasionally the only symptom of blackleg
will be in the roots. When one cuts into the roots they will be dark gray to black if infected.
Count the number of stems with Sclerotinia and with blackleg and
determine the percent of infection.
If 40 stems are examined, multiply the number of infected stems by 2.5 to determine percent infection.
Record the percent disease and use this information in future years to help make crop rotation decisions.
The presence of a high percentage of blackleg indicates a need to lengthen rotations in that and nearby
fields or to plant blackleg resistant varieties within one to three miles of the infested field for the next two
years. The presence of high levels of Sclerotinia in a field is a warning that a fungicide might be needed in
future years in nearby fields if wet weather and saturated soils occur before and at early flower. DO NOT
plant sunflower in Sclerotinia infested fields for at least four or five years. Infection through the roots of
sunflower, leading to Sclerotinia wilt, can occur in almost any year, regardless of weather conditions. This
is in contrast to infections of canola and dry beans, which are from the airborne spores, and are highly
dependent on weather conditions.
Bob Hofbeck, crop consultant, reported that sunflower rust
infections were progressing rapidly last week
in McLean county. Although I have not received other reports, it is likely that rust infections are progressing
rapidly in other areas as well.
Some sunflowers are being sprayed with Folicur at early flowering.
This suggests that the application
may be 3-4 weeks before ray petal wilt. The section 18 label for Folicur indicates that two applications can
be made, and that they should be made at 14 day intervals, raising the question whether a single application
at early bloom will provide control long enough. Data from the manufacturer, Bayer, indicates that the half
life of Folicur is slightly over 14 days. Trying to stretch Folicur efficacy to 21-24 days will be pushing the
envelope, since I would estimate that Folicur levels would be approximately 1/4 of those at the time of
application. If the application at early bloom is made when rust levels are quite low, it may be possible
keep the inoculum level low enough to "get by" for 21 days provided there are not a lot of spores that
blow in from neighboring fields. Decisions on whether to make a second application should take into
consideration whether there are unprotected fields in the area that could be a source of spores, the
anticipated time left to ray petal wilt, as well as current and projected weather conditions.
SCLEROTINIA WILT IN SUNFLOWER
Sclerotinia wilt is beginning to show up in sunflower. Wilt is
caused by root infection, which occurs
when the roots of sunflower come in contact with sclerotia in the soil. Root infections are unique to
sunflower, and are essentially independent of weather. Infections on canola, dry beans, and also
sunflower head rot are due to infections from airborne spores, which are liberated from the Sclerotinia
mushroom bodies which develop from sclerotia in wet weather. Infections from spores are highly
dependent on long periods of wet weather.
Sclerotinia wilt infections will continue to show up into the bloom
period. Infections can spread from
plant to plant by root contact when the crop is in 30 inch rows. Plant to plant spread of Sclerotinia is not
likely to be greater, and it may be even be minimized by solid seeding, since plants are spaced more evenly,
and are not as close as those within a 30 inch row. Studies at Morden, MB indicate that plant to plant
spread begins to decrease when plants are about 12 inches apart, and that little or no spread occurs
when plants are more than 15 inches apart.
Extension Plant Pathologist
PLANT DIAGNOSTIC LAB REPORT
The following is a breakdown of the samples in the lab this week
and the diagnoses.
|Dry beans||Common blight
|Sugarbeet||Aphanomyces root rot|
|Sunflower||Growth regulator herbicide injury
Septoria leaf spot
Fusarium head blight
|Soybean||Phytophthora root rot
Growth regulator herbicide injury
|Peas/oat forage||Harvested too late|
Negative for chemical injury
|Lawn||Helminthosporium leaf spot/
|Tomato||Septoria leaf spot|
|Insect ID||Flea beetle|
UPDATE ON SMALL GRAIN DISEASES
Barley yellow dwarf virus (BYDV): The latest surveys show
that BYDV is readily apparent
in the later maturing barley and wheat in much of the state. Brittany Sund, crop surveyor at the Minot
REC, found some moderate to severe levels in McHenry and Pierce counties this past week. Brittany
also found high incidences of bird cherry oat aphid in these fields. Brian Steffenson, Barley Plant
Pathologist at NDSU, has observed some severely infected late planted barley fields in Ramsey county.
Jerry Schneider, crop surveyor out of the Carrington REC, found a severe infection of BYDV in barley
in Sheridan county, and a severely infected spring wheat field in McIntosh county, as well.
Wheat streak mosaic virus (WSMV): This virus disease was
observed by Brittany Sund in durum
in Williams county and in spring wheat in McLean county. This virus disease is transmitted by wheat curl
mites which need a green grass or wheat or barley crop to survive. Often this disease is associated with
winter wheat plantings or where infected winter wheat volunteers have not been controlled. The
predominant location for wheat streak mosaic observations this year has been in the western counties,
but a few isolated cases have been reported across the state.
Barley stem rust: Crop surveyors are now finding stem rust on
barley in most of the state. Jerry
Schneider found low severity levels of stem rust on barley in Sheridan county. Brian Steffenson found
severity levels of 2% in some commercial barley fields in the northeast district. Stem rust has NOT
been found in spring wheats or durum in North Dakota. South Dakota is reporting some severe
stem rust in susceptible winter wheat cultivars, however.
Oat stem and crown rust: The latest Cereal Rust Bulletin
reported that oat stem rust was reported
from plots and fields in North Dakota, with severities of trace to 20%. Crown (leaf) rust on oats also
is present in oat fields, but the Cereal Rust Bulletin reports that severities are lighter than normal because
cool weather in early spring decreased the amount of infections from buckthorn, the alternate host of
oat crown rust.
Barley leaf rust: Low levels of barley leaf rust are observed
in most late planted barley fields in
North Dakota this year.
Wheat leaf rust: Leaf rust in found in almost all spring wheat fields in North
Dakota. Severity levels
are lowest in the northwest counties, but in other parts of the state, leaf rust has been severe on some
spring wheat varieties (Durum wheats are much more resistant to leaf rust). Severities of 40% or greater
have been observed on flag leaves of 2375' and ACBarrie, but other varieties previously thought
resistant to leaf rust are also showing some levels of infection. Once all the leaf rust evaluations are
compiled and summarized, we should have a better handle on variety response this year.
However, keep in mind that variety performance will not guarantee
variety response in 2000, if leaf rust
again is severe. Bob Bowden, Extension Plant Pathologist at Kansas State, just provided a summary of
the Kansas Rust Report for 1999 on the USDA Cereal Disease Labs web site. He said that several
winter wheat varieties in Kansas showed "significant erosion of resistance compared to last year in
most plots". Surprisingly, Dr. Bowden estimated that wheat yield loss due to leaf rust in Kansas this year
was only 3.4%, despite their relatively heavy infections. Perhaps their rust developed fairly late on their
crops and didnt do too much damage, but it helped provide the inoculum for our infections.
Tan spot/Septoria leaf spots: These fungal leaf spots are
very common in wheat and durum
throughout the state. Durums are not showing leaf rust, but many crops have Septoria infections on
the flag leaves.
Glume blotch: A few fields in Wells and in Logan counties
were observed to have glume blotch
symptoms on the head. Wet weather can splash the Septoria fungus to the grain heads and result in
infection of the glumes. Glume blotch is characterized by a dark discoloration from the glume tip
downward, often with a greyish center, and the fruiting bodies of the fungus may be evident in the
discolored area on the glumes.
Net blotch/Spot blotch on barley: Most barley fields surveyed
have some level of either of these
two fungal leaf spots present. A few fields have had severe infection, but generally leaf spot infections
have not been severe in most of the state.
Black chaff//Melanism: Some wheat varieties in the Prosper and
Casselton nurseries are showing
symptoms of black chaff infection. Some symptoms also have been observed in commercial fields in
eastern ND. Black chaff is a bacterial infection of the glumes. Symptoms consist of dark lines or patches
on the glumes plus dark spots on the awns and possible the stem below the head. Sometimes the bacterial
exudate is visible on the awns and appears shiny. This bacterial disease also can infect the leaves and cause
bacterial leaf blight. When black chaff infection occurs late in the season under dryland conditions, damage
to the grain usually is minimal. However, symptoms can be striking and cause alarm.
Melanism, also called false black chaff, is a condition that
also causes dark discoloration of the glumes,
but it is more uniform than in the case of black chaff, and often a darker discoloration. Also the awns and
stem right below the head dont show the discoloration. Melanism occurs when the environment is both
hot and humid, so we may be seeing some melanism, as well.
Fusarium head blight (scab): Although head scab was observed in
almost every wheat or barley field
in surveys in the east and central and south central districts this past week, severity levels have generally
been very low, and most early fields should suffer only a 1-2% yield loss from this disease. The highest
field severity level so far observed was 6% in a wheat field scouted by Jerry Schneider in Emmons county.
Scab levels may be higher if a producer grew a susceptible variety and had excessive moisture during
flowering and early grain fill. We are seeing higher scab levels in susceptible varieties, for certain planting
dates, in fungicide plots at Langdon, Fargo, and Carrington, but those are sites we wanted to get infection.
The four Extension crop surveyors will be finishing up their
survey efforts this week and next,
concentrating on the late planted fields, to see if some if these diseases, such as scab, have developed more
extensively in the late grain.
Extension Plant Pathologist