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ISSUE 11  JULY 15, 1999

 

POWDERY MILDEW ON PEA

    There have been a number of calls about control of powdery mildew on pea. This year’s concern
is because powdery mildew was common and severe last year on late planted field peas. These peas
did not yield up to expectations and there was a white dust in the air at harvest. I imagine that the white
dust was from spores of the powdery mildew fungus.

    Several sulfur fungicides are registered for powdery mildew control on pea. Sulfur has been used
in Wisconsin (canning peas?) and on the prairie provinces of Canada but we have no data on their
economic return in North Dakota.

    Registered sulfur fungicides include Drexel sulfur (several formulations), Thiolux, Kumulus, Micro
Flo, Terra Sulfur, and Sulfur Six (Helena). Not all sulfur products are registered for peas, so make
certain that the product to be used is labeled for peas. Label rates vary, but summarizing several labels,
it appears that 3-4 lb of active ingredient per acre should be sufficient.

    Do not apply sulfur within 2 weeks of an oil application. Do not apply sulfur in hot weather, as
phytotoxicity may occur.

    Since we have no North Dakota data, we are struggling with recommendations. I will refrain from
making recommendations (which should be research-based), but will provide some suggestions
based on my knowledge of other powdery mildews on other crops and the knowledge of several of
our agronomists on field peas. I believe that sulfur should be applied as soon as powdery mildew is
noted in the field, since it can develop very rapidly. Early detection can be difficult, however, and
may be most successful if done with strong sunlight to your back. Once the lower pods are full length,
but still flat, a fungicide may no longer be economic. It takes about 2-2 weeks from the last open
flower to physiological maturity.

    Most labels indicate that re-application should be done in 10-14 days, yet California data on sugarbeet
powdery mildew indicates that a single application of 4-5 lb ai may provide control for up to 4 weeks.
Re-application might be required if there is a hard rain to wash off the fungicide or if there is significant
new growth that is not protected. A single application of sulfur should cost around $3-$4/A plus
application costs. The economics of more than one application may be suspect without any research data.

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

 

SMALL GRAIN DISEASE SURVEY UPDATE

    Small grain diseases are increasing in early planted grain. Although most early planted grain looks great,
with good yield potential, some of these same crops have substantial leaf spot and leaf rust on the flag
leaf, and a scattering of scab infected heads. Reports from around the state:

    In the southwest, Amy Dukart has seen considerable leaf rust on spring wheats, plus she is now picking
up fields with wheat streak mosaic symptoms and root rot symptoms. A producer from Grant county
also reported severe leaf rust on the spring wheat variety 2398, a variety previously considered resistant
to prevalent races of leaf rust. Amy also sees tan spot consistently on the wheat crops, and spot blotch
on barley
.

    In the northcentral and northwest districts, Brittany Sund consistently has observed tan spot on
lower leaves and middle leaves of the young crop, but severities remain low. She also is observing low
levels of leaf rust on bottom leaves of spring wheat fields in Williams, Pierce, Rolette, and Bottineau
counties. Kent McKay, Area Agronomist at Minot, reports leaf rust in AC Barrie in the Minot wheat
drill strips that were planted in late April.

    In the central and southcentral counties, leaf rust has exploded in some fields. Jerry Schneider
observed 3-10% severities on flag leaves in some spring wheat fields on July 7. At the Carrington REC
on July 13, some varieties in the drill strips had levels as high as 30% severity on the flag leaves. Tan spot,
Septoria, and barley yellow dwarf
symptoms are also common. Scattered head scab infections were
observed on susceptible spring wheat and durum varieties in the drill strips at Carrington, and just a few
infected heads were observed in the barley varieties.

    In fields surveyed by Jerry Ries in northeast counties on July 6-7, wheat leaf rust was commonly
found on middle leaves, but none to only trace amounts were observed on the flag leaves. On July 13,
Jerry found severities of leaf rust from trace to 25% on the flag leaf of spring wheats in Steele county.
Tan spot
and Septoria also were common on flag leaves of post-flowering wheat in eastern counties.
Scattered heads showing symptoms of Fusarium head blight or head scab also were observed in
some fields, but incidences were generally 2% or lower. The completely bleached heads and whitened
leaves and stems associated with root rot symptoms are also being observed in wheat fields. These
symptoms are very noticeable in a field and may be confused with head scab!

    Many fields surveyed the past few days in eastern counties had grain aphids present, with some
having 100% of tillers with aphid infestations. The threat now is to the younger crops.

    Stem rust was observed on a susceptible check wheat variety in plots at Carrington on July 13.
Stem rust also was observed on a single head of Stander barley at Fargo on July 9. Our spring wheats
and durums have resistance to stem rust; a few winter wheat varieties are susceptible. Currently grown
barley varieties are susceptible to race QCC of stem rust. Our single observations do not indicate
a concern about stem rust, but field scouts should report any observations of stem rust
. Stem
rust is a darker red color than leaf rust, the eruptions are generally larger and more elongate, and stem
rust generally may be found on the stem near the base of the plant, but can be found on leaves and awns,
as well.

 

NEW COMPENDIUM OF CORN DISEASES AVAILABLE

    A revised edition of the Compendium of Corn Diseases is now available from the American
Phytopathology Society Press. It is the first revision of this publication in 19 years. The publication
has updated symptom descriptions, pathogen descriptions, and inclusion of newly important diseases.
The publication has 177 color photographs and would be a useful tool for those working with corn.
The cost of this publication is $37.00 and can be ordered from APS Press, toll free number:
1-800-328-7560.

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

 

PLANT DIAGNOSTIC LAB REPORT

    The lab is starting to receive samples, calls, and reports on Fireblight, a bacterial disease of apples,
pears, crabapples, mountain ash, hawthorne, and other plants in the rosaceous family. Fireblight is caused
by a bacterium called Erwinia amylovora, and is characterized by blackening and often crooking of the
end of the stem, referred to as a shepherd’s crook. The bacterium is spread to young, succulent plant
tissue and blossoms by splashing rain and insects, at temperatures above 65E F. It overwinters in a canker
under the bark of these tree species. In the spring, under the proper environmental conditions, the bacteria
will ooze out in plant sap and spread via insects and water. Hail can be of particular concern as a predisposing
factor since it creates fresh wounds that are easily invaded by the bacteria. Symptoms to watch for include
wilting and scorched blossoms and stems. In pears, the leaves may turn black. Fireblight will spread very
rapidly if the weather conditions are favorable.

    Managing fireblight can be tricky. In nurseries and orchards, monitoring disease incidence, noting reports
of the pathogen in nearby locations, and applying copper fungicides, the antibiotic streptomycin (or a
substitute product) at the appropriate time are important. Immediate pruning may also be important.
Copper fungicides, streptomycin, or its substitute may also be used by homeowners as protectants
in the spring. The discrepancy in management is in regard to timing of pruning out infected plant material.
Some reports say to prune out ‘strikes,’ infected blossoms and twigs showing blackening and a shepherd’s
crook, as soon as they are observed. Other reports suggest that it is better to wait until the tree is dormant
to prune out infections. The rationale for waiting until dormancy to prune is that the fresh cuts provide a
prime surface for infection if the bacterium is present. The downside to waiting is that the infection can move
rapidly and may take more of the tree if the infection is not removed. At this time of the year, if infection is
observed in smaller trees or newly planted trees, it would be good to remove the infection now by pruning,
making cuts approximately 8-12 inches below the infected area and disinfecting the cutting instrument with
a 10% household bleach solution between cuts to ensure that you don’t spread the pathogen (bacteria) to a
new cut. If the tree is an established and fairly large tree, it is not as likely that the infection will move so
rapidly in the larger branches and therefore there is a larger window of opportunity to prune out infected
plant material. If this is the case, it might likely be better to wait until the tree is dormant to prune out the
strikes. If you have any questions, please feel free to call the lab.

Cheryl Ruby
Plant Diagnostician
diaglab@ndsuext.nodak.edu


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