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New
Insights into Management of Soilborne Crop Pathogens
Under Direct Seeding: Take-all
R. James
Cook, Endowed Chair in Wheat Research, Departments of Plant Pathology
and
Crop and Soil Sciences, Washington State University, Pullman, WA 99164-6430
Take-all, caused
by the soilborne pathogen Gaeumannomyces graminis var. tritici, is a root
and crown disease of wheat, barley, and related forage and weedy grasses.
The disease was first observed in the Pacific Northwest in 1901 at Albany,
Oregon-one century ago!. The pathogen is thought to be native to this
region. Until the last 25 to 30 years, take-all was known primarily if
not exclusively on wheat under irrigation in southern Idaho, Yakima Valley,
and Columbia Basin, and in high-rainfall areas such as the Willamette
Valley of Oregon and western Washington. In contrast, the disease was
observed in the dryland Pacific Northwest (PNW) in earlier years only
in the wettest parts of the region, such as the Camas Prairie and eastern
Palouse, and then only in years of exceptionally high rainfall. However,
we have continued to see evidence of a wider distribution of take-all
than has been assumed, and this was recently confirmed.
A survey conducted
by graduate student Nathan Ramsey during 1998, 1999, and 2000 revealed
that, of 270 PNW wheat fields planted to winter or spring wheat, where
wheat had been grown either the previous year (high risk) or 2 years earlier
(medium risk), 75% had enough take-all to be limiting to yield in the
year it was sampled (Ramsey, 2001). There was no difference in the amount
of take-all whether the field had been cropped to wheat the previous year
or if the last wheat crop was 2 years earlier. Since most rotations in
the Inland Northwest include winter or spring wheat every other year,
2 years in 3, or every year, we now recognize take-all as one of the most
important diseases limiting yields of wheat in the dryland PNW.
"Dryland"
versus "wetland" take-all
Take-all develops
quite differently on wheat under dryland conditions compared to wheat
grown in high-precipitation or irrigated areas. Since the classic, "textbook"
description of take-all is based on how the disease develops under wet
conditions, the disease has been over looked or "under recognized"
in the dryland PNW-until recently.
Under wet conditions,
such as in western Oregon and western Washington, and under irrigation,
the disease develops as patches of stunted and prematurely blighted plants
(white heads), with the most severe stunting in the centers of the patches
where the plants were infected first. With suitably moist soil, the pathogen
grows from root to root and plant to plant radially outward from centers
of infection in the field. Plants infected last, i.e., those plants on
the edges of the patches, show the least or not stunting, although they
may still ripen slightly prematurely to produce shriveled grain. The other
classic symptom of "wetland" take-all is the black mycelial
growth of the pathogen on and within the seminal and crown roots, into
the crown, and 1-2 inches up the stem. This diagnostic growth up the stems,
which some call the "black stocking," can be seen by pealing
back the leaf sheaths to reveal lower-most internodes of the elongated
stems.
Under dryland conditions,
which includes the great majority of the nonirrigated PNW most years,
the disease occurs on individual plants rather than plants in patches,
and the growth of the fungus from roots upward usually stops in the crown
where the tiller bases are connected to the mainstem. There is no black
stocking symptom with dryland take-all. The "window" of suitably
moist soil when the soil temperature is also favorable (40-60o F) is too
narrow to permit more than limited growth of the pathogen from plant to
plant or upward on the stems. Under these conditions, every plant must
be infected separately from primary inoculum in the soil. On the other
hand, the fact that crown infection is limited to the site where stem
bases of tillers are attached to each other and to the mainstem is still
enough damage to restrict flow of water up the stem and hence can still
cause premature blighting (white heads). However, under dryland conditions,
with each plant infected separately from primary inoculum, we find blighted
plants next to healthy plants much as happens with Fusarium crown rot,
Cephalosporium stripe, and Pseudocercosporella foot rot.
Wetland take-all
is easy to recognize by the patches of plants with white heads and the
black stocking at the lower-most internodes. Dryland take-all can be recognized
by cutting the crown open with a sharp knife to expose the bases of the
stem at the point where they are joined together and to the mainstem.
With Fusarium crown rot, this tissue is typically brown, but with take-all,
it is grey or grey with black mottling.
Management of
Take-all with Crop Rotation
Historically, a 1-year
break out of wheat or barley to a broad leaf crop or to oats or corn (both
non host grass crops), has been considered sufficient for management of
take-all. Again, this has been based on experience with "wetland"
take-all where, once the crop is harvested, the moist conditions of the
top soil while also still relatively warm favors breakdown of infected
roots and tiller bases used by the pathogen as a food base to infect the
next crop. Under dryland conditions in the PNW, the infected roots and
tiller bases last for long time in the soil, and may last even longer
when kept near the soil surface as with mulch-tillage and no-tillage.
In other words, the drier the area in our region, the lower the risk of
take-all in general but also the more likely that the pathogen inoculum
will carry over in a dormant condition through a 1- or 2-year break.
Years ago, keen observers
in Australia noted that take-all was most severe in years of a dry fallow
prior to planting and a wet growing season after planting. The dry fallow
delays or prevents microbial breakdown of the infested roots and crown
tissues so that the inoculum potential of the pathogen remains high in
spite of no hosts, while the wet growing season favors maximal infection
and secondary spread from root to root, plant to plant, and up the stem.
The ability of the
take-all pathogen to survive through 2 years of broadleaf crops in a low
precipitation zone of the PNW is revealed by results from a direct-seed
cropping systems study on the Ron Jirava farm west of Ritzville and led
by William Schillinger. This study compared three cropping systems: continuous
spring wheat; a 2-year spring wheat/spring barley rotation, and a 4-year
safflower/yellow mustard/spring wheat/spring wheat rotation. In the 4th
year of this study, the amount of take-all, although not severe, was the
same on spring wheat whether continuous spring wheat, spring wheat after
spring barley, or 1st or 2nd year spring wheat after back-to-back broad
leaf crops (Table 1; Cook et al, submitted). Neither safflower nor yellow
mustard is hosts of the take-all pathogen, and grass weed hosts were fully
controlled. These results can only be explained on the basis of dormant
carryover of the pathogen in infested wheat residue though 2 years without
a host.
Based on these observations,
we can expect that a 1- or 2-year break to a nonhost crop will be useful
in take-all management mainly or exclusively in areas with annual precipitation
higher than 12 inches, but we are not sure how much higher. Tom Swaintz
asked me to look at one of his wheat fields that had take-all, near Reardon
in a 16-inch precipitation area. Two years earlier, one part of this field
had been cropped to spring barley and the other part had been cropped
to a broadleaf crop before the entire field was planted to two consecutive
crops of wheat. Take-all was present throughout the field in this 2nd
year of wheat, but was distinctly more severe, to the line, where spring
barley had been grown 2 years earlier compared with the part of the field
cropped to a broadleaf crop 2 years earlier. Under these conditions, the
benefits of the broadleaf crop in the rotation extended into a second
wheat crop. The chances of this outcome presumably would also depend on
variations in precipitation from year to year and therefore year-to-year
differences in buildup of the pathogen on host crops and the opportunity
for dormant survival of the pathogen in infested roots and stem tissues
between host crops.
With convincing evidence
that the take-all pathogen can survive 2 years without a host in low-precipitation
areas, the most reliable data on value of alternative crops in a rotation
with wheat for take-all management are from the higher precipitation areas.
Ramsey (2000, 2001) tested the benefits of spring barley compared with
a broadleaf crop rotated with wheat in 2-year rotations in his survey
of 270 fields conducted over the years 1999, 2000 and 2001. Focusing on
fields in areas with > 18 inches of precipitation annually, the average
take-all index for wheat after spring barley was 18.9 compared with an
average of 11.2 for wheat after a broad leaf crop. This difference was
significant at P = 0.5. The take-all index for wheat after wheat in this
same precipitation zone was 16.0, i.e., essentially the same as for wheat
after spring barley.
Independent studies
indicated that a take-all index of 5 was the threshold index for take-all
severity, above which take-all begins to have an affect on the yield.
Thus, while spring barley was like another crop of wheat in these higher
precipitation areas, take-all was still evident on wheat in 2-year broadleaf
crop/wheat rotations.
It is worth noting
that, on barley, the take-all pathogen tends to remain restricted to roots
and extends only modestly or not at all into the crown or up the stems
of adult plants, even under conditions favorable to "wet land"
take-all. Because root tissues are short lived compared with cereal stem
bases as a food base for the take-all pathogen, a 1-year-break between
barley and wheat (i.e., spring barley/broadleaf crop/wheat) will likely
provide better take all control than a 1 year break between wheat crops
(i.e., wheat/broadleaf crop/wheat). Along with control of Cephalosporium
stripe, this fact can help explain the success of the classic 3-year spring
barley/pea or lentil/winter wheat rotation in the Palouse. This rotation
provides 2 full years between wheat crops, for decomposition of wheat
stem bases infested with the take-all pathogen, and 1 full year between
barley and wheat, for decomposition of barley roots infested with the
take-all pathogen.
Another advantage
of spring barley over spring wheat prior to winter wheat is the slightly
longer time (1-2 weeks or longer) between harvest and planting, owing
to the faster maturity of barley. The longer the time between harvest
and planting, the more time for death and disappearance of the take-all
pathogen through decomposition of its food base, provided that soil conditions
are suitably warm and moist for decomposition of cereal residue. The 7-8
months between harvest and planting a spring cereal is even better and
can help explain some of the success in the higher rainfall areas of continuous
spring barley or spring barley/spring wheat systems. Making the most of
this system depends on early elimination of volunteer and grass hosts
of the take-all fungus (the green bridge), much as has been shown for
management of Rhizoctonia root rot.
Management of
Take-all With Continuous Cereals
Rotation with a broadleaf
crop, where ever possible, should always be the first choice for management
of take-all in the higher rainfall areas. However, where broadleaf crops
are not economical, take-all can still be managed while growing continuous
cereals. This is because populations of rhizosphere bacteria suppressive
to this disease build up in response to outbreaks of take-all and continuous
wheat or wheat/barley sequences. The result is that take-all increases
with continuous wheat or wheat/barley during the initial years, peaks
out, and then declines in severity while yields recover. This cycle is
known the world over as "take-all decline."
Take-all decline
has only been documented in situations where take-all can or has become
severe, i.e., in areas with wetland take-all or at the wetter extremes
of areas with dryland take-all. No studies have been done to determine
whether the soil also becomes microbiologically suppressive to take-all
in the low precipitation zones of the PNW. Because the microorganisms
responsible for take-all decline are, themselves, dependent on soil water
potentials at or near field capacity (Howie et al.1987), it is doubtful
that these bacteria represent a potential biological control of dryland
take-all and even more doubtful that they can survive at effective populations
during a year of fallow. The normal pattern observed in the dryland PNW
is for wetland take-all to appear uniformly over the field in the second
and sometimes the first year that the field is irrigated and annually
cropped to wheat after decades of wheat fallow. This early and rapid development
of take-all in response to irrigation is further evidence that the pathogen
is already widely distributed in the field. Take-all decline then follows.
I have documented
the rise and decline of take-all in two plots planted continuously over
many years to wheat or wheat and barley. One was conducted at on the WSU
station at Lind with winter wheat grown under irrigation and conventional
tillage after decades of conventional management in a winter wheat/fallow
sequence. The other was conducted on the Palouse Conservation Field Station
near Pullman with alternating direct seeded spring and winter wheat. In
both cases, take-all decline was complete by about the 15 year of monoculture.
There were obvious improvements in yield already by the 10th to 12th year,
but yields were disastrously low during the years of most severe take-all.
In the experiment at the WSU stations at Lind, for example, yields in
the 7th year of continuous wheat were only half of the yields in fumigated
plots used as checks, and half of the yields achieved without fumigation
in the 15th year of continuous wheat.
The natural drop
in soil pH in response to ammonium forms of nitrogen is thought to favor
or accelerate take-all decline. In this regard, we must pay particular
attention to the possibility of more take-all if or when soils are limed.
One final point on
management of take-all of continuous cereals: assuring adequate phosphorus
availability for the crop can greatly help to limit the amount of this
disease. P-deficient wheat is intrinsically more susceptible to take-all.
With the loss of roots caused by the disease, the ability of the plants
to obtain phosphorous is also then limited, making the plants more susceptible
and aiding the pathogen to become even more aggressive as a parasite.
Phosphorus placement below the seed or with the seed at planting, as done
with direct seeding, can help over come this situation
Basic Research on Take-all Decline
Most of the basic
research aimed at explaining take-all decline has been done at WSU by
ARS scientists David Weller and Linda Thomashow with their students and
postdoctoral associates. This work represents some of the most elegant
research ever conducted in soil microbiology. Briefly, they have shown
that continuous wheat and the consequent development of take-all favors
a shift in make-up of rhizosphere bacteria towards a select group highly
adapted to the rhizosphere of wheat and genetically equipped to produce
an antibiotic, 2,4-diacetylphloroglucinol (PHL), inhibitory to the wheat
take all fungus (Raaijmakers et al., 1997). The ability to make this antibiotic
is a highly conserved trait in soil- and rhizosphere-inhabiting bacteria
world wide, but different strains with this ability are adapted to the
roots of different crops. The strains adapted to roots of wheat represent
a recognizable genetic type that can now be quantified with a DNA-based
test. Using this test, Raaijmakers and Weller (1998) showed that a population
of about 105 of these bacteria per gram fresh weight of root is sufficient
for suppression of take-all. The addition of only about 103 bacteria per
seed, is enough to establish 105 per gram of root in the rhizosphere.
This shows the remarkable ability of these bacteria to multiply in the
rhizosphere of wheat. Most soil bacteria require an initial inoculum dose
of 107 or 108 on the seed in order to achieve a population of 105 in the
rhizosphere.
Field trials conducted
with these rhizosphere-competent strains have produced modest but significant
yield increases (Cook et al., submitted; Mathre et al.1999). Moreover,
these strains are fully compatible with the chemical seed treatments currently
used in the PNW (see Pythium report in these proceedings). Even better,
these strains add value to the current chemical seed treatments, producing
8-10% greater yields and up to 15% greater yields than non-treated seed.
A combination of
early elimination of the greenbridge, placement of fertilizer and especially
phosphorous under the seed at planting, and treatment of the seed with
a combination of chemical and biological seed treatments, has been shown
to elevate yields in direct-seeded cereal-monoculture fields to within
90% of the yields obtained in these same fields in fumigated plots. Without
the biological component, the yields are 80-85% of those in fumigated
plots. Thus far, however, no company has been willing to take on this
biological control technology for treating seed.
Is Take-all Favored
by Direct Seeding?
A great deal of research
has been done to determine whether or not take-all is worse with direct
seeding, but the results have been mixed. In Georgia, where take-all is
a problem in the wheat/soybean double crop system, the disease was more
severe with conventional tillage and seeding compared with direct seeding
(Rothrock, 1987). In contrast, research by graduate student Kevin Moore
carried out with irrigated wheat in the 1970s on the WSU station at Lind
revealed in side-by-side plots that take-all was more severe with direct
seeding that when planting into a prepared seed bed (Moore and Cook, 1984).
Similarly in Australia, side-by-side plots showed more server take-all
with direct seeding than in plots tilled prior to seeding (Roget et al,
1996). It follows that infected plant remains left undisturbed in the
soil will present a higher risk for infection of the next crop than when
this tissue is fragmented into smaller pieces with tillage-like making
kindling out of fire wood. It also follows that direct seeding is like
moving the field into a higher precipitation zone and this we know will
favor take-all. On the other hand, tilling the soil will also distribute
the infested crop residue more uniformly so that more roots of the next
crop will be exposed more uniformly to infection.
Ramsey (2001) looked
at tillage effects on take-all in his 3-year survey of 270 wheat fields
planted to wheat either the previous year or 2 years earlier. He found
no difference in the severity of take-all between conventional and direct
seeding. For example, of fields cropped the previous year to wheat, the
average take-all index was 13.5 for conventional and 13.2 for direct-seeded
fields. Focusing on high-precipitation areas (>18 inches annually),
the average take-all index was 19.4 for conventional systems and 14.1
for direct-seed systems. The take-all indices for wheat from fields in
areas with <18 inches annually were 10.6 for conventional and 11.9
for direct seeding. None of these differences were significant statistically.
Ramsey's results
did not account for likely differences in fertilizer placement between
conventional and direct-seed systems. Virtually all direct seeding includes
phosphorus placement below or with the seed at planting, whereas few conventionally
managed crops are fertilized at the time of planting. Possibly this difference
in use and placement of phosphorous helps offset any additional inoculum
potential of the take-all pathogen in direct seeded fields.
Paired-row Spacing
Widening the rows
opens the wheat canopy to greater warming a drying of the top few inches
of soil-conditions less favorable to take-all. However, widening the rows
also reduces the plant population needed for maximum yield. The Yielder
drill provides a way through a 5/15 inch paired-row spacing to have it
both ways: open the canopy on one side of each row while providing the
same plant density achieved with a uniform 10-inch row spacing..
Research conducted
over several years with a plot drill that permits the openers to be either
paired in a 7/17 inch spacing or spaced uniformly on 12-inch spacing indicates
that, indeed, the amount of take-all can be limited significantly by pairing
the rows (Cook et al, 2000). These studies included experiments with a
fertilizer shank in each seed row so as to remove fertilizer placement
as a variable. All studies were conducted in fields cropped to wheat 4
or more previous years and therefore were considered high risk for take-all.
In addition, the studies were conducted in high-precipitation or irrigated
areas so as to further increase the risk of take-all.
Two of the five studies
that compared paired-row spacing with uniform spacing are described here
as illustrations. One was conducted in an irrigated field near Pendleton
and the other was conducted in a subirrigated site (bottom land) near
Colfax. Although the experiments were conducted in different years in
the mid 1990s, both sites were in the 9th year of continuous wheat when
the respective experiments were conducted. Also, both sites were burned
just prior to laying out and planting the plots.
At the Colfax site,
fumigation was included as a treatment, and straw was returned to individual
plots, so as to provide all combinations of burned + fumigated with no
straw returned, burned + fumigated with straw returned onto burned-fumigated
plots, burned but not fumigated, and burned, not fumigated, with straw
returned. Each of these treatments was then seeded as side-by-side drill
strips of paired and uniform row spacing with four replications. Yields
averaged 95 to 100 bu/A with both paired and uniform row spacing in all
fumigated plots, with our without straw returned to the plots, and in
the nonfumigated treatment left bare-black (no straw returned to the soil).
In contrast, returning straw to the plots after burning resulted in an
average yield of 72 bu/A with uniform row spacing and 82 bu/A with paired-row
spacing. Take-all mainly, but some Rhizoctonia and Pythium root rot, were
severe in nonfumigated plots when covered with straw, and under these
conditions, pairing the rows resulted in 10 bu/A additional yield of winter
wheat.
We have consistently
seen a 20-30 bu/A yield increase with winter wheat in response to burning
in the high-precipitation areas of eastern Washington, but have also shown
consistently that this yield response is nullified by returning straw
to the burned soil surface immediately after planting (Cook and Haglund,
1991). In one such experiment near Pullman, we showed that the average
percentage of adult winter wheat plants with seminal roots destroyed by
take-all was 32, 18, 40, and 12, respectively, in replicated plots not
burned, burned, burned and straw returned, and not burned but fumigated.
These results collectively present a clear picture of less take-all and
more yield in response to burning, but simply returning straw to the soil
surface nullifies the burn effect. In other words, to the extent that
the burn response is due to less root disease, the effect is probably
through the warming and drying effect of making the soil surface bare-black
and is not due to killing the pathogen in soil by the burn.
The experiment at
Pendleton included paired- and uniformly spaced rows of winter wheat over
alternating 24-foot-wide strips of tilled (disked twice) and not tilled
land with stubble burned. Wheat planted into the tilled plots had less
take-all and averaged 9 bu/A more compared to wheat in the direct seeded
plots (109 versus 100 bu/A in tilled and no-till treatments respectively).
These results support the work that suggests that take-all is favored
by direct seeding, where every other management factor is held constant
across the two treatments. When all data for tilled and no-till plots
were combined, pairing the rows had no effect on yield but resulted in
a 25% reduction in white heads and an increase in grain test weight of
0.5 lb/bu, both significant at P = 0.05. The effect of pairing the rows
was real but subtle, being expressed as increased test weight but not
increased yield.
One disadvantage
of opening the canopy through paired-row spacing is great potential for
weeds to grow in the space not planted to wheat. However, with new technology
for weed control, especially with the introduction of herbicide-resistant
varieties, paired-row spacing deserves a close look as one more tool in
management of take-all and other root diseases without burning the straw.
Take-all Prediction
and Risk Assessment-a Look at the Future
With knowledge of
such variables as the recent (past 3-4 years) cropping history, time in
weeks or months between harvest and planting, time between spraying and
seeding for greenbridge control, and annual rainfall, it will be possible
to develop a take-all prediction and risk assessment program for the PNW.
Such a program is currently under evaluation in South Australia (David
Roget and Michael Krause, personal communication) in combination with
a DNA-based test to also provide the information on amount of inoculum
of the take-all pathogen in the soil (Ophel Keller and McKay, 2001). The
DNA-based test is proprietary with the South Australian Research and Development
Institute (SARDI) and licensed to C-Qentec Diagnostics, a subsidiary of
Aventis CropScience. Farmers can obtain a report on the amount of inoculum
of G. graminis var. tritici, Rhizoctonia solani AG 8, cereal cyst nematode,
two species of lesion nematodes, and both Fusarium pseudograminearum and
F. culmorum, for about $A4 per acre. The results for fungal pathogens
are expressed as pico grams (pg) of DNA of the target pathogen per gram
of soil. For the nematodes, the results are expressed as numbers of nematodes
(or eggs) per gram of soil.
In March, 2001, soil
samples from 124 GPS-referenced sites representing 30 acres on the WSU
Cunningham Agronomy Farm were sent to SARDI in Adelaide, Australia, for
analysis of the complete package of pathogens for which their tests apply.
The test was positive for the take-all fungus in only 16 of the 124 samples
assayed, and these were low at 25 pg or less of DNA per gram of soil.
Fifteen pg DNA/g soil is below the level of detection. These low numbers
were surprising, considering that the field had been in spring barley
in 2000 and spring wheat for several consecutive years before that. We
must now test whether the field may be into take-all decline as occurred
in the long-term direct-seeded plot on the nearby Palouse Conservation
Field Station. It is also possible that the long period of no plants between
harvest of the spring barley in 2000 followed by planting spring wheat
in 2001, achieved with excellent greenbridge control in the fall of 2000,
provided time needed for natural disappearance of the inoculum of this
pathogen from the soil. Root assessments confirmed that take-all occurred
in only trace amounts on the spring wheat in 2001, and the yield of 64
bu/A was respectable, considering the lack of moisture during the 2001
growing season.
We will be sending
more soil samples from sites intended for direct seeded crops in 2002
in an effort to gain more experience with this technology and possibly
help guide its introduction into use by growers in the PNW.
Conclusions
In spite of a century
of research on take-all, this disease continues almost unabated, being
subject as much or more to variations in the natural environment and the
natural enemies than to anything we do by way of crop management aimed
at its control. We now recognize that take-all management by crop rotation
is achievable in the higher- but apparently not the lower-precipitation
areas where the pathogen survives in crop residue easily through a year
of fallow and even through 2 years of non host crops. Even in the higher-precipitation
zones of the dryland PNW, including areas with 22-24 inches of precipitation
annually, a 1-year break to a broadleaf crop helps but will not be long
enough to lower take-all below an economic threshold. One possible exception
is the 3-year rotation of winter wheat/spring barley/broadleaf crop, which
allows the necessary 2 years between wheat crops. Barley, being a host
of take-all, is like another crop of wheat when preceding wheat. However,
because the pathogen tends to remain confined to roots of barley, and
not extend into the stems, and because roots decompose faster than stems,
the 1-year break to a broadleaf crop after spring barley and before returning
to winter wheat can provide a significant rotational benefit for take-all
control.
The results on whether
or not take-all is favored by direct seeding have been mixed. When take-all
is studied in side-by-side plots of till and no-till, take-all has consistently
be more severe with no-till (direct-seeding) in both the PNW and Australia.
This is thought to result from a higher inoculum potential of the pathogen
in soil left undisturbed, at least within the old stubble rows, and also
from the wetter and cooler soil environment in fields left covered with
crop residue.
On the other hand,
tilling the soil mixes the inoculum of the pathogen more uniformly into
the soil. Further, direct-seeding is more likely than conventional seeding
to include phosphorous placed directly under the seed or with the seed
when planting, and availability of phosphorous is well known to limit
the amount of take-all. Side-by-side comparisons allow for testing of
a single variable such as till no not till, but direct seeding is a system
that will likely involve a different planting date, different timing and
method of greenbridge control, different method and timing of fertilization,
different depth of seeding, and different row spacing, possibly paired-row
rather than uniform-row spacing. These differences combined with low soil
disturbance will determine the extent to which take-all will flourish
or remain in check. Ramsey's survey shows that take-all is surprisingly
common in conventional systems and that, taking all factors into account,
both systems, because they include frequent crops of wheat, obviously
are favorable to take-all.
Growers in the low-precipitation
area can take solace in the fact that, while rotations and take-all decline
are of little or no use for take-all management in their fields, take-all
itself is limited by the low precipitation and will not be more than a
mild or chronic disease problem, even with continuous spring cereals.
Growers in the higher-precipitation areas can count on benefits from breaks
to broadleaf crops as one approach, or from take-all decline as another
approach. Since broadleaf crops tend also to disrupt take-all decline,
consideration should be given to the use of either crop rotations that
include broadleaf crops, or continuous cereals, but to not alternate between
the two systems. Basic research on take-all decline continues with the
goal of achieving biological control more consistently when applying the
bacteria to seeds and thereby attracting a commercial interest willing
to develop these bacteria into a product.
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