 James
Cook retired in 2005 as Professor of Plant
Pathology and Crop and Soil Sciences and Interim
Dean of the College of Agricultural, Human and
Natural Resource Sciences at the Washington
State University (WSU). His long and productive
career in plant pathology had begun in 1965 when
he was assigned to work on wheat root diseases
for the U.S. Department of Agriculture,
Agricultural Research Service (USDA-ARS),
stationed at WSU, Pullman, Washington.
Teaching students and post graduates was always
an important part of his work. From 1998 – 2005,
he held the R. James Cook Endowed Chair in Wheat
Research, which was established in 1997 with a
$1.5 million gift to the WSU Foundation by the
Washington Wheat Commission. Washington State
University created this Chair to strengthen
research and graduate education in the Plant,
Soil, and Microbiological Sciences.
Prof. Cook received a number of honors through
his career, especially recognizing that he
always worked closely together with growers to
ensure that his research was applicable to the
farming system practiced.
Prof. Cook’s work aimed at wheat health and the
factors that impact plant pathogens, as well as
non-pathogenic microbes. His discoveries and
conclusions gave insight to the whole picture of
factors influencing Root Health, rather than
focusing only on single factors. Root Health is
a topic dear to his heart, as he states himself.
We asked Prof. Cook about his view on the
impact of today’s agronomic practices on Root
Health:
One
factor clearly impacting Root Health is the lack
of rotation in today’s agriculture where
continuous crop monoculture is more and more
common. Traditional crop rotations are no longer
an economical option. In the US Pacific
Northwest (PNW), all different classes of wheat
can be grown, including spring and winter wheat
as well as high grain protein and low grain
protein wheat. Still, rotation is based on
intensive and continuous cereal growing that
includes spring and winter barley.
When I started working at Washington State
University, I had to overcome a number of
theories that were taken for granted. One of
them was that, in monoculture systems, yield is
driven down over the years due to depletion of
soil nutrients. It took some time to prove that
the symptoms of nutrient deficiency that
occurred in continuous monoculture were caused
by an increase of root diseases in this system.
The damaged root system was subsequently unable
to take up the fertilizer available in the soil,
resulting in nutrient deficiency symptoms.
Direct seeding employed in the late 70s is the
other factor that increased pressure from root
diseases. Despite all benefits that this system
has with respect to less soil erosion,
improvement in soil quality and reduction of
fuel costs by saving the energy for tillage, it
favors the soil borne pathogens. With the change
to direct seeding, it was at first believed that
substances from straw left on the soil surface
were toxic or allelopathic to wheat. Our
laboratory found that the same toxic effect
could be observed when dry oatmeal was added to
the soil instead of straw. The symptoms of
stunted wheat and yellow leaves were overcome by
either pasteurizing the soil with moist heat at
45 °C /113 °F for 20 minutes or adding mefenoxam
to the soil. With this, we were able to document
that the stunted yield was not caused by the
straw, but due to soil diseases (Pythium
in this case) that were favored by additional
crop residues on the soil surface. In field
studies we were able to document that the yield
loss and disappointingly poor growth of wheat
when direct seeded into wheat stubble was due to
pathogens in the soil and not in the straw.
What diseases constitute a threat for Root
Health in wheat from an US view and how has this
changed over the past 40 years?
Fusarium root rot became more important with the
introduction of dwarf varieties in the early
1960s. These varieties were able to respond to
additional nitrogen (N), resulting in increased
yield without the danger of lodging.
Unfortunately the higher N uptake correlated to
an increased need for water uptake. The
resulting drought stress of the plants under dry
land conditions was the cause of an increase of
Fusarium root rot because Fusarium is a
root and foot rot pathogen of water-stressed
wheat. By adapting the amount of N fertilizer to
the estimated water availability this problem
could be minimized.
Take-all (caused by Gaeumanomyces graminis)
is an important root disease that is wide spread
and favored by cool and moist conditions. In
monoculture wheat systems with adequate
moisture, it is potentially the dominant root
disease. We learned as others in Europe had
reported that this disease can decline in wheat
monoculture over time due to the establishment
of a suppressive soil. The suppressive soil is
the result of certain antibiotic-producing
strains of Pseudomonas fluorescence that
build-up following one or more Take-all
outbreaks in a field.
Pythium is a pathogen so wide spread that
probably every cup of soil on this world has
Pythium in it. Therefore we started working with
soil fumigation as a research tool. This
treatment allowed us to see what wheat with a
healthy root system looks like. Plant
physiologists suspected that the improved yield
after soil fumigation was because of the dead
microbial biomass releasing additional N that
gave the increased growth response (IGR) of the
wheat plant. However, we were able to document
that of two fumigants that each gave the flush
of N, only one gave the IGR, thereby providing
the first evidence separating the two effects.
Later we recognized that the fumigant that gave
the IGR eliminated Pythium much like
pasteurization of the soil. We also showed that
no fumigant left N unused in the soil.
Pythium is classified as a root nibbler.
It penetrates the roots in the zone of root hair
formation and destroys root tips and root hairs.
Pythium generally likes juvenile cells –
for example, the embryo of a germinating seed.
The introduction of mefenoxam in the early 1980s
as a seed treatment resulted in similar effects
as soil fumigation and supported the awareness
on the impact of this disease. Direct seeded
soils over time have better water drainage than
conventional tilled soils, which can reduce
infections with Pythium over time, as well.
Rhizoctonia root rot caused by Rhizoctonia
solani AG8 in the PNW of the US, is an
increasing problem especially under direct
seeding. This disease can “prune off” the root
like a beaver chews off a tree. Since
Rhizoctonia has a wide host range, crop rotation
is not expected to have a large impact on this
disease. The practice of spraying volunteer
plants with glyphosate two to three days prior
to seeding increased the severity of Rhizoctonia
root rot. The pathogen present in the field
“becomes a millionaire overnight” with the
sudden increase of dying roots already occupied
by the pathogen through parasitism. By spraying
glyphosate four weeks prior to seeding, these
volunteer seedlings are small and therefore
represent less potential food base when they
die, and the decomposers have more time to
destroy this food base and starve Rhizoctonia. I
focused on these four disease pathogens. They
were discovered in this order during my career.
Take-all, Pythium and Rhizoctonia
like cool soils, whilst Fusarium,
especially Fusarium graminearum, is a
pathogen of warmer soils and drought stress.
What is the value of a healthy root, and what
is its role in crop production?
A
healthy root is key for nutrient uptake and
particularly so during early stages of
development. If three out of five seminal roots
get infected or worse, cut off by girdling
lesion, only two are left to provide the
seedling with water and nutrients. Under
phosphorus (P) deficiency, plants become more
susceptible to soilborne diseases. At the same
time, a diseased root results in reduced P
uptake. Zinc and other trace nutrients are
likewise important for disease defense
mechanisms of the plant.
Furthermore, it has been shown that hormones
important to the tops are produced in the root
tips of plants. A root system damaged by a
disease like Pythium or Rhizoctonia
can therefore result in symptoms of hormone
imbalances or deficiencies that have an impact
on the plant’s growth, development, and finally
yield.
An important finding in plant physiology was
discovered by Betty Klepper, plant physiologist
and research leader at the USDA-ARS in Oregon,
US. She found that after a shoot comes out from
the soil, the bud of the axil of the first true
leaf breaks dormancy at the time of appearance
of the fourth leaf on the main stem, initiating
the first tiller. Similarly the bud of the axil
of the second leaf breaks dormancy with
appearance of the fifth leaf on the main stem,
initiating the second tiller and so forth. The
development of leaves and hence tillering is
driven by growing degree days. For the PNW, one
leaf develops and hence another tiller every 100
growing degree days. If, at the time the fourth
leaf elongates, the plant encounters stress such
as nutrient deficiency caused by a diseased root
or drought, the first tiller will be skipped and
never be initiated. In this case, the first
tiller we will see when looking at that plant
might actually be a second or third tiller.
Because the first two tillers have the highest
yield potential, the most important effect of
root health is its effect on tillering. The
number of tillers initiated, plus having the
“right” tillers, has a big effect on final crop
yield.
What needs to be done in order to reach most
of the plant’s potential in a cropping system
with direct seeding and monoculture?
Spray glyphosate four weeks prior to seeding in
order to prevent or eliminate the ”green bridge”
(green growth by volunteer plants or weeds after
harvest before the next seeding). This can help
limit damage caused by pathogens such as
Rhizoctonia.
Spring sown wheat after winter wheat gives an
eight month break that allows reduction of
inoculum.
The application of a seed treatment protects the
embryo and ensures vigorous seedling development
and hence stand establishment. At the same time,
use fresh (new) high quality seeds. Older seeds
can have dead cells; in contact with moist soil,
seed content may leak from those cells to the
soil and stimulate infections with Pythium.
Phosphorus fertilizer placement under the seed
at the time of seeding can help to reduce the
negative effect of root diseases; the P placed
is where it is needed in direct proximity to the
young roots, even if they are diseased. As
mentioned earlier, on one hand P has a positive
effect on the plant’s defense mechanisms. On the
other hand, even if the root is attacked by
disease, there is still enough P available to
ensure further development of secondary roots.
Once the soil begins warming up, the plant can
outgrow its vulnerable phase and overcome the
cooler conditions with higher disease pressure.
Adopting paired row spacing (two rows sown 17 cm
apart alternating with 43 cm spacing, instead of
uniform 30 cm row spacing) showed in our studies
that the resulting greater warming and drying of
the top few centimeters of soil where the
pathogens are most active helped reduce root
diseases.
A combination of these practices can bring
yields up to at least 85% of what can be reached
by eliminating root diseases using soil
fumigation.
Professor James Cook was interviewed by Dr.
Melanie Goll, Network Community Manager for
Syngenta.
Melanie moderates the Global Root Health
Network, a group hosted by Syngenta on LinkedIn®
professional networking services. This group
brings together leading technical experts from
private, public and nonprofit institutions
worldwide to exchange knowledge about Root
Health and the link to plant performance.
For more information, please contact
melanie.goll@syngenta.com or explore the
group on LinkedIn at
http://www.linkedin.com/groups?gid=3773669
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