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Impacts
and Management of Soil Acidity under Direct Seed Systems
- Effects on Soilborne Crop Pathogens
Timothy
C. Paulitz
USDA-ARS, Root Disease and Biological Control Lab, Pullman, WA 99164-6430
Soil
pH can have an enormous influence on the outcome of soilborne diseases
caused by crop pathogens. Some soilborne pathogens have been successfully
controlled by the management of soil pH. For example, clubroot of crucifers
has been controlled by liming the soil to above pH 5.7. At pH 7.8, the
disease is completely checked. On the other hand, common scab of potato,
caused by the actinomycete Streptomyces, can be controlled by dropping
the pH of the soil below 5.2. Unfortunately, none of the soilborne pathogens
of direct-seeded wheat and barley can be controlled to this extent by
manipulating soil pH, nor would it be economical or wise to attempt these
changes on the large acreages of cereals in the Pacific Northwest. However,
cropping practices such as fertilization (the form of N) and lack of tillage
will have a direct influence on soil pH. This shift in soil pH may have
more subtle effects on diseases in direct-seeded crops that must be considered
in any management plan.
The form of nitrogen
affects soil pH in two major ways. First, the addition of ammonium forms
of N tends to acidify the soil. This is due to nitrification- the oxidation
of NH4+ (ammonium) to NO3- (nitrate) by soil bacteria, forming energy
and H+ ions that acidify the soil. However, the form of N may also affect
the pH around the root, to which the pathogen is exposed when infecting
the root. This zone of soil around the root is called the rhizosphere,
and from the disease perspective, this soil is more important than the
bulk soil away from the root. When a plant root takes up NH4+ ions, it
excretes H+ ions back to balance charges, thus acidifying the zone around
the root. When a root takes up NO3-, it excretes OH- ions, making the
root zone more alkaline.
Direct seeding may
have two direct effects on soil pH. Because the soil is not tilled, the
acidification caused by fertilizer application in the top soil layers
is not diluted out by mixing with the more alkaline soil below the fertilizer
zone. Thus, soil pH in the top soil layers may decrease more in direct-seeded
crops compared to conventionally-tilled, at least until the buffering
capacity of the plow layer is reached. On the other hand, soil organic
matter increases in direct-seeded crops over time. This increases the
cation exchange capacity of the soil, buffering the soil by binding H+
to the negatively charged OH and COOH groups on the organic matter. NH4+
ions will also bind to organic matter, reducing acidification by nitrification.
However, the long-term outcome of soil pH in direct seeded crops in the
PNW is still unclear, and probably depends on the buffering capacity of
the parent soil.
How does soil pH
affect diseases? Remember that disease is the outcome of three interacting
components- the pathogen, the plant, and the environment. Soil pH can
indirectly affect diseases by affecting any one of these components. Root
diseases are caused by microscopic soilborne fungi. These organisms form
a network of tiny threads, which can grow through the soil and infect
plant roots. Fungi absorb food as simple molecules from organic matter
or living plants. These molecules must be transported across the membrane
from the outside to the inside of the cell. The fungus expends energy
and uses a proton pump to transport many of these molecules across the
membrane by maintaining a proton (H+) gradient. The external pH (proton
concentration) can affect its ability to take up food. But, in general,
only extremes of pH (greater than 7 or less than 5) reduce the growth
of most fungi. Put another way- at the pH of most agriculture soils, most
fungi are not pH limited in terms of growth. However, pH may influence
the availability of trace nutrients such as iron, zinc or manganezse in
the same way as its availability to plants is affected. Thus, fungi must
work harder to get these less available nutrients.
Soil pH will also affect the host plant. If the pH is too extreme, the
plant will be stressed and may be less resistant to attack by the pathogen.
Soil pH may affect the composition of the root exudates, which attract
the soil borne pathogen. Soil pH will also affect the availability of
nutrients to the plant. Some of these nutrients may be needed for strong
cell walls and resistance to fungi. For example, high levels of available
calcium in more alkaline soils have been implicated in the resistance
to root diseases caused by Pythium.
Finally, pH may affect
the microbial populations that may hold the pathogens in check. It is
well known that fungi are more active in more acid soils, while bacteria
are not as adapted to these conditions. Trichoderma, a biocontrol fungus,
prefers acid soils. Fluorescent pseudomonads, bacteria that have been
implicated in soils suppressive to take-all, prefer more neutral pH soils.
Thus, by shifting soil pH, a natural suppressiveness may be enhanced or
destroyed.
In this paper, I
will address six major diseases of wheat and barley, all caused by soilborne
fungi. In some cases, there is research showing clear-cut evidence for
a pH effect, but in other cases, the research is lacking. Because of the
link between type of N fertilizer and pH, I will also discuss research
on ammonium vs. nitrate fertilizers. With all root diseases, proper placement
of N is also important, to enable the seedling to quickly gain access
to the fertilizer and overcome the lack of absorbtive capacity caused
by a lack of roots.
Take-all- (Gaeumannomyces
graminis var. tritici). With this disease, the evidence is strong
that take-all is more severe in alkaline than in acid soils, and that
disease is reduced when ammonium forms of N are applied, as opposed to
nitrate forms. (Huber and Watson, 1971, Smiley et al. 1973, MacNish, 1980,
MacNish, 1988). In longer-term experiments in Australia at three different
sites over 11 years, less take-all was found in plots fertilized with
ammonium sulfate as aopposed to sodium nitrate. This effect is related
to the pH of the rhizosphere. Smiley and Cook (1973) found the disease
greatly reduced when the rhizosphere pH was below 6.6, but the correlation
with bulk soil pH was poor. The rhizosphere pH was 5.5 for wheat supplied
with ammonium nitrogen was 5.5, compared to 7.5 for plants supplied with
nitrate. The best control occurred with ammonium sulfate, and the addition
of lime negated the control. Take-all decline, a natural suppressiveness
associated with wheat monoculture, takes longer to establish in more alkaline
soils (Cook and Baker, 1983).
Rhizoctonia bare
patch and root rot (Rhizoctonia solani AG-8). There is not much information
on the effect of N or pH on Rhizoctonia root rots of cereals. Most studies
on Rhizoctonia root rots of broad leaf crops have shown that NO3-N results
in less disease compared to NH4-N fertilizers (Huber and Watson, 1974).
The same effect was seen on sharp eyespot of wheat, caused by R. cerealis.
MacNish, an Australian pathologist, looked at the effects of N application
on bare patch in conventional and direct-seeded wheat. He found that cultivation
reduced bare patch, something that has been shown by other researchers.
But the application of N also reduced bare patch in zero-tilled soil.,
and ammonium sulfate resulted in patch development than sodium nitrate
(MacNish, 1985). However, research by Pumphrey et al. (1987) in Pendleton
found that N application and timing of application had no effect on bare
patch. They used ammonium sulfate at planting and ammonium nitrate at
late tillering.
Smiley et al. (1996)
did a 3-year study in the long-term plots at Pendleton, looking at different
rotations, tillage and burning. In the wheat-fallow, they found that application
of N increased the incidence of Rhizoctonia root rot. They applied, 0,
40, or 80 lbs/acre and found more disease at 40, compared to 80 lbs/acre.
MacNish (1988), in long-term cropping systems study in Australia, found
that N application had no effect on Rhizoctonia root rot. It is interesting
that the ammonium reduced soil pH at all sites, but that Rhizoctonia declined
to almost nothing after 7-9 years of continuous wheat cropping. This may
be a case of development of natural suppressiveness developing. In a closely
related disease on turfgrass, brown patch caused by R. solani, plots that
received urea generally had less disease than the nitrate-treated plots
(Fidanza and Dernoeden, 1996a). Low soil pH was weakly correlated with
lower disease levels in one trial in 1993. This may be due to the sulfur
acidifying the soil, rather than acidification from the ammonium. But
in other trials (Fidanza and Dernoeden, 1996b), there was no correlation
with soil pH. The take-home message seems to be lots of conflicting information
with no rule of thumb. The effect of fertilizer type and soil pH do not
appear to be major for this disease, unlike for take-all and Fusarium
crown rot. However, the effect may be very site specific. There is no
literature on the effect of pH on R. oryzae, which is widespread in eastern
Washington (Paulitz, unpublished).
Pythium seedling
and root rot- (Pythium spp). Pythium is another disease on which there
is not much literature on pH effects in the field. There have been some
studies in the lab, showing that at pH 4.8, fewer zoospores attach to
the root than at pH 6.0. (Huyang and Tu, 1998). Pythium produces motile
spores that can swim through wet soil and attach to the root to infect
it. The most detailed study with relevance to the Pacific Northwest was
done by a student of R. J. Cook's in the early 90s (Fukui et al. 1994).
He looked at the disease-producing activity of different inoculum levels
of P. ultimum, in both pasteurized and natural soils. He adjusted the
pH of the natural soils to 4.3-7.6 with sulfuric acid or lime. The optimum
disease activity was from pH 5.0-5.5. There was a slight decline in disease
from pH 5.5 to 6.5, but above pH 6.5, the disease activity dropped significantly.
In one soil where the pH was decreased to 4.3, disease also declined significantly.
The take-home message on Pythium and pH is probably similar to Rhizoctonia-
in the pH ranges of our soils, Pythium is not limited. In fact, Pythium
has optimal activity at the pH ranges of soil that are acidified by ammonium
fertilizers. The literature on N effects on Pythium is also limited. Most
of the work has been done with the use of organic amendments to suppress
Pythium. Pythium is very sensitive to microbial competition, and organic
amendments often increase microbial activity in the soil. Smiley et al.
(1996) found Pythium root rot was more prevalent in sites with inorganic
N fertilizers, as opposed to those fertilized with cow manure or pea vines.
Fusarium foot
rot or crown rot (Fusarium pseudograminearum and F. culmorum). Most
growers are well aware that this disease is favored by increased nitrogen
fertilization and drought stress. The effect of the type of N on this
disease is well-known, based on work done by Cook and Pappendick in the
early 70s. Applications of NH4-N increase disease severity and incidence,
while NO3-N fertilizers decrease the disease (Smiley et al. 1972). This
is similar to Fusarium wilt diseases, which are suppressed by alkaline
soils and nitrate fertilizers (Nelson et al.1981). Smiley et al. (1996)
also found a strong correlation between crown rot and N application, and
the disease was inversely proportional to soil pH, at least in the range
measured (4.3 to 5.3).
Cephalosporium
stripe (Cephalosporium gramineum) This is a disease where soil pH
has a dramatic influence. This soilborne vascular pathogen is more damaging
in acid soils with high moisture. It produces single-celled spores that
infect roots wounded by winter freezing and soil heaving. In a series
of studies by Tim Murray at WSU, he showed that the germination of spores
is not affected by pH (Blank and Murray, 1998), but the production of
spores on wheat straw buried in the soil was greater at acid pH (Murray
and Walter, 1991). Disease increased 5-fold when soil pH decreased from
7.5 to 4.5. The isolation of the pathogen from crown roots was greaterless
at pH 6.7 to 7.2 than at pH 4.7 to 5.9 (Stiles and Murray, 1996). Liming
the soil to increase soil pH from 5.1-5.3 to >6.0 decreased this disease
two out of four years, and there was a significant correlation between
soil pH and infected stems (Murray et al. 1992).
Eyespot of wheat
(Pseudocercosporella herpotrichoides). Only one study has been done
on the effect of N and pH on this disease, with trends similar to that
of Fusarium crown rot. In a study on winter wheat on wheat-fallow, eyespot
incidence increased 2-3 fold when 160 lbs/acre of N was applied, compared
to the non-fertilized plot (Smiley et al. 1996). This can be explained
by the enhanced canopy growth, like planting early, which is well known
to favor eyespot foot rot of wheat.
Conclusions
In conclusion, diseases
caused by soilborne pathogens of wheat and barley can be classified into
two groups- those that are not influenced greatly by pH or type of nitrogen
fertilizer, and those that are. The first group includes the root rotting
pathogens Pythium and Rhizoctonia. These are considered to be generalist
types of pathogens that quickly kill and rot the plant tissue. The second
group includes the crown rotting pathogens Fusarium and Gaeumannomyces
(take-all). These initially infect and colonize the root without causing
massive tissue death, and later move into the crown when the plant is
older. These two diseases show opposite trends- take-all is reduced under
acid conditions, while Fusarium crown rot seems to be increased. One could
speculate that increased soil acidification may increase the risk of Fusarium
crown rot, although N levels and drought stress are probably stronger
risk factors. One can also speculate that liming the soil will increase
take-all. Cephalosporum stripe, a vascular disease, is also influenced
by pH and N, similar to many other vascular diseases. Liming the soil
will decrease the severity of this disease. Thus, the risk of soilborne
pathogens in wheat and barley may be influenced by the potential shift
of soil pH in direct-seeded systems, with the different mix of diseases
depending on the degree and direction of the pH shift in response to the
form of nitrogen, accumulation of organic matter, and whether lime is
used.
References
Blank, C. A. and
Murray, T. D. 1998. Influence of pH and matric potential on germination
of Cephalosporium gramineum conidia. Plant Dis. 82: 975-978.
Cook, R. J. and Baker,
K. F. 1983. The Nature and Practice of Biological Control of Plant Pathogens.
APS Press, St. Paul, MN.
Fidanza, M. A. and
Dornoeden, P. H. 1996a. Brown patch severity in perennial ryegrass as
influenced by irrigation, fungicide, and fertilizers. Crop Sci. 36: 1631-1638.
Fidanza, M. A. and
Dornoeden, P. H. 1996b. Influence of mowing height, nitrogen source, and
ipridione on brown patch severity in perennial ryegrass. Crop Sci. 36:
1620-1630.
Huang, R. and Tu,
J. C. 1998. Effects of hydrogen ion concentration and temperature on attachment
of zoospores of Pythium aphanidermatum to tomato roots. Mededelingen-
Fac. Landbouwk. Univ. Gent 63: 855-859.
Huber, D. M. and
Watson, R. D. 1974. Nitrogen form and plant disease. Annu. Rev. Phytopathology
12:139-165.
MacNish, G. C. 1980.
Management of cereals for control of take-all. Journal of Agriculture,
Western Australia 21: 15-19.
MacNish, G. C. 1988.
Changes in take-all (Gaeumannomyces graminis var. tritici), rhizoctonia
root rot (Rhizoctonia solani) and soil pH in continuous wheat with annual
applications of nitrogenous fertilizer in Western Australia. Aust. J.
Experimental Agricult. 28: 333-341.
MacNish, G. C. 1985.
Methods of reducing rhizoctonia patch of cereals in Western Australia.
Plant Pathology 34: 175-181.
Murray, T. D. and
Walter, C. C. 1991. Influence of pH and matric potential on sporulation
of Cephalosporium gramineum. Phytopathol. 81: 79-84.
Murray, T. D., Walter,
C. C. and Anderegg, J. C. 1992. Control of Cephalosporium stripe of winter
wheat by liming. Plant Dis. 76: 282-286.
Nelson, P. E. Toussoun,
T. A. and Cook, R. J. 1981. Fusarium: Disease, Biology, and Taxonomy.
Penn. State Univ., University Park, PA.
Pumphrey, F. V.,
Wilkins, D. E., Hane, D. C. and Smiley, R. W. 1987. Influence of tillage
and nitrogen fertilizer on Rhizoctonia root rot (bare patch) of winter
wheat. Plant Disease 71: 125-127.
Smiley, R. W. and
Cook, R. J. 1973. Relationship between take-all of wheat and rhizosphere
pH in soils fertilized with ammonium vs. nitrate-nitrogen. Phytopathology
63: 882-890.
Smiley, R. W., Collins,
H. P. and Rasmussen, P. E. 1996. Diseases of wheat in long-term agronomic
experiments at Pendleton, Oregon. Plant Dis. 80: 813-820.
Smiley, R. W., Cook,
R. J. and Pappendick, R. I. 1972. Fusarium foot rot of wheat and peas
as influenced by soil application of anhydrous ammonia and ammonia potassium
azide solutions. Phytopath. 62: 86-91.
Stiles, C. M. and
Murray, T. D. 1996. Infection of field-grown winter wheat by Cephalosporium
gramineum and the effect of soil pH. Phytopathol. 86: 177-183.
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