Tales
from the Annals of Disease
Bruce
Martin and Jim Camberato
Clemson University, Pee Dee Research and Education Center
There have been some devastating disease outbreaks over the past several
years that typically occur rarely – such as bermudagrass rust, Fusarium and
Ascochyta blights, and Dreschlera (Helminthosporium) leaf spots.
Although the organisms that cause these diseases are always present in
the turf environment, minimal disease occurs in healthy turf.
However, stressed turfgrass is susceptible to epidemics of these, and
other, diseases. When rusts, blights, and leaf spots occur with good weather,
one should expect other factors are stressing the turfgrass making it
susceptible to the disease.
Once upon a time on a Coastal golf course there was a mysterious disappearance of ryegrass overseeding in February. Dreschlera leaf spot was identified as the pathogen causing the grass to die. The disease outbreak was unique to that golf course and was not occurring on adjacent courses. Temperature and rainfall were conducive to good ryegrass growth. Inspection of the soil profile revealed a thick thatch accumulation even though the turf had been established for only a few years. Clearly factors other than the disease itself, or the weather, were involved. A routine soil analysis exposed the culprit, excessively low soil pH – less than pH 4 in many areas.
In a place not too far away and in a time not too
long ago
another call was received from a distressed superintendent.
The symptoms were unusually poor bermudagrass growth during spring
green-up. The grass was yellow to
brown in color and had little vigor. Microscopic
examination revealed the spores and pustules of the rust fungus.
This disease occurred on the same course the year before and was
suppressed with fungicide. This
first disease occurrence was considered an anomaly and was not expected to
return. Why then did it reappear?
The soil was infested with sting nematode -- perhaps they were the causal
stress factor? However, nematodes
are usually localized in their distribution and the disease was widespread.
Nematodes were not the main stress factor in this instance.
A routine soil analysis revealed the real villain, excessively low soil
pH – around pH 4.
It was a dark and stormy night
when another tale of woe was told. During
spring transition something was killing both ryegrass and awakening bermudagrass
on recently reconstructed tees. Fusarium
and Ascochyta fungi were plentiful and identified as the cause of death. But why would these rare diseases occur?
A routine soil analysis revealed that the soil pH was excessively low –
around pH 4.
In the three examples above excessively low soil pH
was the stress factor that predisposed the turfgrass to these infrequently
occurring, but devastating diseases. Only
temporary and expensive relief from these diseases can be attained with
fungicides. Fixing the underlying
problem requires liming to maintain proper soil pH. This is simple and inexpensive.
Low soil pH occurs naturally throughout the
Southeast. Many soils that have not
been cultivated have pH’s in the low 4’s.
The exceptions are some Coastal soils high in calcium carbonate from sea
shells or irrigated with high bicarbonate irrigation water.
Sediments dredged from ponds, ditches, and wetlands may have near neutral
pH (around 7) when removed, but low pH develops if the sediments remain aerated.
Oxidation of sulfur compounds in the sediments can result in soil pH in
the low 3’s. Over-application of
elemental sulfur will also have a similar affect on soil pH.
Although oxidation of reduced sulfur compounds can
result in precipitous and rapid declines in soil pH, declining pH in most soils
is typically a more gradual process. The
primary cause of declining soil pH is the application of ammonium containing or
forming fertilizers, such as ammonium sulfate, ammonium nitrate, ammonium
phosphate, and urea. When ammonium
is converted to nitrate by soil bacteria, acidity is released.
Nitrogen sources differ in the amount of acidity they generate.
Ammonium sulfate is two to three times more acid than the other commonly
used ammonium nitrogen sources. Calcium,
potassium, and sodium nitrate do not create any acidity, but actually increase
soil pH.
There are several factors that are detrimental to
turf growth at low soil pH. Toxic
levels of aluminum, hydrogen, and manganese may occur in the soil water.
Aluminum and hydrogen damage grass roots directly, whereas manganese is
toxic to the leaves and stem of the grass. At
low pH, phosphorus availability is reduced and calcium and magnesium levels are
typically inadequate. Deficiencies
of these nutrients may occur. Microbial
activity is also reduced at low soil pH resulting in thatch accumulation and
slowed release of nitrogen from organic sources.
Dependent on the soil type and grass species some or all of these factors
may decrease turf growth.
Grasses vary in their tolerance to acid soils.
Bermudagrass, centipedegrass, and seashore paspalum are moderate to
highly tolerant to acid soils. Tall
fescue, zoysiagrass, and bentgrass are intermediate in tolerance and ryegrass,
bluegrass, and St. Augustinegrass have low tolerance. Generally, soil pH should
be maintained between 5.5 and 6.5 with values below 5.8 appropriate only for the
more tolerant grasses. None of the
commonly grown turfgrasses are tolerant of the pH’s encountered in the above
examples, pH 4 or less.
Maintaining soil pH at favorable levels is a simple
but continual process. Soil sample
annually and apply lime based on the recommendation.
Dolomitic lime is generally preferred because it will provide sufficient
magnesium as well as calcium. Both
are usually low when soil pH is low. Calcitic
lime can be used when soil test shows magnesium to be adequate, but is not
necessary because detrimental effects due to extra magnesium are unlikely.
Dolomitic lime is the least expensive source of magnesium if soil pH
needs to be increased.
Surface-applied lime is slow to react and the affects
move downward into the soil profile slowly.
To hasten the reaction and movement of the lime, incorporation into the
soil by application after core aerification is recommended.
When devastatingly low pH’s are encountered incorporation is required
in order to increase the pH of a significant volume of the root zone.
The smaller the particle size of the lime the faster it will react. The
faster reaction rate of a ground limestone, rather than a pelletized lime, is
particularly important when very low pH’s are encountered.
So, be sure to look to potential underlying factors when unusual diseases become a problem. They are a clue that one or more basic factors (like low soil pH) may be the real culprit.
And after having
soil sampled and limed they all lived happily ever after…………...
First
printed in the Carolinas Green, September-October 1999