Reports on Plant Diseases |
RPD No. 405 - "Helminthosporium" Leaf,
Crown, and Root Diseases of Turfgrasses
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May 1999
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[ Symptoms ] [ Disease Cycle
] [ Control ]
| Helminthosporium leaf, crown, and root
rots are a common and serious group of diseases attacking turfgrasses
in the United States, including Illinois. The causal fungi attack
essentially all turfgrasses as well as numerous pasture, wild
and weed grasses, and small grains.
These diseases reduce vigor and can be very destructive during
wet, humid weather, where the turf is sprinkled frequently,
especially in late afternoon and early evening, in poorly
drained areas, and where it is shady. The more often and longer
grass remains wet, the greater the chance of disease.
Dry periods alternating with prolonged cloudy, moist weather
and moderate temperatures, in addition to close mowing, slow
growth of grass, low fertility, or excessive shade and nitrogen
fertilization, all favor Helminthosporium diseases.
Other factors that may add to disease severity include thatch
buildup, frequent light sprinklings, compaction from excessive
traffic, nematode damage, the use of susceptible grass cultivars,
allowing piles of clippings to remain in place, and applications
of certain postemergence herbicides such as 2,4-D, mecoprop
(MCPP), and dicamba.
Helminthosporium fungi (now generally referred to
as species within the genera Bipolaris, Drechslera
and Exoserohilum) may be responsible for the gradual
browning and thinning (melting-out) of susceptible Kentucky
bluegrass cultivars and other susceptible grasses. As the
disease progresses, large, irregular turf areas may turn yellow,
then brown to straw colored, and subsequently die. In some
instances, the entire turf is lost. Bermudagrass may be severely
attacked by a number of these fungi, causing leaf, crown and
root rots.
The turf may be thinned, or diseased plants may appear in
irregular patches. Fescues, ryegrasses, and bentgrasses may
be similarly affected, especially when maintained under high
levels of nitrogen. Once one or more of these fungi become
established in a turfgrass stand, they remain an ever-present
problem. It is not unusual to find two or more species of
these fungi infecting a single plant or even the same leaf.
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Figure
1.
Melting out in a home lawn.
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Figure
2.
Helminthosporium leaf spot on Kentucky bluegrass leaves.
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Symptoms
This group of diseases produces a variety of symptoms, depending
on the cultivar and species of grass, cultural management
program, season (primarily day length and temperature), weather
conditions, and the fungus or fungi that are present.
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LEAF SPOTS OR BLIGHTS
Small spots or lesions, which vary in color from reddish brown
to purplish black, appear on the leaves from early spring to late
fall. These lesions may increase rapidly in size and become round
to oval, oblong, elongate, or irregular. Their centers often fade
from an ash-white to pale tan or straw color. The spots are commonly
surrounded by a narrow, dark reddish brown to purplish black border
(Figure 2), often described as "eye-spots." Under moist
conditions, one or more lesions may merge and girdle a leaf blade,
causing it to turn yellow to straw-colored, tan, or reddish brown
and die back from the tip. When leaf spots are numerous, leaves
may be completely blighted, wither, and die. Older leaves are much
more susceptible to attack than younger leaves. Depending upon the
species of fungus,.lesions may or may not occur on the leaf sheaths
and stems. However, diseased leaf sheaths will turn from reddish
to purple or brown. Leaf sheath infection is often so severe that
the entire leaf or tiller is girdled and drops prematurely. If the
moisture continues, the disease often progresses inward from leaf
sheath to leaf sheath on a single plant until the plant is killed
above ground and may eventually involve the crown, rhizomes, stolons,
and roots, killing the entire plant. In severe cases nearly all
the leaves and tillers die, resulting in a severe thinning of turf;
hence the name melting-out.
In hot, humid weather, closely clipped bentgrass leaves easily
are girdled, turn reddish brown or dark gray, giving a smoky-blue
cast to infected, irregularly shaped areas in golf or bowling greens.
Such turf appears drought-stressed.
Figure
3.
Helminthosporium crown and root rot.
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CROWN AND ROOT ROTS (MELTING-OUT)
This phase of the disease usually appears first in warm to
hot, dry weather as a reddish brown or purplish black decay
of the stem, crown, rhizome, stolon, and root tissues (Figure
3), sometimes turning reddish brown to chocolate brown or
black when invaded by secondary bacteria and fungi. The feeding
roots on diseased plants are shallow, few in number, or even
absent. Such plants lack vigor and often wilt during midday,
even when soil moisture is abundant. Diseased turf may have
a yellow then brown or drought-injured appearance (Figure
1). The damaged areas may be small and circular to large and
irregular with entire stands of bluegrasses, fescues, ryegrasses,
bermudagrasses, or bentgrasses thinned out or completely destroyed
by severe crown and root rot. This phase occurs most readily
when plant vigor is suppressed by one cause or another, particularly
during hot weather.
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Disease Cycle
The disease cycle for all species of fungi is essentially the same.
The fungi survive from year to year (in periods of very hot or cold
weather) as spores called "conidia" (Figure 4 ) and as
dormant mycelium in dead grass tissues or in infected leaves, crowns,
roots, and rhizomes or solons. During periods of cool, moist weather,
tremendous numbers of conidia are produced on debris, mostly at
temperatures ranging from 38 to 82 F or 3 to 27 C (optimum range
is 60 to 66 or 15 to 18 C) and are carried to healthy leaves and
leaf sheaths by mowers and other turf equipment, air currents, flowing
or splashing water, foot traffic, dragging hoses, animals, and infected
grass clippings.
The conidia geminate in a film of moisture and infect the leaves
either directly or through stomates. Spore germination and infection
of leaves can take place within two hours if the weather is favorable.
These fungi are also capable of saprophytically colonizing plant
debris at or above the soil surface. The fungi produce fresh "crops"
of spores in profusion when dry grass debris is repeatedly rewetted.
The conidia, along with mycelial fragments, spread to new leaf parts
and neighboring plants; thus, the cycle is repeated. New leaf and
leaf sheath infections can occur as long as the weather remains
moist and temperature are favorable. Peak disease development varies
from early- or mid-spring to late September or October, depending
upon the areas and the species of fungus. Some species (for example,
Drechslera poae, D. dictyoides, D. siccans,
and Bipolaris cynodontis), are favored by the cool temperatures
of early spring and fall, while other species such as Bipolaris
sorokiniana, D. erythrospila, and D. gigantea are destructive
primarily only in warm and wet overcast weather in summer. B. sorokiniana
is also very active on old leaves during cool, wet weather in the
fall and early spring.
During warm, dry weather, the leaf lesion phase decreases (except
for B. sorokiniana, D. erythrospila and D. gigantea)
and fungal activity may be restricted to crown, roots, rhizomes
and stolons of diseased plants, killing round to irregular turf
areas (melting-out or crown and root rot). With the return of cooler,
moist weather, leaf infection typically becomes a problem as the
fungi can be seedborne causing seedling blights on new turfgrass
areas.
Disease susceptibility is increased when light intensity is decreased
by shading of trees, shrubs, or buildings; when the mowing height
is lower than that recommended for the grass or grasses being grown;
when turf is fertilized excessively with nitrogen; and when hormonal-type
fungicides or herbicides have been applied to control other diseases
and broadleaf weeds.
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Figure
4.
Disease cycle of Helminthosporium fungi that cause leaf,
crown, and root diseases of turfgrasses (Drawing, Lenore Gray).
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Control
- Mow all turfgrasses at the recommended maximum height for satisfactory
turf use. Avoid close clipping (scalping) at all times. Mow the grass
frequently so that no more than one-fourth to one-third of the leaf
surface is removed at one time. Mow throughout the fall until the grass
stops growing.
- If a dense mat or thatch has formed (more than 1/4 to 1/2 inch thick),
reduce or remove this accumulation by using a vertical mower, power
rake, core aerifier, or a similar machine designed for this purpose.
This operation should be confined to periods of cool weather in spring
and early fall so that turf can recover rapidly. These machines may
be rented at many garden supply centers and tool rental stores.
- Severely diseased turf will recover more rapidly if the soil has
an adequate supply of nitrogen. However over fertilizing may accentuate
disease development. Fertilize on a regular basis to maintain vigor
and steady growth based on a soil test and local recommendations. Avoid
heavy applications of soluble nitrogen fertilizers in hot weather. Follow
a recommended fertilizer program for the grass or grasses being grown
and for their use. Fertilize to maintain as uniform a level of soil
nutrients as possible in the root zone. The three major nutrients–nitrogen
(N), phosphorus (P), and potassium (K)–should be present in sufficient
but not excessive amounts.
- In dry weather, before the turf area is under stress, apply enough
water each time to soak the soil to the depth of the root system. Repeat
as needed, usually every 7 to 10 days, if the weather remains dry. Where
possible, water in the morning so leaf surfaces will dry before dusk.
Apply supplemental water immediately after a light shower during a dry
period. Avoid frequent light sprinkling or water-logging of the soil.
- Sow only high-quality, disease-free seed, sod, sprigs, or plugs from
a reputable dealer. Grow locally adapted, leaf spot-resistant grasses
or combinations (blends and mixtures) whenever possible. Kentucky bluegrass
cultivars that are resistant to leaf spot include: Admiral, America,
Aspen, Banff, Bono, Bristol, Challenger, Columbia, Eclipse, Midnight,
Mona, Somerset, and Trenton.
Perennial ryegrasses rated as resistant to leaf spot or brown blight
include: Birdie II, Citation II, Delray, Diplomat, Manhattan II, Omega
II, Palmer, Prelude, and Repell. Tall fescues with some leaf spot
resistance include Adventure, Apache, Falcon, Jaguar, Mustang, and
Olympic. The resistance to disease breaks down somewhat when turfgrass
is subject to stress.
Resistant turfgrass cultivars listed above should not be considered
all inclusive–many new and valuable cultivars are introduced
each year. When deciding which grasses to buy, check with either your
nearest Extension office or an Extension turfgrass specialist at the
University of Illinois (Urbana) for suggested grass species and cultivars.
- Where shade is dense and air movement is restricted, prune or remove
dense trees and shrubs that border the turfgrass areas. Doing so will
help the grass dry sooner and reduce disease potential. Space landscape
plants properly for allowing adequate air movement and avoiding excessive
shade.
- When establishing a new turfgrass area, provide for both adequate
surface and subsurface drainage. Grade for a slope of 2 to 4 percent
and fill in depressions to achieve a smooth, uniform surface.
- Aerify compacted areas with a power machine. Aerification is a form
of cultivation involving the use of hollow tines, or spoons, to remove
soil cores that leave holes of 1/2 to 3/4 of an inch in the soil to
a depth of 2 ½ to 3 inches. These machines may be rented at many
garden supply and tool rental stores. Aerification should be done in
the spring or fall.
- Follow suggested insect and weed control programs both for your area
and the grasses grown there.
- If the various Helminthosporium diseases cannot be controlled
adequately by cultural practices, fungicide sprays may be needed on
a preventative schedule. See the current Illinois Commercial Landscape
and Turfgrass Pest Management Handbook or Illinois Homeowner's
Guide to Pest Management for the latest fungicide recommendations.
Carefully follow the manufacturer's directions regarding dosage, timing
of application, safety, and other factors. None of the recommended fungicides
will completely control these diseases. Sprays applied at weekly intervals
give better control than those sprayed at two- or three-week intervals.
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For further information concerning diseases of crucifers
and other vegetables, contact Mohammad Babadoost, Extension Specialist in Fruit
and Vegetable Diseases, Department of Crop Sciences, University of Illinois at
Urbana-Champaign. University of Illinois
Extension provides equal opportunities in programs and employment.
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