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University
of Rhode Island GreenShare Factsheets
Turf
Diseases: Leaf Spots & Tip Blights,
Melting Out, Crown & Root Rots
Leaf spots on turfgrass
leaf blades begin as small red to purplish ovals that later
develop tan centers
of dead tissue with darker borders ("eye spots.") The fungi that
cause leaf spots directly penetrate leaf sheaths and blades at
random or enter via mowing wounds which commonly leads to a tip
blight. When turfgrass is succulent from recent nitrogen fertilization
and there is abundant moisture on the leaf blades, numerous leaf
spot infections per blade can occur. The leaf spots may then
coalesce and cause extensive blighting. The fungus may even invade
the crowns and roots, leaving the plants weakened and rotted.
This severe stage is called "melting-out." Large areas of dead
or badly weakened turfgrass may result.
Some fungal species are significant pathogens
only during prolonged wet weather. They invade leaf blades primarily
through mowing wounds causing mostly tip blights as opposed to
the aggressive leaf spot fungi that invade leaf blades randomly
and potentially lead to melting-out.
Leaf spot fungi may be active at a variety
of temperatures. All are associated with frequent moisture on
the leaf blades and high relative humidity.
• Spring: Leaf spots caused by the fungi Drechslera
poae and Drechslera siccans are almost always present
in lawns. Repeated infection and expansion of leaf spots to blights
may lead to "melting out," which is extremely common in Kentucky
bluegrass and perennial ryegrass in early summer. Extensively
infected plants may die, especially when stressed in hot summer
weather.
• Summer: Another species of fungus, Bipolaris
sorokiniana, attacks nearly all turfgrass species and is
most aggressive at high (over 85 degrees F) temperatures and
relative humidity. It may lead to crown and root rots.
• Fall: The same fungi that are active in spring
may cause leaf spots again in fall during rainy weather. Usually
disease is not as severe as in spring, but heavy spore production
in fall will increase disease potential for the following spring.
The fungi that cause leaf spots and melting-out
have simple life cycles. Spores exposed to moisture on leaf blades
for a few hours germinate and infect the turfgrass plants. The
fungus penetrates the leaf tissue, killing cells and absorbing
nutrients. Further spores are produced on diseased tissue only
a few days after the initial infection. Spore production is prolific
on mowing clippings.
In addition to the species listed above, several
other fungi from these two genera cause various leaf spots and
blights which occur on other common lawn turfgrass species. Their
life cycles are essentially identical to those already described.
Several fungi from the genera Ascochyta, Leptosphaerulina and Septoria cause
predominantly tip and leaf blights in prolonged wet weather.
They can usually be identified by the presence of small black
dots (fruiting bodies filled with spores) in dead leaf tissue.
They infect the leaf blades, giving the lawn a ragged appearance,
but are not known to invade the crowns and roots.
Leaf spot and tip blight fungi are present
in every established lawn. They only cause serious damage during
extreme weather conditions involving frequent wetness or when
improper lawn care practices aggravate the disease problem. The
actual identification of the fungus that is causing a leaf spot
is usually not necessary because the cultural and chemical controls
described below are effective against all species.
Keep in mind that mowing
increases infection by leaf spot fungi in the following ways:
a) wounds are made that allow easy penetration
by the fungi, especially when the leaf blades are shredded by
dull mower blades
b) new spores are produced in abundance on returned mowing clippings when leaf
spots are present
c) mowers and feet spread spores.
If only scattered leaf
spots are observed, no significant harm should come to the
lawn. Severe damage is
the result of repeated infection of stressed plants. These
practices should limit disease to an acceptable level:
1) Avoid excessive early spring nitrogen applications
with water soluble fertilizers. The resulting lush growth is
very susceptible to infection by leaf spot fungi.
2) Water infrequently and deeply. Keep leaf
blades dry as long as possible. Never water in the late afternoon
or early evening. Prune landscape plants to allow good air circulation
to promote drying of the turfgrass. Avoid frequent light sprinklings.
3) Mow high to avoid plant stress. Mow frequently
and only cut 1/3 of the blade at a time. Collect clippings when
numerous leaf spots are observed to reduce spore production.
Do not mow when grass is wet.
4) Avoid broadleaf, phenoxy herbicides and
growth regulators during leaf spot epidemics.
5) In damaged areas, reseed with cultivars
resistant to leaf spot and melting-out. Always plant a blend
of resistant cultivars or, preferably, a mixture of resistant
cultivars of different turfgrass species. The genetic variation
of such a mixture reduces the chance of major damage in the future.
Fungicides should not
be applied routinely for leaf spot fungi. If cultural management
has not prevented
severe infection or if prolonged wet weather occurs, several
contact fungicides are available. These fungicides are most effective
when they coat the entire surface of the leaf blades. Applications
must be repeated every 5-10 days depending on rainfall and mowing
frequency. In damaged areas, re-seeding with resistant cultivars
is a better long term solution to leaf spot problems. Turfgrass
suffering from severe "melting-out" and crown and root rot will
not be improved by fungicide applications.
Adapted from the UMass Extension,
2000
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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