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 on turfgrass blade
Leaf spot fungi may be active at a variety of temperatures. All
are associated with frequent moisture on the leaf blades and high
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.
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.
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.
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:
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.
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:
Avoid excessive early spring nitrogen applications with water soluble
fertilizers. The resulting lush growth is very susceptible to infection
by leaf spot fungi.
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.
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
Avoid broadleaf, phenoxy herbicides and growth regulators during
leaf spot epidemics.
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.
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.
from the UMass Extension, 2000