University of Vermont
Summer News Article
Department of Plant and Soil Science
LAWNS: AVOIDING DISEASE DISASTERS
Leonard Perry, Horticulture Professor
University of Vermont
If you have a home, chances are you have a lawn. The master
groundskeeper at Boston’s Fenway Park, David Mellor, provides some tips and
advice in his book The Lawn Bible on how to avoid disease disasters
in your lawn.
Almost all lawn diseases are caused by harmful fungal pathogen
organisms. These are present everywhere, along with beneficial
fungi. These good fungi help keep the bad ones in check, and help to
break down that layer of thatch in lawns. This, in turn, returns
nutrients back to the soil. Proper lawn maintenance helps to promote
these beneficial fungi. It also helps to prevent fungal disease by
keeping grass plants healthy, and conditions not good for fungal
growth. So proper lawn maintenance is the main key to avoiding lawn
diseases. Prevention of these is much easier than trying to cure a
Proper lawn care involves:
--Proper grass selection. Even before starting a lawn, or renovating
one, choose grass varieties appropriate for your area, light conditions, and
soil type. Otherwise they’ll be weak, and more susceptible to
--Proper mowing. This means not mowing grass too short (for most
grasses 3-inch high works well), as this will stress grass. Taller
grass plants are more tolerant to heat and drought stresses.
Similarly, to not stress grass plants, don’t mow more than one-third off at
a time, and make sure mowing blades are sharp.
--Proper watering. Water established lawns deeply and infrequently, if
needed at all. Water in the morning so grass can dry during the day
and not go into the night wet. Improve drainage with an aerator
(available from rental stores) if soils are compacted.
--Proper fertilizing. Only add what is needed, according to a soil
test. Too little, and plants will be stressed. Too much, and
plants are weakened either from excess thatch build up, too much top growth
for the root growth, and excess nutrients washed away, possibly into
--Proper chemical use, or none. Improper use of insecticides,
fungicides, and herbicides can weaken grass, and kill beneficial organisms
that help grasses to thrive and be strong. Use only what is needed,
according to label directions, and only if there aren’t other options.
In spite of your best attempts at good lawn care, problems may
develop. They may not be caused by a disease, but by drought,
pest-damage, improper chemical use, or other. So the first step is to
determine the cause of the lawn problem. Since this can be difficult,
with similar symptoms from several diseases, look closely at the grass
blades, patterns overall in the lawn, and note any recent changes in culture
If any doubt as to the cause, check with your state Extension Plant
Diagnostic Clinic which you can locate through the Northeast Plant
Diagnostic Network (www.npdn.org/nepdn/regional_contacts).
Also check lawn problems with local full-service garden stores (seasonal
stores and chain stores often are of little help), or a local professional
lawn care company (not ones that merely mow).
Here are ten main lawn diseases and cultural ways to help prevent or control
>Snow Mold. Patches of dead grass, either pink or gray, show up in
spring after snow melts. To avoid this disease, mow in fall until
grass stops growing, and avoid excess nitrogen in fall.
>Dollar Spot. Small dead spots a few inches across may appear in
spring and fall with warm days and cool nights. Don’t overwater if
this problem is present, give proper nitrogen fertility, and reduce thatch
>Stripe Smut. With this pathogen, leaves curl up and die, appearing
shredded at leaf tips. You’ll see it mainly in the cool of spring or
fall. Water and fertilize properly, and reduce excess thatch.
>Brown Patch. Circular patches of brown grass appear in summer
during hot, humid weather. Allow grass to dry between watering, and
water early in the day.
>Leaf Spot. With this disease, grass blades have elongated spots
with brown centers and purple edges. Improper watering and too low
mowing during cool, wet weather can lead to leaf spot.
>Powdery Mildew. A white powdery substance on leaves, resembling a
dusting with flour, indicates this disease which loves high humidity and
warmth during summer, particularly in shady areas. Many Kentucky
bluegrass varieties get this disease. If a shady area, improve light
conditions if possible, choose shade-tolerant varieties, and don’t
overfertilize or overwater.
>Rust. With rust, affected areas take on a rusty color; leaf blades start
with yellow spots that develop rusty lumps of spores. You’ll see it on
stressed grass, particularly Kentucky bluegrass or ryegrass during warm,
moist weather. A properly maintained lawn with proper or high mowing
and good fertility, is the way to avoid rust.
>Red Thread. Patches of lawn appear reddish, and a closer look shows
tangles of red fungal threads at the tips of grass blades. Cool, moist
weather favors this pathogen, particularly on fine fescue and perennial
ryegrass, or lawns that are poorly fertilized.
>Fusarium Blight. This disease starts as small light to reddish brown
circular spots which, as they enlarge, have characteristic live green grass
in their centers. Drought-stressed grass with shallow roots is
particularly prone to fusarium. Water properly, don’t overfertilize,
aerate if excessive thatch is present, mow at the proper height, and reseed
with resistant varieties.
>Pythium Blight. This also is known as “grease spot”, as small patches of
grass look slimy when wet. Dry, they’ll be light to reddish
brown. Use proper watering, particularly during warm, humid weather,
and correct a too high soil pH (too alkaline).
Keep in mind through the season that for a lawn disease you need the
pathogen (often present), a vulnerable grass plant (i.e. stressed or weak
from improper culture), and the right conditions for the fungus to
thrive. Unless you have all three, your lawn won’t get diseased.
You’ll find many more tips on growing and caring for lawns in David Mellor’s
Return to Perry's Perennial