By Dr. Thomas T. Yamashita
Every summer, managers of golf courses, parks and school yards, as well as homeowners, struggle to keep their lawns green and healthy. This is especially the case in years with extended exposure to temperatures above 95 degrees Fahrenheit.
Except for grasses such as Bermuda, barnyardgrass and wild rye, typical lawn grasses will literally shut down their food making machinery (photosynthesis) when temperatures climb above 95 degrees. While photosynthesis slows down, the energy and food using reactions (respiration) accelerate. Thus, as a heat wave continues for an extended period, lawns are affected at the physiological level.
The resulting growth from this type of exposure is oftentimes weak and of low density and integrity. The take home message is that these resultant, low integrity, pre-weakened tissues can be easily colonized by a long list of soil-borne and foliar pathogens.
Overwatering during heatwaves often creates conditions perfect for Pythium blight and other diseases caused by water molds.
In the mid-90s, coming off a 7-year drought, we experienced a sudden burst of heavy rains in 1995, and again during the winter and spring of 1996. This extended period of wetness allowed many disease-causing organisms to proliferate. One group which thrives under wet conditions are the water molds, such as Pythium and Phytophthora. Another soil-borne fungus which can grow under moist soil conditions is a member of the mushroom family, Rhizoctonia.
The populations of these pathogens had built up considerably in 1995 and 1996. With the hot summer weather, many superintendents and homeowners stepped up their irrigations. When a browning of the lawn was observed, it was typically interpreted as being due to a lack of water, and they responded with more irrigation. Many of these light brown patches, however, were the initial symptoms of infections with Pythium, Rhizoctonia, or other moisture-loving pathogens.
Thus, the additional irrigations merely created a better environment for enhanced infection and spreading. Whenever divergent groups of pathogens are causing problems (such as the water molds of the class Phycomycetes and Rhizoctonia of the class Basidiomycetes), it is best to use a broad-spectrum fungicide or a combination of more targeted materials.
One of the most accessible fungicides sold in nurseries for the home gardener is a broad-spectrum fungicide called Daconil.
Daconil controls both Pythium and Rhizoctonia, and several other water molds. When combatting Pythium blight, it is important to use it as both a foliar spray and a light soil drench. Most turf soils we have examined are also low in potash and calcium. It is important to supply these elements annually (e.g. 20 lbs per 1000 sq. ft. of potassium sulfate, and the same for calcium sulfate). Spread the potash and calcium evenly over the lawn and immediately incorporate with sprinkle irrigation.
Ahead of an anticipated heat wave, it is important to apply a foliar spray of calcium. For Central Californians, unless you live on the west side of the San Joaquin Valley, you’ll probably have to add a very small quantity of boron to the spray as well (e.g. about 2 oz. of a 6% to 8% calcium liquid, with 1 tablespoon of a 10% boron liquid and 2 tablespoons of a spreader, all mixed into a 2.5-gallon sprayer). If the heat wave continues, spray every 4 to 5 days until the hot weather subsides. Be sure to spray the Daconil fungicide separately.
The time of day matters as well. Avoid irrigations after 7 pm in the summer. Try to do most of your sprinkling in the morning. If you are in the throes of a heat wave, you may have to lightly sprinkle around 10 am and again at 3 pm, a process referred to as ‘syringing.’ Syringing helps cool off overheated lawns.
You may be interested to know that the Daconil fungicide used to control Pythium blight and Rhizoctonia brown patch will also control rust, that orange-colored disease symptom on blades of grass you frequently see around this time of year.