Lawn rust hits weak grass but is treatable

By Bev Johnson | The Daily Journal

Published Saturday, August 11, 2007

Petunia is having a hissy fit. Bunkey's nice new white Keds are now orange. It only took a trip outside to discover the culprit. A walk through the Kentucky blue grass lawn raised a cloud of orange dust. The lawn had been infected with rust.

Turf grass rust comes in several forms. Perennial ryegrass gets crown rust. Our blue grass lawns get stem, stripe and leaf rusts. Even at that, many of these rusts will have many races that are specific to certain cultivars of grass. If you look closely at infected grass, the leaves will have a yellow or red cast.

The individual blades will appear chlorotic, or light green to yellow, with yellow-orange or black spots. Under magnification, orange spores will be spotted on the infected grass blades.

So, what happened to Bunkey's lawn to cause it to turn against him? Weak grass. It can be caused by mowing too low.

In the heat of the summer, three inches is not too high. The taller grass shades the roots protecting them from the heat. Slow growing grass due to poor nitrogen fertility, shading or not enough water make grass more susceptible to rust.

Now, to correct his problem, Bunkey will have to fertilize his grass with a high nitrogen fertilizer, then water the snot out of it. The goal is to have the grass grow well enough to need weekly mowing. Fertilization can't be done too late in the season or the lawn will be too succulent and predispose it to snow mold and winter injury.

If he had used more common blue grass seed, which is more genetically diverse than the newer "improved" cultivars, Bunkey may not have had the problem. Using seed of just one improved cultivar means that if one blade gets rust, they all will.

With the more common seed, there are many different cultivars so only the most susceptible will get rusty.

There are chemicals that will kill the rust. Banner, Rubigan, Boyleton and Eagle are sterol inhibitors. Three others are not recommended because they are used at very low rates and extended application intervals. Heritage, Compass and insignia are more apt to make the rust resistant rather than kill it off. If you do have rust, cut the grass tall, fertilize early with a high nitrogen fertilizer and don't be stingy with the water. Most plants, grass, trees and ornamentals need at least an inch of water a week.

If you need to prioritize, water any trees planted in the last five years and mulch around them to keep the soil moist. Then water the mature trees, vegetable garden, flowers and grass last. Grass can survive with the least amount of water. Not watering it at all may lead to the death of your lawn.

Bev Johnson is a master gardener for West Otter Tail County.