Q: If rust is observed on zoysiagrass in fall is it worthwhile to treat with a fungicide or is it better to wait until spring and then apply the fungicide? Does rust have activity during winter cold season? What fungicide would you recommend?
A: Read this from Texas A&M:
In the fall, applications of Banner, Daconil or Bayleton are required to prevent rust on zoysia lawns.
And this from Purdue University:
The powder is actually millions of microscopic spores produced by a fungus called rust. There are several different rust fungi that cause rust. The most common one on Kentucky bluegrass, annual bluegrass, fescues and ryegrasses is Puccinia graminis. A separate species, Puccinia zoysia can infect zoysia grass.
Rust becomes a problem when grass plants are growing slowly. When grass plants are growing fairly rapidly, leaf tissues are removed by mowing at relatively frequent intervals, and the disease does not become apparent. With grass plants that are growing slowly, the fungus has sufficient time (7-14 days) to produce the microscopic spores in infected leaf tissue. These spores are then wind-blown or splashed by rain or irrigation to other leaves, where new infections can occur. Consequently, the disease can become very severe when certain weather conditions occur when the grass is growing slowly.
Leaf infections occur most frequently when days are dry and windy followed by heavy dew formation at night. The dry, powdery spores are easily disseminated by wind currents.
Rust, by itself, rarely kills a grass plant, unless other stress factors are involved. Rust infected plants are weakened. When the disease continues into late fall, infected plants may become more susceptible to winter injury. Young seedlings are highly susceptible, and proper water and fertility management may be required for early fall seedings.
The rust fungi rarely survive the winter in Indiana. The disease organisms survive winters in infected tissues in the southern and southwestern states. Spores of the fungi are wind-borne in spring and summer from those areas and the disease moves northward into Indiana and surrounding states, usually in July and August.
Control of rust in the home lawn is best accomplished by fertilizing and irrigating, as needed, to promote grass growth. Do not promote excessive growth. Water infrequently, but deeply. Irrigate during the early part of the day. Irrigate at a time that will permit complete leaf dryness before dew formation. Watering in the evening will increase the length of time that free moisture is on the leaves and will increase the chances of infection. Mow frequently and collect clippings when possible. Several fungicides will aid in the control of rust, but multiple applications are generally required, and you might choose to achieve control via management practices first. You may find products containing chlorothalonil or mancozeb being sold under various trade names at garden supply stores or nurseries.