High Yield Grass Seed Production and Water Quality Protection Handbook

Disease Management

Automated weather stations such as the one
pictured here will ultimately enable growers
to integrate rust-limited cultural practices
with weather-based fungicide applications.

Although rust can occur on any of the
grass crop species grown in the Willamette
Valley, it is most severe on tall fescue
(pictured here) and perennial ryegrass.

Spring fungicide treatments for purple
eye spot in orchardgrass can help
prevent losses from this potentially
harmful leaf spot disease.

If leaves are wet while temperature is
above the minimum threshold for rust
infection (35 F) during the night and
early morning, infection is likely. The
warmer the temperature, the more
infection can occur

Once infection has occured, the rate of
rust development depends primarily on
temperature and doesn't require leaves
to be wet. On perennial ryegrass, the rust
fungus can complete a generation in 10
days when average temperatures are
60 F, but requires about 40 days at 40 F.

Grasses grown for seed are afflicted by a variety of disease agents, which can attack the roots, leaves, seed head and seed of the plant. Although the pathogens may attack specific crops and they each have unique peculiarities in their life cycles, there are basic management practices that apply to most diseases and provide general disease control.

In the establishment of a new field, sanitation is very important. The field should be free of disease-bearing residues or pathogen propagules. In a rotation schedule, it is best to avoid grasses, including cereal grains and corn, for at least two years.

Propagules such as blind seeds, ergot and seed galls will not survive longer than two seasons in the absence of a flowering host. The seed used to establish the field should be of high quality and free of pathogens or disease. In particular: tall fescue, perennial ryegrass and annual ryegrass should be free of blind seed; orchardgrass should be free of Anguina; Kentucky bluegrass should be free of ergot; and all grass seed should be free of pathogenic Fusarium species.

The recommended practices to maintain a healthy, vigorous stand apply to disease control since a healthy plant is less likely to succumb to disease. Following the recommended application rates for nitrogen and other nutrients is especially important for blind seed control. Practices that maintain appropriate soil pH, good soil drainage and avoidance of soil compaction are beneficial.

Good weed control can contribute to good disease control. Weeds such as Poa species (e.g. bluegrass) can support early build up of diseases such as ergot. Weeds of Agrostis species (e.g. bentgrass) which are allowed to flower and set seed can support increased seed gall. Weed grasses growing along field borders, roadways and ditches can support diseases such as blind seed, choke, ergot and seed gall.

Spray or cut bordering grasses to prevent seed heads from developing and maturing. At harvest, it is important to remove as much seed from the field as possible.

Avoid late harvest which could result in excessive seed shatter and return of seed to the soil.

Combines should be adjusted to remove lightweight seed. In-field pre-cleaning of seed should be avoided. These practices are especially important for seed diseases such as blind seed, where the fungus over-winters in the infected seed. The more infected seeds that are returned to the field, the greater the potential for disease the following year.

Field burning has a long history of success for disease control. In the absence of open burning, propane burning may reduce some diseases such as choke, but is less effective as a general disease control tool compared to open field burning. Baling residues to remove the straw is considered beneficial in that infested residues are removed. However, it is not yet established if leaving finely chopped residues in the field to decompose during the winter will increase disease. We believe there may be some potential for increase of some fungal pathogens such as Fusarium, although to date we have not yet documented any increase in disease where residues were left in the field.

Leaf spot diseases

In the spring, grasses such as orchardgrass are susceptible to leaf spot diseases such as purple eye spot. Fungicides are available for control and recommended rates are detailed in the PNW Plant Disease Management Handbook. The diseases are most active during cool, wet conditions and are less troublesome later in the season when warmer and dryer conditions prevail. In some years, environmental conditions will be more favorable for disease development than in others. It is not fully established, however, if these diseases result in a significant reduction in seed yield.

Disease caused by rust fungi can cause serious damage to a grass seed crop if not controlled. Fungicides are available for control but timing of applications is important for optimum control (see section on rust control).

Rust control

The most serious disease threat to grass seed production in the Willamette Valley is stem rust. It can occur on any grass crop species, but is most severe on perennial ryegrass and tall fescue. First-year stands generally are more heavily rusted than established stands.

It is not necessary to completely eliminate stem rust in order to avoid disease losses. Research in progress suggests that yield reductions do not occur unless rust levels exceed approximately 2 percent by the last week before harvest. It takes careful management, however, to keep rust from increasing to damaging levels once it becomes established in the field. The primary management practice for stem rust is the use of triazole and strobilurin fungicides. To help prevent the pathogen from developing resistance to these materials, use of the different fungicide types should be rotated during the season.

Date of epidemic onset

The date of rust epidemic onset, and its ultimate severity, varies substantially from one year to another due to the strong influence of weather conditions on the disease. Warm, dewy nights and mornings present the greatest hazard of infection. An active research program is under way at the USDA-ARS National Forage Seed Production Research Center to implement a weather-based disease advisory system that will help crop managers know when a spray is necessary and when it is not. Most effective and efficient management will be achieved through a combination of field scouting for early rust appearance, consultation of a weather-based advisory for best spray timing, and application methods that provide thorough coverage.

Although no cultivar is resistant to stem rust, a cultivar of lower susceptibility may show a later onset of the rust epidemic and therefore need fewer fungicide applications in the season. For first-year stands, late fall planting dates appear to lead to less rust in the spring than do early fall planting dates. Any factors that can reduce exposure of the crop to infection in the fall will likely result in less overwinter survival of the pathogen, and therefore less intensive rust in the spring. At present, however, fall application of fungicide is not recommended.

Ultimately, it will be possible to integrate rust-limiting cultural practices with weather-based fungicide application to avoid yield loss while reducing fungicide use to the minimum necessary. More detailed information is available at http://nfsprc.usda-ars.orst.edu.

Stephen Alderman and William Pfender; USDA, Agricultural Research Service Cynthia Ocamb; Oregon State University Extension Service