Current weather conditions in addition to conditions we experienced last fall and winter indicate heavier rust pressure this season. The spring epidemic of rust develops from infections that have survived the winter. Also, rust populations are greatly influenced on fall planting dates. The earlier the plantings, especially spring plantings, carry more rust through the winter than the later planted stands. Warmer winters which we have experienced this year favor an earlier epidemic.
Bill Pfender of OSU has worked on developing and validating the Grass Stem Rust Estimator seen below. USDA determines on March 15, the amount of rust present in stands that were planted in Mid September at the Hyslop Research Farm in Corvallis. On March 15, active rust pustules per foot of row sampled equaled 1. Last year at the same date, the number was .05 pustules per foot of row. The previous high rust epidemic was 2005 and the count on March 15 was 22.4 pustules per foot of row. 2002 was 12.0, and 2000 was 4.0.
All the scouting work we do is important because individual fields and individual varieties may have higher levels of rust than observed in the research plots, but the plots do give a good early indication of the severity of the rust epidemics that we can expect. We are already finding some rust in "ideal" situations.
As a preventative treatment, it is a good practice to include a fungicide in your programs when making a growth regulator application on ryegrass and fescue fields.
USDA Rust Model April 28, 2010
(Click on the graph above to enlarge)
The lower graph shows the model's calculation for plant growth, which is very important in rust development. The graph shows the lengths, in inches, of: (red line) sheath number 2, (green line) flag sheath, and (blue line) flower head plus stem. The modeled timing of emergence for these plant parts, which should match approximately your field observations, can be adjusted by altering the date in the Date of flag sheath emergence box near the top of the web page. Dates refer to the time when approximately 25% of tillers show the indicated development. The graph also shows the number of possible generations of rust development (purple line, right-hand y axis), which gives an indication of the reproductive potential of rust disease under this year's weather conditions.
The model makes two estimates of active rust levels (left y axis): those that are visible (red line), and a total (green line) which includes visible rust as well as infections that will become visible only after the latent period elapses. Dead, non-sporulating pustules are not included in the rust estimates. Visible disease levels above about 2000 cause economic loss to the seed crop. To avoid reaching these levels, we estimate that fungicides should first be applied when the green (total infections) line first crosses the purple (action threshold) line. Additional sprays should be applied at intervals of 14-17 days or more, when the green line again reaches the action threshold. Please note that this action level is provisional, and we have not yet verified it across a range of circumstances. The daily infection factor (blue bars, with scale on the right y axis) is a measure of the favorability of daily weather for rust infection. Maximum fungicide action will occur when fungicide is applied within a few days of high-infection weather.
Talk with your CPS fieldman about fungicide timings and overall rust control is your grass seed crops this year. With the unexpected early stripe rust pressure in wheat, we may be in for high disease year.