Perennial ryegrass growing degree days (GDD)
Perennial ryegrass GDD’s will be tracked during the 2026 growing season with comparisons to the previous eight years. The accumulation of GDDs will begin after the snow has melted from the perennial ryegrass fields and continue through swathing. A base temperature of 32 °F will be used for perennial ryegrass (T-Base = 32 °F).
- Year to date GDD = 1,676 (Table 1)
- GDD last week June 12-28 = 218 or 31.1/day; long term average = 212 or 30.3/day
- GDD forecast for the next 10 days = 381 or 38.1/day
- Average GDD accumulation for first week of July = 265 or 37.9/day
- The 10-day forecast suggests above average temperatures for the first week of July. Projected GDD is 38.1/day compared to the long-term average of 32.9/day
| Year | 2026 | 2025 | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2026 vs. 2025 |
|---|---|---|---|---|---|---|---|---|---|---|
| March | 0 | 0 | 0 | 0 | 0 | 131 | 30 | 0 | 0 | 0 |
| April | 191 | 229 | 296 | 93 | 95 | 236 | 183 | 211 | 184 | -38 |
| May | 619 | 812 | 653 | 959 | 649 | 640 | 600 | 548 | 815 | -193 |
| June* | 866 | 932 | 859 | 1,064 | 959 | 1,007 | 995 | 919 | 1,007 | N/A |
| July | N/A | 1,040 | 1,119 | 985 | 1,104 | 1,174 | 1,179 | 1,067 | 1,100 | N/A |
| Total | N/A | 3,013 | 2,927 | 3,101 | 2,807 | 3,188 | 2,987 | 2,745 | 3,106 | N/A |
General crop condition
Last week the temperatures were close to the long term average, which is a welcome change from the hot temperatures in late May and early June. The moderation of daily temperatures in the last few weeks should be positive for ryegrass pollination and the beginning of seed filling stage. Spring seeded ryegrass is now heading and has been shedding pollen. Heavy pollen clouds have been observed in several perennial ryegrass fields. Perennial ryegrass sheds pollen in the mid-morning. As these pollen clouds drift across ryegrass fields, it looks like dust from gravel roads.
After pollination, the next critical stage in perennial ryegrass growth and development is the seed filling stage. As mentioned in previous newsletters, to maximize seed fill and test weight it’s important to protect the entire seedhead with a fungicide to control rust and other fungal diseases. As of last week, ryegrass rust was not observed at the U of MN Magnusson Research Farm in ryegrass areas that did NOT receive a fungicide treatment. Field scouting for rust will continue this week.
Crop management
Historically, in northern MN environmental conditions, crown rust has been observed after 1,500 and leaf and stem rust after 1,900 GDD. Rust spores travel on low level jet stream winds from southern states into the perennial ryegrass production areas of northern MN. Perennial ryegrass is heading, and many fields are in full head extension. To maximize perennial ryegrass seed yield, it is important to protect the entire ryegrass seedhead from diseases that can reduce the photosynthetic area of the seedhead. The following are strategies for rust control in perennial ryegrass post heading.
- Scout ryegrass fields for rust a couple times a week as in favorable environmental conditions rust can develop and increase rapidly and this disease can “explode” in a few days.
- If a fungicide has been applied with a previous trip across the field, apply a fungicide when the last fungicide is about to “run out.” The number of days for disease protection will depend upon the fungicide used and product rate.
- Fungicide applications when the ryegrass seedhead is fully extended will provide protection of the entire seedhead. A full rate of Priaxor or Quilt Excel at full head extension should provide protection for 21 to 28 days. As of June 28, the year-to-date accumulated GDD was 1,676. The new 10-day GDD forecast is for 38.1/day. If that forecast holds true by the end of the July 4 weekend, GDD accumulation will be 1,941 which will be in the beginning of the historical window for leaf and stem rust infection in perennial ryegrass seed production fields.
Pest management
Crown Rust and Leaf & Stem Rust
Crown and leaf and stem rust are two leaf diseases that can cause economic yield losses in perennial ryegrass seed production. Crown rust pustules are yellow to orange red in color while leaf and stem rust are red in color. In northern MN conditions we typically see Crown rust after 1,500 and leaf and stem rust after the accumulation of 1,900 GDD.
Crown rust can infect perennial ryegrass plants, photo 3. The alternate host of crown rust in buckthorn and local crown rust infections can occur from crown rust spores from buckthorn, photo 4. Buckthorn is a common shrub that can be found in many areas of Minnesota.
Leaf and stem rust is the most destructive fungal disease in perennial ryegrass seed production. Spores that attack the seed head will reduce seed weight as the rust will decrease the photosynthetic area of the plant, photo 1. If not treated with a fungicide, untreated leaf and stem rust can consume the entire plant, photo 2. Research conducted by U of MN scientist have documented up to 80% ryegrass seed yield loss from heavy leaf and stem rust infections.
Armyworm and Braconid Wasp
Last week a total of 118 armyworm moths were captured in six traps in perennial ryegrass seed fields. The average moth capture was 19.6/trap with an average of 2.8/night. The range of moth capture was 8 to 29/trap.
Braconid wasps are an example of a beneficial insect in perennial ryegrass fields. They are harmless to humans and act as natural pest control by laying their eggs inside or on harmful pests like armyworm larvae. A picture of cocoons on the stem of perennial ryegrass, see photo 5.
Next week’s newsletter will be released on July 7.
