Disease prevention was probably about the last thing on anyone’s mind during the extended drought and hot temperatures experienced in the Southeast this summer. These conditions, however, are when one pathogen can gain an advantage – Chrysorhiza zeae. This pathogen is very different from your typical Rhizoctonia pathogen. It occurs under hot and dry conditions, especially when fertility is low. This is in contrast to Rhizoctonia solani, which infects warm-season grasses in the fall when temperatures are cool and moisture is high, causing large patch. Chrysorhiza zeae causes mini-ring, which is also known as leaf and sheath spot of bermudagrass. It also can cause disease on creeping bentgrass greens, but symptoms often occur in conjunction with brown patch.
Quite a few golf courses in the Southeast are seeing lots of miniature rings popping up with the onset of cooler weather. Although symptoms are appearing now, infection occurred weeks or months ago. Symptoms can lag infection, making curative disease control very difficult, if not impossible. The rings become apparent when bermudagrass growth slows because new growth no longer masks the underlying dead leaves. There have been some important findings recently around fertility that can help prevent this disease. Added attention on fungicide placement can help in disease control efforts.
- Nitrogen: Clemson and North Carolina State University research indicates that switching to a urea-based nitrogen fertility program helps prevent mini-ring. It has been observed at some courses that converting all nitrogen applications to 100% urea – with no ammonium and no nitrate – prevented outbreaks of mini-ring. To prevent mini-ring, this change in the fertility program has to be implemented well in advance of infection.
- Masking Symptoms: In areas of the Southeast where bermudagrass loses its color or goes dormant, use of colorants or pigments can help hide or disguise the ring symptoms.
- Fungicides: Certain fungicides with activity on Rhizoctonia solani, such as thiophanate-methyl, have no effect on Chrysorhiza zeae. DMI- or sterol-inhibitor fungicides, while effective on the pathogen, can also impart growth regulating effects on the bermudagrass, potentially reducing growth and recovery.
- Spraying Methodology: Chrysorhiza zeae resides under dead or dying leaf sheaths, making curative control of mini-ring very difficult. Applications of fungicides need to be targeted into the thatch at the soil surface, not the foliage. This may entail higher water volumes or light irrigation after application.
Mini-ring is not a devastating disease on putting greens like spring dead spot, Pythium blight and root rot, or Bipolaris leaf spot. However, when symptoms occur the visuals can be very alarming. Outbreaks year after year can also lead to turfgrass loss and poor spring transition. In areas like Florida, where bermudagrass does not go dormant and rounds are highest during the winter months, curative control and bermudagrass regrowth and recovery is essential.
There are lessons to be learned when this disease occurs that can help with future prevention. Contact the USGA Green Section for further details and help.
Southeast Region Agronomists:
Chris Hartwiger, director, USGA Course Consulting Service – chartwiger@usga.org
Steve Kammerer, Ph.D., regional director – skammerer@usga.org
Addison Barden, agronomist – abarden@usga.org