Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant and Environmental Science

Committee Chair/Advisor

Dr. Lambert B. McCarty

Committee Member

Dr. Steven N. Jeffers

Committee Member

Dr. James P. Kerns

Committee Member

Dr. Julia L. Kerrigan

Committee Member

Dr. Joseph A. Roberts


Mini-ring is a disease in ultradwarf bermudagrass (UDBG) [Cynodon dactylon (L.) Pers. × C. transvaalensis (Burtt-Davy)] putting greens caused by Waitea zeae (Voorhees) J.A. Crouch & Cubeta, (formerly Rhizoctonia zeae). Symptoms typically resemble frog-eye patches that are 10 to 40 cm in diameter with a bronze to orange outer ring and green center. In the southeastern United States, mini-ring symptoms appear in late-summer and generally persist until UDBG dormancy in late-fall. Mini-ring is often problematic in UDBG when nitrogen (N) fertility is reduced to manage organic matter production and improve putting green performance and perceived green speed. While W. zeae is most frequently reported as the causal agent, other species of Waitea have been isolated from UDBG exhibiting mini-ring symptoms. Waitea zeae causes visible leaf lesions and basal sheath rot in other turfgrasses; however, in UDBG, dieback of leaf tissue occurs in the absence of leaf lesions and sheath rot. Although W. zeae has been isolated from UDBG leaf tissue throughout the growing season, it is unclear if other plant tissues—e.g., root, rhizomes, and stolons—may be possible infection courts and when W. zeae infection most likely occurs. The objectives of these studies were to: I) investigate the impact of N source and N rate on mini-ring disease development and severity in UDBG; II) determine what plant tissues W. zeae can infect and when infection is most likely to occur; and III) collect and characterize isolates of Waitea spp. recovered from symptomatic UDBG putting greens.

To study the impact of N on mini-ring disease severity, ammonium sulfate (AMS) [(NH4)2SO4] and urea (CH4N2O) were applied weekly to ‘P18’ (MiniVerde) and ‘TifEagle’ UDBG at rates of 4.9, 9.8, and 14.7 kg N ha-1. Mini-ring severity increased with increasing rates of AMS whereas disease symptoms in plots treated with urea remained relatively low.

Cores from a UDBG putting green located in Florence, SC were collected monthly from June to October in 2016 and 2017. Isolation of W. zeae occurred in all months, isolation frequencies were greatest in August and September.

In a growth chamber bioassay, UDBG roots, stolons, and leaves were infected by W. oryzae or W. zeae when inoculum when growing medium was infested with representative isolates.

Nineteen Waitea species isolates were recovered from UDBG putting greens expressing mini-ring symptoms in North Carolina and South Carolina. Isolates of W. prodiga, W. oryzae, and W. zeae represented 5, 16, 79% of isolates collected. Isolates were characterized by sequencing the rDNA-internal transcribed spacer region, and these sequences clustered with Waitea species isolate sequences deposited in GenBank and previously described. These studies demonstrate the primary causal agent of mini-ring in UDBG is W. zeae, while other species of Waitea are likely involved to a lesser extent.

An effective mini-ring management strategy should include regular N applications using N sources other than AMS, such as urea, to promote UDBG growth and recuperative potential and applications of fungicides mid-summer before symptom development. Fungicides should be applied in a manner that encourages movement of active ingredients into the putting green rootzone to reduce W. zeae infection of UDBG roots and stolons.

Author ORCID Identifier




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