Dollar Spot and Gray Leaf Spot Severity as Influenced by Irrigation, Chlorothalonil, Paclobutrazol, and a Wetting Agent

Abstract

Creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw.] and perennial ryegrass (Lolium perenne L.) are commonly grown fairway grasses that are susceptible to dollar spot (Sclerotinia homoeocarpa F. T. Bennett; DS) and gray leaf spot [Pyricularia grisea (Cooke) Sacc; GLS.] diseases, respectively. This field study assessed the influence of two irrigation regimes (light and frequent nighttime versus deep and infrequent morning irrigation) and six chemical treatments including: chlorothalonil (tetrachloroisophthalonitrile), paclobutrazol [(2RS,3RS)‐1‐(4‐chlorophenyl)‐4,4‐dimethyl‐2‐(1H‐1,2,4‐triazol‐1‐yl)pentan‐3‐ol; PB], wetting agent (polymeric polyoxyalkylene 95% oxoalkonyl hydroxyl polyoxlalkane diyl 5%; WA), chlorothalonil + PB, chlorothalonil + WA and chlorothalonil + PB + WA on DS and GLS severity over a 3‐yr period. Dollar spot was more severe in late summer in creeping bentgrass that received deep and infrequent versus light and frequent irrigation in 2002 and 2004 and disease severity was negatively correlated with volumetric soil moisture (r = −0.77 to −0.96). Soil moisture levels above 0.25 cm3 cm−3 were associated with an improved ability of chlorothalonil, PB, and WA to suppress DS, but they had no effect on GLS. In 2002, GLS rapidly and severely damaged even fungicide‐treated plots, and there was no irrigation effect. In 2004, gray leaf spot was more severe in light and frequent nighttime irrigated blocks versus deep and infrequent morning irrigated blocks. Chlorothalonil (8.0 kg a.i. ha−1) provided effective GLS control when applied on an 8‐d interval in 2004 but only in infrequently irrigated blocks where disease pressure was less.