Gray leaf spot caused by Pyricularia grisea has become an important disease of perennial and annual ryegrasses in the Northeast. This disease is most often seen in late summer and early fall in this area on higher cut ryegrass in roughs, athletic fields, and lawns. The pathogen also attacks many genera of warm season turfgrass, as well as Festuca species. Symptoms first appear as small, water-soaked lesions which become necrotic. The leaf spots can vary in color, size, and shape but are most often gray to light brown with a dark border. Older lesions are sometimes surrounded by a yellow halo. Lesions may coalesce into irregular shapes and cause complete blighting of individual blades. Blighted leaves may be twisted into a characteristic "fish-hook" shape and covered with grayish conidia and conidiophores. Infected patches may remain small or if the environmental conditions are favorable, expand into large, irregularly shaped necrotic areas. Symptoms may resemble drought or heat stress.
P. grisea survives unfavorable periods as dormant mycelium in infested leaves and plant debris. Conidia are produced prolifically under warm, humid conditions and spread by wind, water, and equipment to start new infections. The optimal temperature for disease development is 82-90 °F and epidemics may continue into October, subsiding after a heavy frost. Leaf wetness is also a critical factor in epidemic development. Gray leaf spot is more serious in newly established plantings, in the shade, and on higher cut turf. Disease severity increases with nitrogen levels;water-soluble sources of nitrogen promote both rapid growth and disease development. Other stress factors such as herbicides, plant growth regulators, soil compaction, and drought may predispose the turf to gray leaf spot.
- Minimize periods of leaf wetness by watering in mid-day. Water thoroughly and as infrequently as possible, avoiding moisture stress.
- Prune trees and shrubs to reduce shade and improve light penetration and air circulation
- Avoid excessive nitrogen fertility. Keeping nitrogen fertility low (0.5 lb N/1000 sq ft) during the summer months reduces turgrass susceptibility. Slow release forms of nitrogen do not stimulate disease development.
- Reduce mowing heights.
- On golf courses, monitor roughs where disease is most likely to start.
- Where gray leaf spot has been previously diagnosed, begin applications of preventive fungicides in mid-July.
- Replace ryegrass species with less susceptible species (bentgrass and bluegrasses).
Management with Fungicides
During periods of high disease pressure, only fungicides with high efficacy are recommended. Excessive applications of strobilurin (Group 11) and benzimidiazole (Group 1) fungicides risk the development of fungicide resistance development in the pathogen population. Tank mixes are highly recommended for this disease. Follow accepted fungicide resistance management practices when treating P. grisea.
For a listing of fungicides currently labeled to manage this disease, refer to the Disease Management chapter of UMass Extension's Professional Guide for IPM in Turf for Massachusetts.
Written by: M. Bess Dicklow