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Dr. Watkin's report also reveals that turfgrass species differ in their susceptibility to diseases influenced by nitrogen. One notable difference is brown patch injury on tall fescue compared to perennial ryegrass. Higher levels of nitrogen caused brown patch injury to be significantly worse on a turf-type tall fescue. However, the same higher nitrogen levels reduced the incidence of brown patch on two cultivars of perennial ryegrass tested.
The key to turf disease prevention, from the standpoint of fertility, is balance, explains Dr. Watkins. A ratio of N:P:K of 3-1-2 is a good rule of thumb during the growing season. Too much nitrogen promotes excessive, succulent growth. Plant cell walls become thinner and easier to penetrate by fungi. Nitrogen also increases a plant?s production of carbohydrates and glutamine, which favors germination of fungal spores.
For these reasons, turf advisors normally recommend keeping nitrogen levels balanced and just high enough to prevent chlorosis. This level varies according to the species and cultivar. A rate of 2 lbs./1,000 square feet exhibited good color and vigor without significant brown patch injury for the turf-type tall fescue.
Too little nitrogen can also impact turf disease incidence. Some nitrogen is needed to provide turfgrass plants with the vigor to resist or recover from disease injury. "Nutrient deficient turfs lack vigor and this allows the fungus to colonize new tissues," said Watkins. "The recuperative potential of nitrogen-starved turf is poor. Slow-growing, undernourished turf is prone to severe rust injury."
For example, bentgrass required moderately high levels of nitrogen to hold off dollar spot. Perennial ryegrass needed even more nitrogen to ward off crown rust during periods of high dew. Too much nitrogen induces brown patch in turf-type tall fescue.
The best defense against turfgrass diseases is a balanced fertility program with specified levels of nitrogen at certain times of the year.