Impacts of Sustained Use of Dairy Manure Slurry and Fertilizers on Populations of Pratylenchus penetrans under Tall Fescue.

Various manures and composts have been reported to reduce population densities of plant-parasitic nematodes. Dairy manure slurry is often used as a primary source of nitrogen for forage crops. This study was conducted to determine the effects of dairy manure on population densities of Pratylenchus penetrans parasitizing tall fescue. Beginning in 1994, dairy manure and inorganic fertilizer were applied after each harvest (2 to 4 times/year) at rates of 50 and 100 kg NH-N/ha; control plots were not treated. Nematode populations in soil and roots were determined at 19 sample dates during the fourth (1997), fifth (1998), and sixth (1999) years of manure and fertilizer applications. The sustained use of dairy manure and fertilizer increased population densities of P. penetrans. Our results contrast with many previous studies demonstrating that application of manures decreases population densities of plant-parasitic nematodes. Frequent applications of moderate amounts of manure to a perennial grass crop may have prevented the development of nematode-toxic levels of ammonia or other toxic substances such as nitrous acid or volatile fatty acids. Two years with no additional manure applications were required for P. penetrans population densities to return to levels similar to fertilized or untreated soil.