Influence of papermill processing residuals on saprophytic growth and disease caused by Rhizoctonia solani

Abstract

A greenhouse experiment was conducted to determine the effect of the decomposition of papermill processing residuals (PPRs) on the saprophytic growth of Rhizoctonia solani and pathogenesis on snap beans ( Phaseolus vulgaris) planted at 0, 14, 28, 42 or 56 days after addition of PPR at 25 g kg −1 soil. Soils were incubated for 236 days to determine the long term effects of PPR on R. solani. A reduction in mean hypocotyl lesion area was observed for beans plants planted 0, 14 and 56 days after residuals addition. Disease in the amended treatment never exceeded that of a nonamended control. Rhizoctonia populations increased from 4.0 to 9.1 propagules g −1 amended soil through the first 14 days. After 149 and 236 days, Rhizoctonia populations declined to 3.3 and 2.8 propagules g −1 soil, respectively. Rhizoctonia populations decreased slowly in nonamended soil from 3.8 propagules g −1 soil (initial population) to 1.6 propagules g −1 soil after 236 days. Populations in the amended soil were significantly higher than those of the control throughout the experiment. PPR addition resulted in increased rates of soil respiration which declined rapidly after 20 days from 0.113 mg CO 2–C g −1 soil day −1 (initial rate) to 0.049 mg CO 2–C g −1 soil day −1. Over the following 173 days the respiration rate declined to 0.011 mg CO 2–C g −1 soil day −1. The respiration rate in nonamended soil remained relatively stable and significantly lower than that of the amended soil throughout the incubation. PPR treatment resulted in the immobilization of soil N. Soil NO 3–N decreased from 16 to 0 mg kg −1 soil 14 days after residuals addition. Soil NH 4 was significantly lower than that in the control from day 14 to day 70. Net N mineralization rates became apparent approximately 84 days after PPR incorporation. Increased CO 2 evolution and immobilization of N, associated with amended soil, may have contributed to disease suppression. Plant elemental content indicated that nutrient uptake was largely unaffected by PPR addition. However, plants grown in amended soil produced less biomass and contained less Ca and N ( p<0.05) than plants grown in the nonamended soil. The observed suppressive effect of decomposing PPR in soil is complex and may be related to nutrient deprivation.