Relationship of Sclerotium oryzae: Inoculum levels, stem rot incidence and severity, yield of rice in California, and bacterial populations recovered from S. oryzae sclerotia and rice stems under different rice residue management practices

A continuous year experiment was initiated in 1993 in Colusa County to evaluate the effects of alternative residue management strategies on overwintering sclerotia of Sclerotium oryzae, stem rot incidence and severity, and yield. After the first year, the number of sclerotia. recovered from winter flooded plots was less than from winter non-flooded plots. Yearly, the number of sclerotia recovered was greatest in the rolled subplots and smallest in the burned subplots. By 1997, % viable sclerotia, in the burned plots was less than half in other subplots. Yield increase occurred in three out of four years in the winter-flooded plots.;In 1994 a replicate trial was established in Butte County. In two of three years, there was a significant reduction in total sclerotia recovered from the winter-flooded plots. Yearly, the number of sclerotia recovered was greatest in the incorporated and smallest in the burned subplots. The highest yields were from the winter-flooded plots in 1996 and 1997 and the burned subplots in 1997. Results suggest winter flooding is the best alternative to burning for stem rot management.;There is a need to predict disease potential in fields since burning may be allowed when high disease potential is apparent. Correlation analyses showed total sclerotia per gram of soil (TS) was positively correlated to disease incidence, and disease incidence (DI) and disease severity (DS) were highly correlated throughout the growing season. TS and VS were negatively correlated to yield. DS and DI in the previous year were highly correlated to TS and viable sclerotia per gram of soil (VS) in the following year's seedbed. Disease assessment in the previous season shows promise for determining disease potential in the following season allowing timely management decisions.;Fourteen bacterial genera were recovered at the Colusa site from the sclerotial surfaces of Sclerotium oryzae and rice stems during the growing season and the off season from plots with various management regimes. Seventeen genera were recovered from the trial in Butte County. Pseudomonas, Methylobacterium, Bacillus and Acidovorax were most common at the Colusa site, and Micrococcus, Pseudomonas, Chryseobacterium , and Acidovorax were most common at the Butte site.