2.7.12
RELATIONSHIP BETWEEN SUPPRESSION OF FOOT ROT OF BARLEY (FUSARIUM CULMORUM) AND CELLULOLYTIC ACTIVITY IN ARABLE SOILS

IMB KNUDSEN, PH RASMUSSEN, J HOCKENHULL, D FUNCK JENSEN

Plant Pathology Section, Department of Plant Biology, The Royal Veterinary and Agricultural University, Thorvaidsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark

Background and objectives
Changes in soil organic matter caused by, e.g. tillage and crop rotation, can affect disease suppression and the activities of various soil enzymes. Specific organic amendments, such as mulching with straw have been shown to influence soil suppressiveness and soil cellulolytic activities [1]. The purpose of this study was to investigate the relationship between soil suppressiveness towards foot rot of barley and the cellulolytic activities of P-glucosidase (EC 3.2.1.21), cellobiohydrolase (EC 3.2.1.91) and endocellulase (EC 3.2.1.4) in arable soils.

Materials and methods
Field-moist soil samples from six organically and four conventionally farmed soils were collected in the early spring of 1995 and 1996. None of the soils had received any tillage, organic inputs or other fertilizer treatments for at least 6 ;month prior to sampling. Soil samples were taken from a depth of 6-13 ;cm as undisturbed blocks (70X80X115 ;mm3), from nine intersections of a 30X30 ;m2 grid and stored at 2C in sealed plastic boxes until use. Disease suppressiveness was investigated in bioassays consisting of pots with test soils mixed with sand, and barley seeds inoculated with a conidial suspension of Fusarium culmorum (1.5X106/ml). Pots with sand served as the conducive control. After 19 days, disease severity was evaluated on the barley seedlings on a scale from 0 to 4 and the percentage of disease suppression was calculated. The cellulolytic activities of o-glucosidase, cellobiohydrolase and endocellulase were measured by the use of three different fluorogenic substrates. The soil activities were expressed as Vmax values obtained by using the Hanes-Wolf transformation of the Michaelis-Menten equation.

Results and conclusions
No correlation between soil suppressiveness and the farming systems was obtained. The same was true for the cellulolytic activities. Significant correlations were found between soil suppressiveness and the activities of beta-glucosidase and cellobiohydrolase, where soils representing the highest disease suppressiveness had the highest activities. In addition, the activities for beta-glucosidase and cellobiohydrolase were significantly correlated to soil contents of total C and N, soil moisture and CEC of the soils. None of the cellulolytic activities were correlated to specific microbiological factors, including microbial C and N biomass and total number of fungi. These findings support previous results suggesting that disease suppression primary is associated with the abiotic fraction of the soils. The present study indicates that increased soil cellulolytic activities may play a role in suppression <1>Fusarium foot rot of barley.

References
1. Martens DA, Johanson JB, Frankenberger WI 1992. Soil Science 153, 53-61.