5.2.44
SUPPRESSIVE EFFECT OF ANTAGONISTIC AGENTS ON RHIZOCTONIA ISOLATES ON LETTUCE AND POTATO IN ARGENTINA FIELD PLOTS

L GASONI1, J COZZI1, K KOBAYASHI2, V YOSSEN3, G ZUMELZU3 and S BABBITT4

1Instituto de Microbiologia y Zoologia Agricola, INTA, CC 25, 1712 Castelar, Argentina; 2Faculty of Agriculture, Hokkaido University, Kita-Ku, Sapporo 060, Japan; 3Facultad de Ciencias Agrahas, Universidad Nacional de Cordoba, Argentina; 4Facultad de Agronomia, Universidad de Buenos Aires, Argentina

Background and objetives Potential antagonistic agents are often applied on the basis of inhibition of the pathogen in growth chamber or greenhouse experiments. Results based on simulations may have little relevance to the complex relationships that occur in the field. Application of the biocontrol agents directly into the field is problematic due to the difficulties of detecting large and homogeneous numbers of inocula in plots. In this paper we report the preliminary results of 2 years of field evaluations on antagonist performance in pathogen suppression.

Materials and methods
The following bacterial agents: Bacillus cereus (isolates 96 and 238), B. pumilus (235), Pseudomonas fluorescens (6) and one isolate of Trichoderma harzianum (Th-1) were selected as the most promising strains from our greenhouse screening program [1]. The experiments were conducted in Cordoba (Argentina). A detailed study of pathogens in naturally infected soils and damaged hosts, performed during 1995, revealed the presence of different strains of Rhizoctonia solani complex. The field plots (3 m long x 4 rows for lettuce and 7 m long x 3 rows for potato) included treatments with different agents, non-treated and chemical fungicide-treated controls. Seeds of lettuce and potato pieces were coated with powder formulations combining antagonistic strains, carriers and cover powders [2]. At seeding time, biocontrol isolates were also applied as liquid inocula (containing approximately log 8 cfu/ml) to reinforce the suppressive effect. Evaluation of plant stand was made weekly during three months.

Results and conclusions
Plant stands evaluated during 1996 showed increases of 30 and 46% in plots treated with B. cereus, isolates 238 and 96, respectively, compared to the non-treated control. In plots treated with P. fluorescens the increase of stand plants was 27.6%, while in those treated with T. harzianum it was 20% and with B. pumilus, 15%. In the experiment conducted during 1997, natural infection in non-treated plots was not significantly higher. The difference in performance by year can be explained by climatic conditions and the effect on the pathogen. The plant stand in potato crop showed increases of 24.9 and 11.9% in plots treated with B. cereus, isolates 96 and 238, respectively, compared to the non-treated control. In plots treated with B. pumilus it was 26.4% and with T. harzianum, 27.9%, while withP. fluorescens the increase was only 7%. Formulations with B. pumilus and with T. harzianum were consistently the best for both crops. The most important consideration in the final evaluation of these inoculants was the prevention of serious stand losses. Moderate stand losses generally did not reduce yields, because individual plants grew rapidily and compensated for variations in stand density.

References
1. Gasoni L, Cozzi J, Kobayashi K, 1997. Zeitschrift fur Pflanzenkrankheiten und Pflanzenschutz (in press).
2. Cozzi J, Gasoni L. 1997. Proceedings of the Fourth International Workshop on Plant Growth-Promoting Rhizobacteria, pp. 468-471.