Aim: To develop an appropriate formulation of the deleterious rhizobacterium Pseudomonas trivialis X33d and to evaluate its effectiveness to reduce brome growth. Methods and Results: Two formulations of Ps. trivialis X33d, a semolinakaolin granular formulation (Pesta) and talc-kaolin powder, were prepared and their effectiveness in reducing brome growth was evaluated. Both brome suppression and cell viability of X33d were higher in Pesta granular formulation than in talc-kaolin powder one. The impact of storage temperature and the addition of adjuvants (sucrose and oil) to the granular formulation of X33d were assessed in order to improve the shelf life of the formulation. The longest viability was found in formulated product supplemented with adjuvants and stored at 4°C. The effect of Pesta granules supplemented with adjuvants and stored for 6 months at 4°C on brome and wheat growth under controlled and greenhouse conditions was evaluated. The X33d formulation in Pesta increased the growth of wheat and reduced brome growth. Conclusion: Our results indicate that Ps. trivialis X33d formulated in Pesta has potential as a bioherbicide to control brome. Significance and Impact of the Study: Because of the impracticality of applying bacterial cell suspension on a large scale, the use of Pesta granules of X33d against brome could help in achieving a sustainable agriculture application of a bioherbicide.

The rhizobacterial strain X33d was previously shown to suppress the growth of the weed great brome (Bromus diandrus Roth.). The aim of this work was to identify X33d, characterize its physiological activities, assess its specificity on different non-target crops, and its impact on the growth and the root architecture of great brome and durum wheat (Triticum durum Desf.) grown alone and together. Based on 16S rDNA sequencing, X33d was identified as Pseudomonas trivialis. The specificity assay, performed on a mixture of soil/sand/peat, highlighted the suppressive activity of P. trivialis X33d against great brome and the promoting effect on most of the considered crops, especially durum wheat. Although the growth of durum wheat on quartz sand was unaffected, P. trivialis X33d suppressed the growth and affected the root architecture of great brome, especially when co-seeded with durum wheat. Great brome plants inoculated with X33d and co-seeded with durum wheat showed low root biomass, short root systems and low surface area, volume and number of tips. Moreover, P. trivialis X33d synthesized indole-acetic acid (IAA) that could be involved both in great brome growth suppression and durum wheat growth promotion. Our results indicate that P. trivialis X33d could be exploited as a potential biocontrol agent against great brome without affecting the durum wheat growth. These results are discussed in relation to the competitive capability of great brome towards durum wheat.

Italian thistle (Carduus pycnocephalus L.) is a common and increasingly important weed in Tunisia. It is also problematic in the western United States and a target of biological control. In surveys conducted in northern Tunisia from 2003 to 2005, Italian thistle plants in many locations were found diseased by rust. Eighty-five isolates of rust were collected from Italian thistle during these surveys. Each isolate was collected from a single plant and stored individually as mixtures of urediniospores and teliospores at 4°C or in liquid nitrogen. Urediniospores and teliospores of all isolates were similar in morphology and matched the description of Puccinia carduorum Jacky (3). Isolate B1003 (BPI No. 878207), collected from Béja, Tunisia, was arbitrarily selected for further study. Comparison of internal transcribed spacer (ITS) regions of B1003 (GenBank Accession No. EF050059) with other ITS sequences indicated a 97% similarity to P. carduorum (GenBank Accession No. PCU57351) from Carduus nutans subsp. leiophyllus. Eight Italian thistle plants, grown from seeds collected in Béja, were inoculated in the 3- to 5-leaf stage with urediniospores of B1003 by spraying plants with an aqueous suspension of urediniospores at 106 spores per ml with approximately 0.03 ml of surfactant until they were thoroughly wet. Four plants were sprayed with water plus surfactant only. All plants were covered with plastic bags and placed in a growth chamber at 18/20°C night/day temperatures. Bags were removed 24 h after inoculation and plants were monitored daily for symptoms. Plants sprayed with water plus surfactant only did not develop symptoms. Six inoculated plants developed disease symptoms similar to those observed on samples collected during the surveys. White flecks appeared within 7 days of inoculation and developed into brown pustules 10 days after inoculation. Pustules enlarged and produced urediniospores until they covered both sides of diseased leaves. One month after appearance of symptoms, diseased leaves turned yellow and died. Urediniospores from these plants were used to inoculate six plants each at the 2- to 5-, 6- to 8-, and >8-leaf stages. All plants became diseased and produced uredinia. Plants in the 2- to 5-leaf stage were more severely diseased than other plants. P. carduorum was introduced to the United States in a field test for control of C. nutans subsp. leiophyllus (musk thistle) and has become established in a number of states (1). An isolate of this fungus has also been found from C. tenuiflorus (slender-flower thistle) in California (4). However, neither isolate causes substantial disease on Italian thistle from California (2,4). Isolates of P. carduorum from C. pycnocephalus in Greece, Italy, and Turkey caused little disease on most Italian thistle collections from California (2), indicating variability in susceptibility among plants as well as in virulence among rust isolates from different geographical locations. To our knowledge, this is the first report of P. carduorum parasitizing Italian thistle in Tunisia. Tests will be conducted to determine the host range of this isolate among C. pycnocephalus collections and other species. References: (1) A. B. A. M. Baudoin and W. L. Bruckart. Plant Dis. 80:1193, 1996. (2) W. L. Bruckart and G. L. Peterson. Phytopathology 81:192, 1991. (3) D. B. O. Savile. Can. J. Bot. 48:1553, 1970. (4) A. K. Watson and K. Brunetti. Plant Dis. 68:1003, 1984.

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