Comparison of chemical treatments for reducing epiphytic Pseudomonas savastanoi pv. savastanoi populations and for improving subsequent control of olive knot disease

Abstract

The economic impact of the olive knot disease caused by Pseudomonas savastanoi pv. savastanoi (Psv) has dramatically increased in recent years, especially in high-density plantations. Here we report chemical control trials, performed in an olive grove planted with two susceptible cultivars, Arbequina and Picudo, over a four-year period. The effect of treatments made with copper oxychloride, cuprocalcic sulfate plus mancozeb or acibenzolar-S-methyl on the Psv populations and subsequent appearance of the disease were evaluated. Both copper treatments reduced the proportion of samples where Psv was isolated, and greatly reduced Psv populations in trees of cv. Picudo. The effect of copper on Psv populations was observed after the first application, but the greatest differences between copper-treated and untreated plants were observed in the third year, after five copper applications. No resistance to copper was detected in the remaining epiphytic Psv populations in treated plants. The average number of knots per plant was significantly lower in copper-treated plants than in untreated plants. By contrast, acibenzolar-S-methyl did not reduce either Psv populations or the disease during the study. These results support the usefulness of copper treatments for olive knot management. Results indicate that two copper treatments should be performed regularly under an integrated control program, especially in high-density groves, since their efficiency was demonstrated not only in decreasing olive knot incidence, but also in reducing Psv populations.

Introduction

Olive knot disease, caused by Pseudomonas savastanoi pv. savastanoi (Gardan et al., 1992) (hereafter Psv, according to Vivian and Mansfield (1993)) can lead to severe damage in olive groves. Psv infections in fresh wounds of olive trees (Olea europaea L.) start with a small cavity due to the collapse of immediate plant cells and are more frequent on trunks and branches, and rare on leaves and fruits. Subsequently, a proliferation of tissue follows up the periphery of the cavity resulting in the knot development (Smith, 1920, Surico, 1977). The endopathogenic life cycle of this bacterium in olive plants has recently been studied in detail (Rodríguez-Moreno et al., 2009). Hypertrophy and hyperplasia of plant tissues on infected wounds is due to the bacterial production of auxins and cytokinins (Smidt and Kosuge, 1978, Comai and Kosuge, 1980, Surico et al., 1985, Iacobellis et al., 1994, Rodríguez-Moreno et al., 2008). Olive knot disease is also dependent on hrp/hrc genes (Sisto et al., 2004). Psv can survive from one season to another inside the knots and may produce bacterial exudates during wet periods (Horne et al., 1912, Wilson, 1935). Consequently, bacteria can spread by wind, rain or insects and produce knots through new wounds caused mainly by leaf scars, hail, frost, pruning or harvesting (Wilson, 1935, Hewitt, 1938, Ciccarone, 1950, Quesada et al., 2010). Psv also has a resident phase colonizing the surface of symptomless stems and leaves, and these epiphytic Psv populations show seasonal shifts (Ercolani, 1978, Ercolani, 1991, Lavermicocca and Surico, 1987, Quesada et al., 2007). They may also serve as inoculum reservoir for subsequent infections (Wilson, 1935, Quesada et al., 2010).

Due to the economic impact of the olive knot disease, growers require adequate control methods to overcome its negative repercussions on the yield and even on olive fruit quality (Schroth, 1973, Quesada et al., 2010), because few commercial cultivars are significantly tolerant to olive knot disease (Penyalver et al., 2006). Disease management in the field is based on preventive procedures, because it is difficult to eradicate infections once established. As advised by the European and Mediterranean Plant Protection Organization (EPPO–OEPP), new olive groves should be established using Psv certified plant material (EPPO, 2006). When the disease is already present in established groves, branches with knots should be properly removed to minimize bacterial inoculum load (Wilson, 1935, Teviotdale and Krueger, 2004, Quesada et al., 2010). Besides, harvesting and pruning should always begin from healthy trees, and the number of wounds produced should be minimized (Wilson, 1935). Damage is more severe in new plantations with a high tree density and frequent severe pruning, because the small distance between plants facilitates bacterial dissemination and makes it more difficult to establish efficient olive knot disease control protocols (Tous et al., 2007).

Several studies suggest that the management of epiphytic Psv populations will likely lead to lower incidence of olive knot disease (Ercolani, 1978, Ercolani, 1991, Lavermicocca and Surico, 1987, Quesada et al., 2007, Quesada et al., 2010). In this context, a chemical control program using copper compounds was suggested many years ago, based on field observations in California (Horne et al., 1912, Wilson, 1935). Currently, common practices employ just one post-harvest application of copper; however, this provides only minimal protection against the disease, thus additional sprays in spring are needed to substantially improve its control (Teviotdale and Krueger, 2004). To our knowledge, there is no available information about the effect of copper compounds on the population dynamics of epiphytic Psv, the possible appearance of copper resistance, or about its role in decreasing olive knot incidence under Mediterranean conditions. In the aforementioned area, olive crop cultivation has increased dramatically in recent decades, probably due to the knowledge of the benefits of olive oil consumption for human health (Visioli and Galli, 1998, Luchetti, 2002). On the other hand, induction of systemic acquired resistance by chemical compounds has not been explored yet as an alternative control method for olive knot disease, although it has been evaluated in other diseases caused by plant pathogenic bacteria (Vallad and Goodman, 2004, Walters et al., 2005). Given the abovementioned reasons, this study evaluates the effects of repeated preventive treatments with two copper compounds and one plant resistance inducer on: (a) population dynamics of epiphytic Psv; (b) appearance of bacterial copper resistance; (c) incidence of olive knot disease; (d) influence on trees vigour; (e) fruit yield, over a four-year period.