Methods to control plant disease may exclude the pathogen from the host, improve the resistance of the host, directly protect plants from pathogens, and/or eradicate or reduce pathogen inoculum (Agrios, 1997). The decision process to determine which control method, or combinations of control methods, is best suited for a particular disease must be based on knowledge of the pathogen life cycle and the interaction between host and pathogen (Lozano and Wholey, 1974). Hence, methods used to control walnut blight have generally focused on reducing pathogen inoculum and protecting plants from infection.
3.5.1 Copper and non-copper treatments
The most effective bactericide treatments for reducing populations of X. arboricola
pv. juglandis are those products containing metallic copper (Miller and Bollen, 1946; Mulrean and Schroth, 1982). The most widely used copper products contain copper oxychloride, copper hydroxide, cuprous oxide or Bordeaux mixture (Mew and Natural, 1993). Copper is an essential element for enzymes involved in cell respiration (Garcia-Horsman et al., 1994); however, above a certain concentration copper damages DNA and lipid membranes and becomes toxic to the cell (Hoshino
et al., 1999; Muller et al., 2000; Finney and O‟Halloran, 2003). Alternative chemicals to copper for controlling walnut blight have not been identified, with bactericidal sanitation treatments and antibiotics proving to be either ineffective or unreliable against walnut blight (Miller and Bollen, 1946; Belisario and Zoina, 1995; Lang et al., 2006).
3.5.2 Amendments to copper sprays
Copper applied alone does not always provide effective control of walnut blight. The toxicity of copper is reduced by the low solubility of copper bactericides (Arman and Wain, 1958), and by copper ions forming complexes with organic compounds on plant surfaces (Menkissoglu and Lindow, 1991). However, the addition of ethylene- bisdithiocarbamate fungicides (EBDC‟s), such as maneb and mancozeb, to copper has increased control of walnut blight in comparison to applications of copper alone. In Tasmanian orchards, Lang et al. (2006) found blight incidence was significantly reduced when copper-based products were combined with mancozeb. Similarly, Buchner et al. (2001) reported that the addition of EBDC‟s increased the efficacy of copper-based products for the control of walnut blight in northern California. The toxicity of copper has also been enhanced by the addition of EBDC‟s for the control of X. campestris pv. vesicatoria in pepper and bacterial speck caused by
Pseudomonas syringae pv. tomato (Conlin and McCarter, 1983; Marco and Stall, 1983). The chelating ability of EBDC‟s is thought to increase copper toxicity as copper ions are prevented from complexing with other organic substances (Cooksey, 1990). The addition of iron, in the form of ferric chloride, to copper hydroxide has also shown to enhance the toxicity of copper to X. arboricola pv. juglandis, although the incidence of walnut blight was not significantly reduced (Lee et al., 1993). These findings indicate the importance of combining amendments with copper-based products to increase the toxicity of copper to the walnut blight pathogen.
3.5.3 Timing of copper-based sprays
Reducing or excluding the initial inoculum is a suitable strategy for the control of monocyclic epidemics. Blight incidence of fruits in monocyclic epidemics was entirely dependent upon the amount of initial inoculum in seven of ten orchards in California (Adaskaveg et al., 2000). Similarly, Ninot et al. (2002) found that populations of X. arboricola pv. juglandis in Spanish orchards were highly variable in spring and relatively constant during summer, leading them to suggest that chemical treatment to reduce population growth is best concentrated during spring rather than summer. Nino et al. (2002) found that three sprays applied during the two to three week period of budburst did not significantly increase disease incidence and
severity on fruits in comparison to a total of seven sprays i.e., three budburst sprays and four sprays applied at weekly intervals after the budburst period. In northern Californian orchards, disease incidence on fruits was significantly reduced after application of two sprays in which copper-based sprays were applied one and two weeks after budburst (Lindow et al., 2004). Furthermore, during a 14-year study conducted in northern Californian orchards, a maximum of three sprays applied during the budburst to early post-blossom period often provided satisfactory control of blight (Miller and Bollen, 1946). These results suggest that bactericide sprays applied during budburst may reduce pathogen inoculum, protect primary infection courts and reduce disease incidence during monocyclic epidemics.
Bacterial populations generally increase exponentially under conditions favourable for multiplication (Stall et al., 1993), and a direct relationship has been observed between the numbers of X. arboricola pv. juglandis cells in buds and the mean number of diseased fruits (Lindow et al., 2004). In a study conducted over 5 years in Californian orchards, wet spring seasons led to polycyclic epidemics and increased the severity of disease (Adaskaveg et al., 2000). Thus, multiple applications of copper are generally applied after the budburst period in an attempt to reduce the rate of disease increase. However, high rainfall may limit copper persistence on trees (Ninot et al., 2002). Rainfall of 40 to 90 mm per week reduced copper levels on sour cherry leaves by more than one half, leaving sub-lethal levels for the control of
Pseudomonas syringae pv. morsprunorum (Olsen and Jones, 1983). Therefore, to improve control of walnut blight, the persistence of copper on walnut tissues after rainfall warrants investigation.
In polycyclic epidemics, the inoculum can be multiplied many times during the growing season, requiring further control applications during the growing season. However, copper tolerant strains of the walnut blight bacterium are present in French and Northern Californian walnut orchards that have a history of high copper use (Gardan et al., 1993; Lee et al., 1993); these bacteria have been found to accumulate copper ions in the periplasm and outer membrane of the cell (Cooksey, 1990). Furthermore, high levels of copper within soils can lead to crop losses in walnuts (Radix and Seigle-Murandi, 1993): concentrations of less between 80 to 110 mg
copper per kg of soil in orchards have led to decline in soil biota (Martin, 1986; Paoletti et al., 1995). Ninot et al. (2002) found that application of 14 kg of copper per hectare for 3 years led to an 80% increase of copper in the top 10 cm of soil in comparison to application at 7 kg per hectare. Hence, the use of intensive copper spray regimes may have potential long-term detrimental effects on plant performance and the frequency of copper tolerant strains of X. arboricola pv. juglandis.
In northern Californian walnut orchards, temperature and extended wetness periods have been identified as critical environmental factors in walnut blight epidemics (Adaskaveg et al., 2000); this knowledge has been used to develop XanthoCast™, a model that utilises wetness period duration and temperature for calculating the cumulated daily risk of disease. In field trials conducted over 9 years, copper-based spray programmes timed according to host phenology i.e., the budburst period, and XanthoCast™ predictions after the budburst period, have predicted the same number, or a reduced the number, of sprays when compared to a calendar-based spray regime, while providing similar disease control (Adaskaveg et al., 2009). These results indicate the potential for reducing the number of copper sprays in Tasmania.
3.5.4 Biological methods
Xanthomonad phages are abundant in nature and occur together with host bacteria in soil and lesions of diseased plants (Hayward, 1964; Stolp and Starr, 1964). In New Zealand walnut orchards, bacteriophages were readily isolated from the top 2.5 cm of soil underneath the trees where X. arboricola pv. juglandis was isolated (McNeil et al., 2001). As a consequence, the potential use of bacteriophages as bio-control agents against X. arboricola pv. juglandis have been trialed in New Zealand orchards; however, further research is required for this method to be implemented into an integrated disease management strategy (Jenkins et al., 2010).