Phytophthora species are known to cause important diseases in agriculture (Kurbetli, 2013). The factors that determine the severity of the damage caused by Phytophthora can be biotic [e.g., insects and other pathogenic microorganisms that act as antagonists or synergists (Huber, 2002)] and abiotic [e.g., pH, free water, temperature, and mineral nutrition such as N (Huber and Haneklaus, 2007; Mur et al., 2016)]. Specifically, N can have both a beneficial and harmful impact on the diseases caused by Phytophthora species (Erwin and Ribeiro, 1996). It affects the incidence of root decomposition caused by Phytophthora parasitica Dastur. in citrus, Phytophthora sojae Kaufm. & Gerd. in Glycine max L., Phytophthora cinnamomic Rands. in Persea americana Mill., and the incidence of late blight in Solanum tuberosum L. caused by Phytophthora infestans (Mont) De Bary. (Elmer and Datnoff, 2014; Erwin and Ribeiro, 1996; Huber and Graham, 1999). However, questions regarding how the N source and dose affect disease incidence remain mostly elusive. Previous studies have associated N source and dose as factors that might affect the interaction between Phytophthora species and the host (Elmer and Datnoff, 2014; Erwin and Ribeiro, 1996; Huber and Graham, 1999; Huber and Thompson, 2007; Huber and Watson, 1974). However, this effect is inconsistent and extremely dependent on the pathogenic species and the corresponding host (Huber, 2002; Huber and Haneklaus, 2007).
The high N inputs used in walnut production (Gupta et al., 2012; Simorte et al., 2001; Weinbaum and Van Kessel, 1998) and the high susceptibility to P. cinnamomi reported in Juglans regia L. (Guajardo et al., 2017, 2019) highlight the need to investigate this interaction further. The importance of assessing the severity of the disease is enhanced by the worldwide presence of this pathogen. For example, Guajardo et al. (2017) reported that 93.3% of the walnut orchards in Chile show damage caused by oomycete species. There have been similar cases in France, Italy, Hungary, and Iran, in which species of Phytophthora have been recovered from infected walnut roots (Kurbetli, 2013). In our study, we aimed to test whether N can control Phytophthora cinnamomi attack in growing J. regia saplings. First, we conducted a preliminary in vitro assay to elucidate the effect of the dose and the source of N on the mycelial growth of P. cinnamomi. Then, we studied the effect of different doses of N and inoculation with P. cinnamomi on the root and canopy growth of J. regia saplings grown under greenhouse conditions.
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