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Excessive nitrogen (N) use in agriculture has been associated with increased severity of the damage caused by  Phytophthora  species. In this study, we investigated the impact in vitro and in vivo of N about  Phytophthora cinnamomi . The preliminary in vitro assay showed the effect of different N sources on the mycelial growth of  P. cinnamomi . This assay indicated that ammonium nitrate (NH 4 NO 3 ) and ammonium sulfate [(NH 4 ) 2 SO 4 ] allowed for greater control of  P. cinnamomi  mycelia in comparison with calcium nitrate [Ca(NO 3 ) 2 ] and potassium nitrate (KNO 3 ) when used with 1000 ppm N. The in vivo assay showed the severity of  P. cinnamomi  in 5-month-old  Juglans regia  saplings grown under greenhouse conditions. We selected NH 4 NH 3  as the source for N for the greenhouse assay, considering the inhibitory effect on the ingrowth of  P. cinnamomi  and the intensive use of this fertilizer in agriculture. Walnut saplings were fertilized with 0, 35, 70, 140, 210, and 1050 ppm N and were inoculated with zoospores of  P. cinnamomi  45 d after the application of nitrogen treatment (DAA). They were harvested at 90 DAA. We found that a 70-ppm N fertilization reduced the development of  P. cinnamomi , resulting in lower root and canopy damage indices (DIs) than the unfertilized inoculated treatments and fertilized treatments greater than 140 ppm. The results of the in vitro and in vivo assay agree that increased N concentrations were associated with reduced mycelium growth of  P. cinnamomi , providing further evidence that N fertilization can mitigate this disease. Greater root and canopy damage was observed in saplings fertilized with 1050 ppm N, regardless of whether they were inoculated with  P. cinnamomi , as a result of N phytotoxicity (verified through foliar analysis). In contrast, inoculated and unfertilized saplings (N0) also showed high root and canopy DIs associated either with the inoculation with  P. cinnamomi  or the no fertilization treatment. We postulate that 70 ppm N is the best fertilization rate for  J. regia  saplings because the positive effects of N on growth are maximized and the damage caused by  P. cinnamomi  is mitigated.

Impact of Nitrogen Fertilization on Phytophthora cinnamomi Root-related Damage in Juglans regia Saplings