Nitrogen fertilization effect on olive leaf spot and olive anthracnose incidence in the olive tree
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Nitrogen has a great influence on the canopy development of growing plants and
on the composition of plant tissues. Nitrogen fertilization is often associated with
increased susceptibility of plants to pests and diseases, and rational fertilization is usually
reclaimed as a cultural measure to mitigate their effects. Two field trials were conducted
in rainfed olive groves during the years 2017 and 2018, one of the cultivar Madural and
the other of cv. Cobrançosa. Three (0, 40 and 120 kg N ha-1) and four (0, 20, 40 and 120
kg N ha-1) nitrogen rates were tested, respectively in 'Cobrançosa') and 'Madural'. In the
leaves, the level of incidence of the olive leaf spot (Spilocaea oleagina (Cast.) Hugh.) was
visually assessed and after the leaves had been incubated with NaOH (5%). The presence
of diseases in the fruit, such as anthracnose (Gloeosporium olivarum Alm.) were also
recorded. Olive leaf spot appeared with relatively high incidence in 'Cobrançosa' in 2017
(8.0 to 54.0%) and 2018 (14.0 to 44.7%) and in Madural in 2018 (24.5 to 36.9%), but not
related to nitrogen fertilization. Olive anthracnose was relatively high in 2017 in 'Madural'
(8.7 to 15.3%) and 'Cobrançoa' (9.7 to 31.3%) and relatively low in 2018 in 'Madural' (2.0
to 8.0%) and Cobrançosa (0.7 to 4.7%), but also unrelated to nitrogen fertilization. These
results suggest a reduced influence of nitrogen fertilization on the incidence of olive leaf
spot and olive anthracnose, although the high experimental variability suggests that other
causes are still to be sought.
