Speaker
Description
1. Introduction
The olive is the main tree crop in regions with Mediterranean climate, being cultivated on extensive areas in most countries of Southern Europe. Olive leaf spot, caused by the fungus Venturia oleaginea, represents one of the most important foliar diseases for the olive tree crop worldwide. Dispersal of the fungus occurs mainly through spores of the pathogen that are washed away by raindrops in spring and autumn. Infected leaves remain asymptomatic for weeks or months, depending on the environmental conditions (Viruega et al., 2013). After this incubation period, affected leaves show spots, circular leaf bundles and necrosis of the central nerve underside. Eventually, disease progression results in the defoliation of infected leaves, which has a direct impact on crop productivity. On the contrary, the impact of the presence of Venturia oleaginea in the photosynthesis and transpiration rates of infected leaves is unknown. This study aims to fill this gap by evaluating the gas exchange of asymptomatic and symptomatic leaves as compared with that of control unaffected leaves.
2. Materials and methods
Leaf-level measurements of stomatal conductance (gs), net photosynthesis (Pn) and clorophyll fluorescence were conducted in a field experiment with ‘Picual’ trees using a gas analyzer (CIRAS2, PPS Co. Ltd., UK) and a porometer/fluorometer (LI-600, LI-COR Biosciences GmbH Nebraska, USA). Measurements were performed in 2023 on three dates between late winter and early spring. 80 fully expanded leaves were selected and labelled for the measurements in each date, 40% of them showing visible symptoms of olive leaf spot. After the measurements, the same leaves were excised and immersed in a solution with 5% of NaOH for 30 min in the lab. This procedure was used for detecting latent infections, which allowed us to group the leaves into three categories (‘unaffected’, ‘asymptomatic’ and ‘symptomatic’). Differences among leaf categories were evaluated with an analysis of variance (ANOVA). Treatments were compared according to the Least Significant Differences at P ≤ 0.05. When ANOVA assumptions were not fulfilled, the Kruskal-Wallis test was used, and leaf categories were compared by Dunn's test at P ≤ 0.05. In all cases, measurements performed under low irradiance conditions (active photosynthetic radiation < 1000 μmol m-2 s-1) were excluded from the analysis.
3. Results
On average, symptomatic leaves showed lower gs and Pn values (≈20% and ≈24% lower, respectively) as compared with unaffected ones, but statistical differences were not systematically found. Leaves with asymptomatic infections showed intermediate values, not being statistically different from those recorded in unaffected leaves in most cases. Besides, V. oleaginea infections generally had no impact on chlorophyll fluorescence.
4. Discussion
This study represents the first thorough study evaluating the impact of olive leaf spot on the gas exchange of symptomatic and asymptomatic leaves of olive trees. Our findings revealed a relatively consistent reduction of gs and Pn in the leaves exhibiting visible symptoms. However, the differences concerning unaffected leaves were not always statistically significant. The decrease in Pn detected in olive leaves showing visible symptoms of Venturia oleaginea (i.e., symptomatic) may have an impact on biomass production and yield. However, the lack of consistent reductions in asymptomatic leaves points to defoliation being the primary driver of yield loss by the disease. Further research would be needed to obtain a clearer picture in any case.
5. References
Viruega, J. R., Moral, J., Roca, L. F., Navarro, N., Trapero, A. (2013). Spilocaea oleagina in olive groves of southern Spain: survival, inoculum production, and dispersal. Plant disease, 97(12), 1549-1556. https://doi.org/10.1094/PDIS-12-12-1206-RE
| Keywords | Photosynthesis; Olea europaea; Olive leaf spot; Stomatal conductance |
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