Satellite monitoring of bio-fertilizer restoration in olive groves affected by Xylella fastidiosa subsp. pauca

Abstract

Xylella fastidiosa subsp. pauca (Xfp), has attacked the olive trees in Southern Italy with severe impacts on the olive agro-ecosystem. To reduce both the Xfp cell concentration and the disease symptom, a bio-fertilizer restoration technique has been used. Our study applied multi-resolution satellite data to evaluate the effectiveness of such technique at both field and tree scale. For field scale, a time series of High Resolution (HR) Sentinel-2 images, acquired in the months of July and August from 2015 to 2020, was employed. First, four spectral indices from treated and untreated fields were compared. Then, their trends were correlated to meteo-events. For tree-scale, Very High Resolution (VHR) Pléiades images were selected at the closest dates of the Sentinel-2 data to investigate the response to treatments of each different cultivar. All indices from HR and VHR images were higher in treated fields than in those untreated. The analysis of VHR indices revealed that Oliarola Salentina can respond better to treatments than Leccino and Cellina cultivars. All findings were in agreement with in-field PCR results. Hence, HR data could be used to evaluate plant conditions at field level after treatments, while VHR imagery could be used to optimize treatment doses per cultivar.

Figure 1

Total monthly precipitation (Meteo-data were collected from the Nardò meteo-station close to the fields Nardò A, Nardò B as well as Galatone, and from the Otranto one which is close to Cannole) (mm) between March to August from 2015 to 2020 for (a) Nardò meteorological station—Ionian Coast and (b) Otranto meteorological station-Adriatic coast.

Figure 2

Total spring–summer precipitation (mm) from 2015 to 2020 for (a) Nardò meteorological station—Ionian Coast and (b) Otranto meteorological station—Adriatic coast.

Figure 3

Maximum daily temperature values for 2013, 2019 and 2020, (a) Ionian coast; (b) Adriatic coast.

Figure 4

S-2 NDVI trend from each experimental field with or without restoration treatment. (a) Galatone; (b) Nardò A; (c) Nardò B; (d) Cannole. Starting of the restoration treatment and pruning dates are indicated. Fields (a), (b), (c) located on the Ionian coast and field (d) situated on the Adriatic coast.

Figure 5

Cannole: (a) Image detected from drone in June 2022, after treatments. In green polygon, treated field surrounded by untreated fields; (b) tree with Xfp symptoms. Oliarola Salentina and Cellina di Nardò cultivars are planted both in treated and untreated fields.

Figure 6

NDVI trend for each year (2015–2017) from S-2 date: (a) Galatone; (b) Cannole.

Figure 7

NDVI trend for each year (2018–2020) from S-2 date: (a) Galatone; (b) Cannole.

Figure 8

NDVI trend for each year (2015–2017) from S-2 date: (a) Nardò A; (b) Nardò B.

Figure 9

NDVI trend for each year (2018–2020) from S-2 date: (a) Nardò A; (b) Nardò B.

Figure 10

Pearson correlation values between S-2 spectral indices (2015–2020), temperature (min, max, mean values) and total rain precipitation (a) from March to May; (b) from June to August in all treated and untreated fields.

Figure 11

Galatone window images from HR S-2 (7.1), VHR Pléiades (7.2) and VHR automatically classified (7.3). Boundaries of treated (green) and untreated (red) fields are overlaid in the scenes.

Figure 12

NDVI mean value for Galatone (Ionian coast, column a) and Cannole (Adriatic coast, column b) from: all S-2 pixels in the field (the first line); only all Pléiades tree crown pixels in the fields (second line); Pléiades tree crown pixels of different sample trees (third line).

Figure 13

NDVI mean value for test fields, Nardò A (column a) and Nardò B (column b), same Ionian coast, from: all S-2 pixels in the field, first line; all tree crown pixels only in VHR Pléiades images, second line; tree crown pixels of different sample trees in VHR images, third line.

Figure 14

NDVI mean values from different cultivars, i.e., Leccino, Ogliarola Salentina, Cellina di Nardò, with respect to untreated trees in Galatone.

Figure 15

Scatterplot of NDVI and OSAVI field mean values from S-2 and (a) index mean values from only tree crown pixels from Pléiades images; (b) index mean values all over Pléiades pixels from available treated and not treated fields.