Soil and Leaf Ionome Heterogeneity in Xylella fastidiosa Subsp. Pauca-Infected, Non-Infected and Treated Olive Groves in Apulia, Italy

Abstract

Xylella fastidiosa subsp. pauca is responsible for the "olive quick decline syndrome" (OQDS) in Salento (Apulia). The main epidemiological aspects of the syndrome are related to the pathogen spread and survival in the area, and to the biology of the insect vector. The assessment of the macro and microelements content (i.e., ionome) in soil and leaves could provide basic and useful information. Indeed, knowledge of host ionomic composition and the possibility of its modification could represent a potential tool for the management of diseases caused by X. fastidiosa. Therefore, soil and leaf ionomes of naturally infected, not infected, and zinc-copper-citric acid biocomplex treated trees of different areas of Apulia and the bordering Basilicata regions were compared. We observed that soil and leaf ionomic composition of olive farms growing in the pathogen-free areas north of the Salento Barletta-Andria-Trani BAT (Apulia) and Potenza PZ (Basilicata, Apulia bordering region) provinces is significantly different from that shown by the infected olive groves of the Salento areas (LE, BR, TA provinces). In particular, a higher content of zinc and copper both in soil and leaves was found in the studied northern areas in comparison to the southern areas. This finding could partly explain the absence of OQDS in those areas. In the infected Salento areas, the leaf ionomic profile resulted as being markedly different for the biocomplex treated compared to the untreated trees. A higher zinc content in leaves characterized treated with respect to untreated trees. On the other hand, among the not-infected trees, Xylella-resistant Leccino showed higher manganese content when compared with the higher pathogen sensitive Ogliarola salentina and Cellina di Nardò. According to these results, soil and olive leaf ionome could provide basic information for the epidemiologic study and possible control of X. f. subsp. pauca in Apulia.

Keywords: epidemiology; inductively coupled plasma atomic emission spectroscopy analysis; macronutrients; micronutrients; olive quick decline syndrome; pathogen control.

Figure 1

PLS-DA scoreplot (A), importance of variables ranked by VIP (Variable Importance in Projection score) (B) and box plot for the discriminant elements (C) obtained for soil samples, with a VIP score > 1. (Figure 1B: the colored boxes on the right indicate the relative increase/decrease of the corresponding variable in each group). BAT: Barletta-Andria-Trani; LE, BR, TA: Lecce, Brindisi, Taranto provinces (Apulia); PZ Potenza province (Basilicata, Apulia bordering region).

Figure 2

PLS-DA scoreplot (A), importance of variables ranked by VIP (Variable Importance in Projection score) (B) and box plot for the discriminant elements (C) obtained for olive leaf samples. (Figure 2B: the colored boxes on the right indicate the relative increase/decrease of the corresponding variable in each group). TR (farm treated with zinc–copper–citric acid), NTR (farm not treated and infected by Xylella fastidiosa subsp. pauca) and NI (farm not infected by Xylella fastidiosa subsp. pauca).

Figure 3

OPLS-DA scoreplot (A), importance of variables ranked by VIP (Variable Importance in Projection score) (B) and box plot for the discriminant elements (C) obtained for olive leaf samples for treated (TR) and not treated (NTR) infected olive leaves.

Figure 4

OPLS-DA scoreplot (A), importance of variables ranked by VIP (Variable Importance in Projection score) (B) and relative box plot for the discriminant elements (C) for TR (from Cellina di Nardò and Ogliarola salentina, all Lecce districts) and not infected samples (NI) olive leaves from Brindisi, Taranto and Barletta-Andria-Trani and Potenza provinces.

Figure 5

Mean concentration of sodium (Na, mg kg⁻¹) within olive tree leaves obtained for infected-not treated (NTR) and not-infected (NI), and the corresponding soil sample sites.

Figure 6

Box plots concentration for manganese olive leaf (A) and soil (B) samples of infected olive trees of Cellina di Nardò, Leccino and Ogliarola salentina cultivars, located in Taranto province (see also Table 1).

Figure 7

Expansion of Apulia region in Italy. Sample areas are showed by the tree symbol.