MANGANESE AND BRITTLE LEAF DISEASE OF DATE PALM TREES
A. Namsi, M. Montarone, P. Serra, O. Ben Mahamoud, M.L. Takrouni, A. Zouba, O. Khoualdia, J.M. Bové, N. Duran-Vila
Five oases affected by “brittle leaf disease”, or “maladie des feuilles cassantes” (MFC) of date palms, and one healthy oasis were chosen in the southern part of Tunisia for mineral analyses of leaflets from adult and juvenile palms, as well as of the soils surrounding the palm trees. In the MFC-affected oases, the following trees were selected: apparently healthy trees (AH), and trees with mild (DS1), moderate (DS2), and severe (DS3) symptoms. Healthy trees (H) were selected in the healthy oasis. Groups of three trees of each category were selected, and the soils from the three trees of each group were mixed. Manganese was the only element for which significant differences were seen between adult leaflets from healthy trees and those from MFC-affected trees. The adult leaflets from the healthy trees (H) had the highest Mn content whereas leaflets from the symptomatic trees DS1, DS2, and DS3 had the lowest Mn contents with values significantly smaller than those for leaflets from H and AH trees. Most trees were growing in soils with a content of exchangeable Mn of around 3 mg/kg or lower, regardless of their disease status. However, the trees of one MFC-affected oasis were growing in soils with a content of exchangeable Mn of around 4.5 mg/kg or higher, regardless of their disease status. From these observations, it is inferred that MFC is apparently not related to the content of exchangeable Mn in the soils of the oases studied, but is inversely correlated with the Mn content in the leaves. The soils of the oases studied were alkaline, with conditions that favour formation of insoluble Mn compounds and therefore render the element unavailable to palm trees. Leaflets from MFC-affected palm trees have been shown to contain MFC-specific RNAs (MFC-RNAs). Dot-blot hybridization analysis, using a bifunctional DNA probe that detects the MFC-RNAs, gave positive signals with all preparations from adult symptomatic leaflets collected from DS trees in the affected oases. Some AH trees also gave positive signals, confirming that these symptomless trees are only apparently healthy. No hybridization signals were observed with preparations from symptomless leaflets collected healthy oasis (H) or from the Germplasm Bank of Degache where the disease has never been observed.