Evaluation of the tolerance of seven citrus rootstocks to Phytophthora gummosis under saline conditions

The combination of biotic and abiotic stresses in semiarid regions results in severe losses in citrus production. Neglecting the interactions that may occur between these factors would lead to the over or underprediction of the performance of rootstock cultivars under natural conditions and therefore to their erroneous ranking. In this study, seven citrus rootstocks, including two new Poncirus trifoliata hybrids, were evaluated for tolerance to Phytophthora gummosis after a short exposure to salt stress. The inoculation was made at stem level with a pathogenic isolate of Phytophthora citrophthora collected at the experimental field of INRA in Kenitra, Morocco. The results of this study highlighted a significant influence of salinity on the length of stem necrosis caused by Phytophthora attacks. Interestingly, the new Poncirus trifoliata hybrids F6 and F18 expressed high levels of tolerance to gummosis under saline conditions similar to those observed in the 'Gou Tou' sour orange (Citrus aurantium L.) and Sunki mandarin (Citrus sunki Hort. Ex. Tan.) checks, whereas pomelo (Citrus grandis) and citrumelo 'Winter Haven' (Citrus paradisi Macf. x Poncirus trifoliata L. Raf.) were ranked as sensitive. The length of stem necrosis was positively correlated with the accumulation of chloride in the leaves, but was independent of stem water content. This finding supports the hypothesis that the fungal infection is favored by the specific effects of salts rather than their osmotic effects.