Xylem ultrastructural modification and the possible participation of phenolic compounds in the natural defense or resistance mechanisms of olive plants infected with Verticillium dahliae Kleb. were studied. Microscopic study showed that the mycelium propagated and passed from one element to another through the pit. The formation of tyloses and aggregates contributed to obstruction of the xylem lumen. In vivo changes in the levels of these phenolic compounds in infected olive plants and their antifungal activity against Verticillium dahliae Kleb., as revealed by in vitro study, strongly suggest that they are involved in natural defense or resistance mechanisms in this plant material, the most active being quercetin and luteolin aglycons, followed by rutin, oleuropein, luteolin-7-glucoside, tyrosol, p-coumaric acid, and catechin. .

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Phytophthora citrophthora causes serious losses in Citrus fruits through brown rot lesion. The effect of infection with P. citrophthora on Citrus sinensis (cv. Valencia Late) fruits was studied, with particular reference to the levels of the flavanones hesperidin and isonaringin and the polymethoxyflavones sinensetin, nobiletin, tangeretin, and heptamethoxyflavone, because flavonoids are most probably involved as natural defense or resistance mechanisms in this genus. Changes in the levels of these flavonoids were detected after infection. The hesperidin and isonaringin contents fell by 13 and 67%, respectively, whereas the contents of their corresponding aglycons, hesperetin and naringenin, increased, suggesting the hydrolyzing effect of this fungus on the glycosylated flavanones. The heptamethoxyflavone, nobiletin, sinensetin, and tangeretin levels increased by 48, 28, 26, and 24%, respectively. The in vitro study revealed that these compounds acted as antifungal agents, the most active being the aglycons (naringenin and hesperetin), followed by the polymethoxyflavones and flavanone glycosides. The participation of these flavonoids in the defense mechanism of this Citrus species is discussed.

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Grapevine trunk diseases (GTD), caused by a wide range of different fungi, are responsible for decline and productivity losses in vines at all growth stages. Grapevine responses to fungal attack include morphological and physiochemical defence mechanisms in the vascular system to reduce fungal infections. However, the extent to which these responses could control further spread by GTD‐fungi in the xylem vessels is poorly known. This study shows the formation of tyloses inside xylem vessels of diseased grapevines, as well as extracellular ligninolytic activities [lignin peroxidase, manganese peroxidase (MnP) and/or laccase] exhibited by some GTD‐fungi isolated here from symptomatic grapevines. In particular, Botryosphaeriaceae spp. and Phaeoacremonium minimum showed all three lignin‐degrading enzymatic activities. We also examined whether selected vine phenolic compounds, often located in the vascular system in response to fungal infection, could affect the lignin‐degrading activity from those GTD‐fungi as well as fungal colonisation. We found that phenolic compounds appeared to inhibit MnP activity, in addition to reducing fungal growth by causing anomalies in the hyphae morphology. Our results support that affected grapevines can initiate the tylosis formation in order to constrain fungi in the xylem vessels, while highlight the complementary action of the phenolic compounds to inhibit the fungi growth and colonisation. Phenolic compounds are therefore likely to have important role in alternative strategies for preventing trunk diseases.