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SUMMARY

Xylem sap was extracted from cut branches of stone-fruit and pip-fruit trees at different times of the year, and analysed for nitrogen and carbohydrate content. Growth of Stereum purpureum was measured in the different sap samples in vitro. Nitrogen and carbohydrate levels in xylem sap from peach trees were highest in late winter and early spring, and these samples supported most growth of S. purpureum. At full bloom the levels of nitrogen and carbohydrate in xylem sap from stone-fruits (peach, nectarine, plum, cherry) were several times greater than in sap from pip-fruits (apple, pear), and the growth of S. purpureum in sap from stone-fruits was approximately ten times that in sap from pip-fruits.

The results suggest that seasonal variations in susceptibility of stone-fruits to silver-leaf disease, and the greater susceptibility of stone-fruits than of pip-fruits, are due to differential ability of their xylem saps to support growth of S. purpureum.