Development of Lesions Caused by Phytophthora cinnamomi in the Secondary Phloem of Eucalyptus marginata

Abstract

Phytophthora cinnamomi Rands invaded the secondary phloem of inoculated roots and stems of Eucalyptus marginata Donn. ex Sm. For 12 months after inoculation, lesion development was followed in coppice stems. As lesions extended, the phloem or inner bark became discoloured due to the accumulation and oxidation of polyphenols. Starch also was deposited in the necrotic phloem. The primary wall material of sieve tubes and associated parenchyma was hydrolysed but fibres were unaffected. Fungal spread was most rapid in the outer phloem tissue where cells were loosely packed and characterized by many expanded parenchyma cells. Fungal invasion of the inner phloem resulted in cambial kill. Roots were not girdled by the fungus in the first 12 months after inoculation, as they resisted tangential spread of the fungus more effectively than coppice stems. Lesions were contained once necrophylactic (wound) periderms formed in the bark. Although the necrophylactic periderm restricted fungal activity during winter and spring, the fungus did 'break-out' in summer and invade new areas of phloem in 50% of the inoculated roots and stems. Summer lesion extension was usually associated with kino production: a series of kino veins reflected the intermittent activity of the fungus. Once the characteristics of typical lesions were recognized, interpretation of root lesions resulting from natural infections was possible.