Excised Twig Assay for the Study of Apple Tree Crown Rot Pathogens In Vitro

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Abstract
Dormant, excised apple twigs were used to determine pathogenicity and relative virulence of pythiaceous fungi [Phytophthora cactorum, P. megasperma, Pythium spp.] isolated from apple trees with crown rot symptoms. Twig segments were inserted vertically into a test isolate culture growing on cornmeal agar plus agar in a storage jar. After incubation, the twig segments were removed from the agar and stripped of their periderms, and the length of necrosis was measured. Dormant twigs were preferable to active twigs. Fungal contamination on the agar surface was minimized by amending the agar with pimaricin at 10 mg/l. The method can be used to determine the pathogenicity of different species of pythiaceous fungi, to compare relative virulence of isolates within a species, and possibly to determine relative resistance of different apple scion and rootstock cultivars.

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