Allelochemicals Biologica... - Name

8
DAVID E. WEDGE
AND BARBARA J. SMITH
inhibition zone dimensions produced by active metabolites and control standards
against each pathogenic strain tested. Chemical profiles provide valuable information
for the rapid selection of specific antifungal metabolites with unique activity against
fungicide-resistant pathogens and identify new compounds with novel mechanisms
of action.
Using modifications of the methods described previously, bioautography
techniques were successful in allowing us to track the path of naturally occurring
fungitoxic compounds present in strawberry leaves. Our studies indicated that
concentrations of fungitoxic compounds vary between anthracnose resistant and
susceptible cultivars and are present in different amounts in vegetative tissues of
different ages. Using leaves of the anthracnose-susceptible cultivar Chandler and the
anthracnose-resistant cultivar Sweet Charlie, we isolated and demonstrated the presence
of three antifungal compounds. While the mechanism of strawberry anthracnose
resistance is unknown, results from this study indicate that anthracnose resistance in
strawberry may depend on the concentration of two constitutive antifungal compounds
and the elicitation of a third compound in younger leaves.
These two constitutive antifungal compounds were exhibited in both ‘Chandler’
and ‘Sweet Charlie’ plants but ‘Sweet Charlie’ plants produced approximately 15
times more antifungal activity than ‘Chandler’ plants. Fungal growth inhibition
associated with extracts from ‘Chandler’ plants appeared to be temporary. A third
compound, detected exclusively in ‘Sweet Charlie’ plants, was produced only after
young leaves were sprayed with a commercially available elicitor of antifungal compounds
(Vincent et al., 1999).
The antifungal activity of 32 naturally occurring quinones of four major classes:
1,4-naphthoquinones, 1,2-naphthoquinones, 1,4-benzoquinones, anthraquinones, and
other miscellaneous compounds from our natural products collection were tested for
antifungal activity using bioautography. Bioautography allowed for the rapid evaluation
of quinones which demonstrated good to moderate antifungal activity against
Colletotrichum spp. Colletotrichum fragariae appeared to be the most sensitive species
to quinone-based chemistry, C. gloeosporioides of intermediate sensitivity, and C.
acutatum was the least sensitive species to these naturally occurring compounds
(Meazza et al., 2003).
Bioassay-directed isolation of antifungal compounds from an ethyl acetate extract
of Ruta graveolens leaves yielded two furanocoumarins, one quinoline alkaloid, and
four quinolone alkaloids, including a novel compound, 1-methyl-2-[6'-(3'’,4'’methylenedioxyphenyl)hexyl]-4-quinolone.
Antifungal activities of the isolated
compounds, together with 7-hydroxycoumarin, 4-hydroxycoumarin, and 7methoxycoumarin
which are known to occur in Rutaceae species, were evaluated
using bioautography and microbioassay procedures. Four of the alkaloids had moderate
activity against Colletotrichum species, including a benomyl-resistant C. acutatum.
These compounds and the furanocoumarins 5- and 8-methoxypsoralen had moderate
activity against Fusarium oxysporum. The novel quinolone alkaloid was highly active
against Botrytis cinerea. Phomopsis species were much more sensitive to most of the