Phytophthora vignae, causal agent of stem and root
rot of cowpea (Vigna unguiculata), was reported for the
first time in Sri Lanka. The pathogen was found in cowpea
field soils from 3 of 5 geographic regions sampled. Only
one site however, had plants exhibiting disease symptoms.
Of the eight cowpea varieties grown in Sri Lanka,
four were shown to be relatively resistant; all other
legumes inoculated were completely resistant.
Two morphologic and physiologic races of P. vignae
were identified among the 24 isolates recovered, based on
differential pathogenicity on cowpea varieties.
Bacteria isolated from field soils, and other known
bacterial biocontrol agents, inhibited P. vignae in
culture, but only three Sri Lankan isolates considerably
suppressed the disease in greenhouse tests. Volatile
substances produced by most bacteria inhibited mycelial
growth and sporangial production by P. vignae. The
increased pH of the exposed medium suggested the
involvement of ammonia. Volatile inhibitors were produced
by these bacteria in soil, but only with added substrate;
Strain DF-3101 also reduced oospore germination in soil.
Cowpea plants inoculated with the VA mycorrhizal
(VAM) fungus Glomus intraradices in P. vignae-infested
soil were larger than non-mycorrhizal plants, but only at
low levels of the pathogen. VAM colonization was reduced
at high levels of the pathogen, and root infection by the
pathogen was reduced by VAM.
The fungicides metalaxyl, fosetyl-Al, Banrot, and
Manzate-200DF reduced in vitro mycelial growth, but at
different concentrations. Sporangia formation and
germination, and oogonia formation by P. vignae, was
reduced significantly by metalaxyl and fosetyl-Al. In
greenhouse tests, metalaxyl, even at low concentrations,
reduced disease; Fosetyl-Al was effective at high
concentrations; Manzate-200DF was effective as a soil
drench but not as a foliar spray; Banrot effectively
reduced disease at 50 mg a.i./L. Exposure of a bacterial
biocontrol agent to these fungicides in vitro did not
affect its capacity to subsequently produce volatile
inhibitors, but exposure to 10 ug/ml of metalaxyl and 50
ug/ml of Manzate-200DF reduced its capacity to
subsequently inhibit mycelial growth of P. vignae. / Graduation date: 1991
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37166 |
| Date | 04 October 1990 |
| Creators | Fernando, W. Gerard Dilantha |
| Contributors | Linderman, Robert G. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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