Summary. Brassica cultivars Ebony and Indian mustards,
and Rangi rape residues reduced the soil level of
Tylenchulus semipenetrans by up to 76% compared
with unamended soil, and in a greenhouse reduced levels on the roots of orange
(Citrus sinensis) seedlings.
Paratrichodorus lobatus reached high levels in pots
containing unamended soil but was not detected in pots containing amended
soils. However, Pythium ultimum was isolated more
frequently from roots, and propagule numbers of Pythium
spp. were significantly higher in amended soils. Soil amendment did not affect
growth of orange seedlings, and the benefits from reduced nematode levels may
have been negated by the increase in pythium infection.
In field experiments at 3 citrus orchards cleared for replanting, and at 1
established orchard between tree rows, brassica cultivars were grown
in situ (20 kg seed/ha) as green manure crops.
Highest crop production was at a site with heavier soil under drip irrigation,
where Ebony and Yellow mustards produced 13–15 kg fresh
weight/m2. Although soil levels of
T. semipenetrans were reduced by 79–91% by
incorporation of green manures, brassica cultivars including Ebony, Indian and
Yellow mustards, and Humus and Rangi rapes, were no more effective than were
self-seeding weeds. At 1 site, incorporation of a poor stand of Ebony mustard
(but not of weeds) produced higher soil levels of
T. semipenetrans; Paratrichodorus
sp. numbers declined after mustard incorporation at this site but increased
after weeds. Growth and soil incorporation of either brassicas or weeds
increased soil levels of Pythium spp., but fusarium
levels were decreased by incorporation of weed and Indian mustard residues.
Growth of citrus measured at 2 field sites did not differ between soils
amended with brassica or weed residues.
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