Effect of brassica and weed manures on abundance of Tylenchulus semipenetrans and fungi in citrus orchard soil

Abstract

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.