EFFECTS OF PLANT-PARASITIC NEMATODES AND RHIZOSPHERE MICROORGANISMS ON THE GROWTH OF WHITE CLOVER (TRIFOLIUM-REPENS L) AND PERENNIAL RYEGRASS (LOLIUM-PERENNE L)

A pot trial was carried out to study the effects of plant-parasitic ne matodes and rhizosphere microorganisms on the growth of perennial ryeg rass (Lolium perenne) and white clover (Trifolium repens). Plants were grown together in Horotiu sandy loam (Vitric Hapludand). The treatmen ts consisted of: untreated field soil (UT); soil frozen to - 20-degree s-C to reduce nematodes (FR); soil fumigated with chloroform to kill m ost soil organisms (FU); and fumigated soil inoculated with a layer of frozen soil to reintroduce organisms present in frozen soil (FUI). Th e mean wet weights of white clover and ryegrass grown in UT soil for 6 -9 weeks were only 10 and 60%, respectively, of those grown in FR soil and 5.8 and 56%, respectively, of those grown in FU soil. The severe growth reduction of white clover in UT was attributed to early invasio n of roots by two nematode species, Heterodera trifolii and Meloidogyn e hapla, which were detected in stunted 7-day-old seedlings. In the FR and FUI treatments, the freeze-thaw process appeared to have killed t he second stage juveniles but not the eggs, which hatched to release m ore juveniles to invade white clover roots. Plant growth in FR, and to a lesser extent in FUI treatments, was reduced in comparison with FU, where there was no nematode invasion. The bacterial numbers in surfac e-sterilized roots were unaffected by any of the treatments. This sugg ests that the entry of bacteria into root tissues independent of the w ounding caused by nematodes. The mean bacterial numbers of the medians (means of log cfus across treatments and plant species of the median numbers across replicates) for ''total'' bacteria, fluorescent pseudom onads, Gram-negative bacteria and Gram-positive bacteria from surface- sterilized roots were 4.26, <2, 2.94 and 3.08 for the four treatments, respectively. The most common bacterial genera identified were Pseudo monas and Bacillus. Fungi isolated from surface-sterilized roots inclu ded Fusarium oxysporum, Codinaea fertilis and many sterile fungi, with F. oxysporum being the most common identified fungus in FU and FUI tr eatments. C. fertilis was killed by the soil freezing process. F. oxys porum did not appear to be associated with a decrease in dry matter pr oduction of either white clover or ryegrass, but C. fertilis may have adversely affected the ryegrass dry matter production in the treatment UT.