Role of N2-fixation in the sustainability of the ponded grass pasture system

Ponded pastures in northern Australia produce green fodder in the seasonally dry winter period and may be employed to reduce grazing pressure on dryland pastures. The soils under ponded pasture, currently 26,000 ha in Queensland, are inherently infertile. This study was conducted to determine if a non-symbiotic association between bacteria and grass roots was responsible for the supply of N to ponded grasses. Intact soil-plant cores were obtained from a new ponded pasture of Aleman (Echinochloa polystachya) and an 8-year-old pasture of Hymenachne (Hymenachne amplexicaulis). Nitrogenase (N2-ase) activity was measured using the acetylene reduction assay and bacteria were selectively isolated to N-free malate medium from root segments of the most active plants. N2-ase activity of the intact soil-plant cores ranged from 76 to 380 g N ha−1 d−1 for the Aleman pasture and from 5 to 179 g N ha−1 d−1 for the Hymenachne pasture. Assays on excised roots showed the greatest activity on adventitious roots formed on the submerged nodes of Hymenachne stems. No major differences in colony morphology were detected in N2-fixing bacteria isolated from the roots of the two grasses. An association appeared to exist between bacteria and the roots of both grasses with most of the N for the young Aleman pasture being fixed N, whereas the fixed N supply for the older Hymenachne pasture was supplemented by the mineralization of organic N

Effect of organic amendments and solarisation on Fusarium wilt in susceptible banana plantlets, transplanted into naturally infested soil

Despite extensive research since pathogenicity was first established in 1919, no cultural or chemical control strategy has proven effective against Fusarium wilt of bananas. The efficacy of cultural control is attributed to the suppression of pathogen activity. Yet, amending naturally infested soil with aged chicken manure has been shown to enhance disease severity, without any change in the activity of the pathogen Fusarium oxysporum f. sp. cubense (Foc) in the soil. In this study, the effect of amending soil with composted sawdust, and of solarising soil, was compared with the effect of amending soil with chicken manure. Bioassays comparing the activity of Foc in the soil with the extent of invasion of banana pseudostem tissue by Foc were used to investigate why strategies targetting pathogen survival have not proven successful in controlling this disease. The enhancement of Foc invasion of the banana plantlets was reproduced with the addition of chicken manure to the naturally infested soil. However, changes in the activity of Foc in the soil were not associated with changes in the frequency of invasion of the plantlets. Invasion of banana pseudostems in the sawdust and solarisation treatments was not significantly different from invasion in the respective control treatments, despite a reduction in the activity of Foc in the sawdust-amended soil and an enhancement in the solarised soil. Moreover, the increase in Foc activity in the solarised soil recorded during the bioassays occurred despite the effectiveness of solarisation in reducing the survival of Foc in pre-colonised banana root tips buried in the soil. Changes in the frequency of invasion were associated with changes in the availability of mineral nitrogen, particularly ammonium N. These results suggest that the physiological response of banana cultivars to ammonium N may be associated with their susceptibility to Fusarium wilt. Accordingly, cultural strategies for controlling Panama disease will only be effective if they enhance the ability of the host to resist invasion

Soil receptivity and host-pathogen dynamics in soils naturally infested with Fusarium oxysporum f. sp. cubense, the cause of Panama disease in bananas

Disease severity associated with the pathogen Fusarium oxysporum is generally thought to be proportional to the population of fungal propagules in the soil. However, results from studies using naturally infested soil are contradictory, implicating host predisposition to disease. In this study, soil was amended with chicken manure to investigate the interdependence between the activity and invasiveness of the pathogen, and the incidence of Panama disease in susceptible banana plantlets. Two soil types naturally infested with either race 1 or race 4 of the pathogen, and cultivars Lady finger and Grande Naine, were used. Pathogen activity was measured by burying root tip segments for 5 days, then calculating the frequency of isolation of Fusarium from the segments. Pathogen invasion was measured by transplanting banana plantlets into trays of amended and unamended soil for 4 weeks, then calculating the frequency of recovery of Fusarium from each pseudostem. Amending both soil types with chicken manure enhanced both pathogen invasion and disease incidence. However, pathogen activity was not correlated with either parameter. We postulate that the addition of chicken manure is predisposing banana plantlets to Panama disease, by reducing the efficacy of the host wound response

A foliar rating system for comparing the resistance of banana cultivars grown as tissue-cultured plantlets in the laboratory to Fusarium wilt

A foliar rating system was developed to assess the progress of Fusarium wilt (Panama disease) caused by Fusarium oxysporum f. sp. cubense in seven banana cultivars differing in their resistance to race 1 of the pathogen. Plantlets were transplanted into unamended soil naturally infested with the pathogen, soil amended with urea and soil amended with aged chicken manure. A corm invasion score was also developed to assess the accuracy of the foliar symptom score as an indicator of cultivar resistance. On the basis of foliar symptom scores alone, the response of five of the seven cultivars in the chicken manure treatment corresponded to their known field response. However, the response of the other two cultivars, both susceptible to the pathogen in the field, fell into two categories. One had a high foliar symptom score and a correspondingly high corm invasion score, whereas the other had a low foliar symptom score and a high corm invasion score. Breeders need to be aware of the two categories of susceptible response, if inferior breeding material is to be rejected early on in a breeding program