Identity of Fusarium species associated with collar rot and wilt in passion fruit (Passiflora edulis)

Background: Despite the immense contribution of passion fruits to people’s livelihood on a global scale, the crop’s productivity remains low owing to fungal diseases causing up to 100% loss. Fungi are highly variable and the identity of species or variates responsible for recently devastating passion fruit wilt and collar rot diseases had not been characterized. This study was aimed at identifying pathogens causing wilt and collar rot symptoms in passion fruits. Methodology: Fungi were isolated from diseased samples collected from three locations in Central Uganda to identify Fusarium spp associated with collar rot and wilting of passion fruit. This was established by differentiating mycelium pigmentation on Potato Dextrose Agar (PDA), examining slides at X40 magnification under a light microscope for specific macro and microconidia, and amplification with specific Transcription Elongation Factor-1α, TEF 1α primers for identification of Fusarium spp. Results: It was revealed that wilting was associated with a single species, out of 6 selected isolates from the suspected wilted plant, 3 were Fusarium spp associated with the disease in the field but only one of these isolates was proved to be a pathogenic type Fusarium oxysporium. Collar rot was associated with one pathogenic Fusarium spp out of the 6 selected isolates. Conclusion: The results indicate that collar rot and Fusarium wilt are each caused by specific strains of Fusarium pathogens. Recommendation: The identification of pathogenic Fusarium in farmers’ orchards is a starting point for designing effective disease management measures against the predominant fungal pathogenic variants in passion fruits. 

The role of lianas for the structure and function of tropical rainforests of the Congo basin

Tropical forests constitute an important terrestrial biome, playing a vital role for the global carbon cycle and providing human population with valuable goods and services. Within this biome, many plant life forms coexist, including trees, shrubs, herbs, epiphytes, and lianas. Compared to trees, lianas have traditionally received less attention in ecological research. In recent years, however, there has been an increasing interest on the probable role of lianas in forest functioning. The general focus of these studies is to understand the mechanisms of liana diversity, distribution and coexistence with other life forms, but also to elucidate the effects of lianas on ecosystem processes and biodiversity. However, most of these studies on lianas have been conducted in the Neotropics. In Africa, which encompasses the second largest block of tropical forest, baseline field data on the ecology of lianas are cruelly lacking for many regions. Consequently, a comprehensive comparative analyses of liana research between tropical regions is hampered by the limited number and geographic range of the studies. In this thesis, I describe liana communities and analyzed a number of structural/functional characteristics of lianas, in most cases comparative to trees, along different environmental gradients in the tropical forest in central Congo basin. I mainly focus on structural and functional shifts in lianas along different forest gradients, including productivity (forest types), successional, and elevational gradients. As far as species diversity and distribution are concerned, I found that liana diversity was higher in old-growth edge forests, but lower in secondary forests than in mixed old-growth forests. This suggests that a moderate level of disturbance in tropical forests of the central Congo basin can increase and facilitate liana diversity, particularly in forests edges. However, the expansion of forest fragmentation due to agriculture activities may hinder the recovery pathways of tropical forest in the central Congo basin with a potential loss of liana diversity. I further demonstrated a progressive colonization of tropical forests by lianas after agricultural abandonment with a peak density at the intermediate successional stage. I also found low liana diversity, but high liana density in the monodominant old-growth forest, suggesting that closed canopy forests in the central Congo basin can also harbor a high number of lianas, and this high density of lianas might be driven by other abiotic and biotic factors than disturbance. In terms of functional strategies, I demonstrated the flexibility of liana functional strategies along a secondary succession, allowing liana species to coexist with tree species. The responses of lianas and trees to changes with succession converged in terms of specific leaf area (SLA) and leaf nitrogen content (LNC), but diverged in terms of leaf phosphorus content (LPC). These patterns in functional composition most probably resulted from environmental filtering, induced by a change from nitrogen to phosphorus limitation as the succession progressed to a mature forest. I further showed that lianas operate at relatively constant intrinsic water- and nitrogen- use efficiencies along an elevational gradient as opposed to trees, suggesting the existence of a functional divergence of water and nutrient use strategies between lianas and trees along elevation. This finding shows that lianas appear to be less impacted than trees by the increasing level of drought stress, and nutrient limitation with increasing elevation. On the effect of lianas on trees, I found a negative association between the presence of lianas on trees and the tree diameter increment in the old-mixed forest, suggesting a potential negative effect of lianas on tree growth. I further showed that larger tree individuals and slow-growing tree species harbored relatively more lianas in both the old-growth mixed forest and the monodominant old-growth forest in the central Congo Basin. This finding suggests that the exposure time could be an important driver of liana infestation, and that slow-growing tree species are more prone to liana infestation. However, I also found that the colonization and level of infestation of trees by lianas were not associated with tree leaf functional traits. This may imply that the negative effect of lianas on trees in the central Congo basin might not vary with tree functional identities, but mainly with tree structural attributes. Altogether, these findings provide new insights that can advance our understanding of mechanisms that control the abundance and distribution of lianas as well as liana and tree coexistence in the central Congo basin, an area of the tropics that has been understudied. Furthermore, the collected data on liana structural attributes and functional traits that can serve as basis to parametrize vegetation models capable of predicting the impact of lianas on the future of the Congo Basin carbon balance

Lianas rapidly colonize early stages of tropical forests, presumably through leaf trait diversification

Questions: Ecological succession is the process during which ecosystems recover after disturbances. Studies investigating community reassembly during tropical forest succession have rarely compared lianas to trees. We addressed two questions: (1) how do changes in stem density, total basal area and species richness of lianas and trees compare throughout a secondary succession, and to what extent does the relative basal area of lianas change along a secondary succession; and (2) how do the successional trajectories of functional community trait values of lianas and trees compare? Location: Yoko forest reserve, central Congo basin. Methods: Using univariate Bayesian modeling techniques, we analyzed differences in successional pathways between lianas and trees in terms of community structure, and functional assembly in a replicated chronosequence spanning from young to old-growth forests. Results: We found divergent structural trajectories between lianas and trees along the forest chronosequence. The stem density of lianas peaked at the intermediate stage, while that of trees almost linearly decreased from early to late stages of succession. The basal area of lianas increased at a higher rate than that of trees, which translated into a marginal increase of liana relative basal area over succession. On the contrary, we observed a lower rate of increase in species richness for lianas than trees over succession. We found a progressive convergence in the responses of lianas and trees to changes with succession in terms of specific leaf area and leaf nitrogen content, but a diverging response in terms of leaf phosphorus content. These functional composition patterns most probably resulted from environmental filtering, induced by a change from nitrogen to phosphorus limitation as the succession progressed to a mature forest. Conclusions: These findings underscore the rapid colonization of tropical forests by lianas after agricultural abandonment, presumably by deploying a more diverse leaf economic spectrum early in succession

Soil nutrient depletion and tree functional composition shift following repeated clearing in secondary forests of the Congo Basin

The Congo Basin's rapidly growing population still largely depends on shifting cultivation for both energy and food security. This nexus of population growth and ecological impact will continue to exacerbate landscape degradation in the coming decades. To quantify the effects of land-use intensity on soil nutrient stocks and the functional composition of young regrowth forest in the Congo Basin, we used fallows of different ages that had been subjected to a varying number of clearing cycles. We show that repeated clearing substantially affected soil cation stocks, reducing total K, Mg and Ca in the upper 20 cm of soil by roughly 20% per clearing cycle. Additionally, we show that plant-available nitrogen (ammonium and nitrate) and phosphorus decline in the topsoil with increasing land-use intensity. Furthermore, the tree functional composition of young fallows changed after repeated clearing cycles: we observed a decrease in abundance of pioneer species and an increase in nitrogen fixing species early in succession. Variation in soil total nutrient stocks was decoupled from changes in vegetation, and soil plant-available nutrients only marginally explained tree functional composition changes. We conclude that land-use intensity substantially affects both soil total and plant-available nutrients in a shifting cultivation system, as well as the functional composition of the regenerating vegetation. However, compositional changes of the tree community are only partly driven by land-use intensity effects on soil plant-available nutrients

Lianas and trees exhibit divergent intrinsic water-use efficiency along elevational gradients in South American and African tropical forests

Elevational gradients provide excellent opportunities to explore long-term morphological and physiological responses of plants to environmental change. We determined the difference in the elevational pattern of foliar carbon isotope composition (δ13C) between lianas and trees, and assessed whether this difference arises from changes in photosynthesis or stomatal conductance. We also explored the pattern of nutrient limitations with the elevation of these two growth forms. We conducted inventories of lianas and trees using standardized techniques along elevational gradients in Ecuador and Rwanda. We determined the values of several foliar traits including δ13C and chemical traits in dominant liana and tree species. We set up Bayesian linear mixed-effect models to quantify the effects of elevation and these two growth forms, and the difference of the effect of elevation between the two growth forms on each of the foliar traits. We found consistent growth form specific divergences in foliar δ13C and carbon to nitrogen ratio (C:N) responses to elevation. While we noted a meaningful increase in foliar δ13C and C:N with elevation for trees, lianas did not exhibit such a trend. Foliar δ13C and C:N remained relatively constant for lianas along the transects. The physiological processes at the basis of foliar carbon isotope fractionation shift differently in lianas and trees along elevation. Lianas operate at relatively constant intrinsic water- and nitrogen- use efficiencies with elevation as opposed to trees. Altogether, the study suggests the existence of a functional divergence of water and nutrient use strategies between lianas and trees along tropical elevational transects.Funding provided by: European Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000781Award Number: 63764

Lianas and trees exhibit divergent intrinsic water-use efficiency along elevational gradients in South American and African tropical forests

Aim: Elevational gradients provide excellent opportunities to explore long-term morphological and physiological responses of plants to environmental change. We determined the difference in the elevational pattern of foliar carbon isotope composition (δ13C) between lianas and trees, and assessed whether this difference arises from changes in photosynthesis or stomatal conductance. We also explored the pattern of nutrient limitations with the elevation of these two growth forms. Location: The study was conducted in two mountain forests situated in the Neotropics and Palaeotropics. Time period: August–September 2015 and August–October 2016. Major taxa studied: Lianas and trees. Methods: We conducted inventories of lianas and trees using standardized techniques along elevational gradients in Ecuador and Rwanda. We determined the values of several foliar traits including δ13C and chemical traits in dominant liana and tree species. We set up Bayesian linear mixed-effect models to quantify the effects of elevation and growth form on each of the foliar traits , and the difference of the effect of elevation between the two growth forms (lianas and trees). Results: We found consistent growth form specific divergences in foliar δ13C and carbon to nitrogen ratio (C : N) responses to elevation. While we noted a meaningful increase in foliar δ13C and C : N with elevation for trees, lianas did not exhibit such a trend. Foliar δ13C and C : N remained relatively constant for lianas along the transects. Main conclusions: Lianas operate at relatively constant intrinsic water- and nitrogen-use efficiencies with elevation compared with trees. Altogether, the study suggests the existence of a functional divergence of water and nutrient use strategies between lianas and trees along tropical elevational transects