Jacques Avelino

Plant species diversity for sustainable management of crop pests and diseases in agroecosystems: a review

Farmers are facing serious plant protection issues and phytosanitary risks, in particular in the tropics. Such issues are food insecurity, lower income in traditional low-input agroecosystems, adverse effects of pesticide use on human health and on the environment in intensive systems and export restrictions due to strict regulations on quarantine pests and limits on pesticide residues. To provide more and better food to populations in both the southern and northern hemispheres in a sustainable manner, there is a need for a drastic reduction in pesticide use while keeping crop pest and disease damage under control. This can be achieved by breaking with industrial agriculture and using an agroecological approach, whose main pillar is the conservation or introduction of plant diversity in agroecosystems. Earlier literature suggest that increasing vegetational biodiversity in agroecosystems can reduce the impact of pests and diseases by the following mechanisms: (1) resource dilution and stimulo-deterrent diversion, (2) disruption of the spatial cycle, (3) disruption of the temporal cycle, (4) allelopathy effects, (5) general and specific soil suppressiveness, (6) crop physiological resistance, (7) conservation of natural enemies and facilitation of their action against aerial pests and (8) direct and indirect architectural/physical effects. Here we review the reported examples of such effects on a broad range of pathogens and pests, e.g. insects, mites, myriapods, nematodes, parasitic weeds, fungi, bacteria and viruses across different cropping systems. Our review confirms that it is not necessarily true that vegetational diversification reduces the incidence of pests and diseases. The ability of some pests and pathogens to use a wide range of plants as alternative hosts/reservoirs is the main limitation to the suppressive role of this strategy, but all other pathways identified for the control of pests and disease based on plant species diversity (PSD) also have certain limitations. Improving our understanding of the mechanisms involved should enable us to explain how, where and when exceptions to the above principle are likely to occur, with a view to developing sustainable agroecosystems based on enhanced ecological processes of pest and disease control by optimized vegetational diversification.

Effects of phosphite on phytoalexin accumulation in leaves of cowpea infected with Phytophthora cryptogea

Leaves of susceptible cowpea seedlings (cultivar TVu645) inoculated with Phytophthora cryptogea and floated on buffer solution rapidly develop water-soaked spreading lesions. When floated on buffered phosphite solution (2·44 mol ml−1), lesion development is reduced and lesions are restricted within 24 h of inoculation. At the time of lesion restriction, concentrations of phosphite in lesions and the surrounding healthy tissues are 0·86 and 0·75 mol g−1 fresh weight, respectively. At these concentrations, mycelial growth of P. cryptogea is inhibited by only 26 and 23% in vitro. Within 48 h of inoculation, phosphite concentration in the necrotic tissues is 2·5 times that in the healthy surrounding zones but never exceeds the concentration of the external solution. The level of kievitone reaches a maximum of 134·5 g g−1 fresh weight in phosphite treated leaves within 24 h of the inoculation, compared with a concentration of 57·2 g g−1 fresh weight in untreated infected tissues. The concentration of kievitone which accumulates in arrested lesions at 24 h is more than the concentration required to inhibit P. cryptogea by 90% in vitro (100·9 g ml−1). Phosphite treatment also induces high levels of phaseollidin at the inoculation sites, exceeding the in vitro ED90 by 48 h after inoculation. Low amounts of vignafuran also accumulate in and around the lesion. These results support the hypothesis that the host defence reactions are involved in the mode of action of phosphite.

Landscape context and scale differentially impact coffee leaf rust, coffee berry borer, and coffee root‐knot nematodes

Crop pest and disease incidences at plot scale vary as a result of landscape effects. Two main effects can be distinguished. First, landscape context provides habitats of variable quality for pests, pathogens, and beneficial and vector organisms. Second, the movements of these organisms are dependent on the connectivity status of the landscape. Most of the studies focus on indirect effects of landscape context on pest abundance through their predators and parasitoids, and only a few on direct effects on pests and pathogens. Here we studied three coffee pests and pathogens, with limited or no pressure from host-specific natural enemies, and with widely varying life histories, to test their relationships with landscape context: a fungus, Hemileia vastatrix, causal agent of coffee leaf rust; an insect, the coffee berry borer, Hypothenemus hampei (Coleoptera: Curculionidae); and root-knot nematodes, Meloidogyne spp. Their incidence was assessed in 29 coffee plots from Turrialba, Costa Rica. In addition, we characterized the landscape context around these coffee plots in 12 nested circular sectors ranging from 50 to 1500 m in radius. We then performed correlation analysis between proportions of different land uses at different scales and coffee pest and disease incidences. We obtained significant positive correlations, peaking at the 150 m radius, between coffee berry borer abundance and proportion of coffee in the landscape. We also found significant positive correlations between coffee leaf rust incidence and proportion of pasture, peaking at the 200 m radius. Even after accounting for plot level predictors of coffee leaf rust and coffee berry borer through covariance analysis, the significance of landscape structure was maintained. We hypothesized that connected coffee plots favored coffee berry borer movements and improved its survival. We also hypothesized that wind turbulence, produced by low-wind-resistance land uses such as pasture, favored removal of coffee leaf rust spore clusters from host surfaces, resulting in increased epidemics. In contrast, root-knot nematode population density was not correlated to landscape context, possibly because nematodes are almost immobile in the soil. We propose fragmenting coffee plots with forest corridors to control coffee berry borer movements between coffee plots without favoring coffee leaf rust dispersal.

Topography and crop management are key factors for the development of american leaf spot epidemics on coffee in costa rica.

ABSTRACT We monitored the development of American leaf spot of coffee, a disease caused by the gemmiferous fungus Mycena citricolor, in 57 plots in Costa Rica for 1 or 2 years in order to gain a clearer understanding of conditions conducive to the disease and improve its control. During the investigation, characteristics of the coffee trees, crop management, and the environment were recorded. For the analyses, we used partial least-squares regression via the spline functions (PLSS), which is a nonlinear extension to partial least-squares regression (PLS). The fungus developed well in areas located between approximately 1,100 and 1,550 m above sea level. Slopes were conducive to its development, but eastern-facing slopes were less affected than the others, probably because they were more exposed to sunlight, especially in the rainy season. The distance between planting rows, the shade percentage, coffee tree height, the type of shade, and the pruning system explained disease intensity due to their effects on coffee tree shading and, possibly, on the humidity conditions in the plot. Forest trees and fruit trees intercropped with coffee provided particularly propitious conditions. Apparently, fertilization was unfavorable for the disease, probably due to dilution phenomena associated with faster coffee tree growth. Finally, series of wet spells interspersed with dry spells, which were frequent in the middle of the rainy season, were critical for the disease, probably because they affected the production and release of gemmae and their viability. These results could be used to draw up a map of epidemic risks taking topographical factors into account. To reduce those risks and improve chemical control, our results suggested that farmers should space planting rows further apart, maintain light shading in the plantation, and prune their coffee trees.

Shade Tree Spatial Structure and Pod Production Explain Frosty Pod Rot Intensity in Cacao Agroforests, Costa Rica

Vegetation composition and plant spatial structure affect disease intensity through resource and microclimatic variation effects. The aim of this study was to evaluate the independent effect and relative importance of host composition and plant spatial structure variables in explaining disease intensity at the plot scale. For that purpose, frosty pod rot intensity, a disease caused by Moniliophthora roreri on cacao pods, was monitored in 36 cacao agroforests in Costa Rica in order to assess the vegetation composition and spatial structure variables conducive to the disease. Hierarchical partitioning was used to identify the most causal factors. Firstly, pod production, cacao tree density and shade tree spatial structure had significant independent effects on disease intensity. In our case study, the amount of susceptible tissue was the most relevant host composition variable for explaining disease intensity by resource dilution. Indeed, cacao tree density probably affected disease intensity more by the creation of self-shading rather than by host dilution. Lastly, only regularly distributed forest trees, and not aggregated or randomly distributed forest trees, reduced disease intensity in comparison to plots with a low forest tree density. A regular spatial structure is probably crucial to the creation of moderate and uniform shade as recommended for frosty pod rot management. As pod production is an important service expected from these agroforests, shade tree spatial structure may be a lever for integrated management of frosty pod rot in cacao agroforests.

Primary and Secondary Yield Losses Caused by Pests and Diseases: Assessment and Modeling in Coffee

The assessment of crop yield losses is needed for the improvement of production systems that contribute to the incomes of rural families and food security worldwide. However, efforts to quantify yield losses and identify their causes are still limited, especially for perennial crops. Our objectives were to quantify primary yield losses (incurred in the current year of production) and secondary yield losses (resulting from negative impacts of the previous year) of coffee due to pests and diseases, and to identify the most important predictors of coffee yields and yield losses. We established an experimental coffee parcel with full-sun exposure that consisted of six treatments, which were defined as different sequences of pesticide applications. The trial lasted three years (2013–2015) and yield components, dead productive branches, and foliar pests and diseases were assessed as predictors of yield. First, we calculated yield losses by comparing actual yields of specific treatments with the estimated attainable yield obtained in plots which always had chemical protection. Second, we used structural equation modeling to identify the most important predictors. Results showed that pests and diseases led to high primary yield losses (26%) and even higher secondary yield losses (38%). We identified the fruiting nodes and the dead productive branches as the most important and useful predictors of yields and yield losses. These predictors could be added in existing mechanistic models of coffee, or can be used to develop new linear mixed models to estimate yield losses. Estimated yield losses can then be related to production factors to identify corrective actions that farmers can implement to reduce losses. The experimental and modeling approaches of this study could also be applied in other perennial crops to assess yield losses.

Towards a Collaborative Research: A Case Study on Linking Science to Farmers’ Perceptions and Knowledge on Arabica Coffee Pests and Diseases and Its Management

The scientific community has recognized the importance of integrating farmer’s perceptions and knowledge (FPK) for the development of sustainable pest and disease management strategies. However, the knowledge gap between indigenous and scientific knowledge still contributes to misidentification of plant health constraints and poor adoption of management solutions. This is particularly the case in the context of smallholder farming in developing countries. In this paper, we present a case study on coffee production in Uganda, a sector depending mostly on smallholder farming facing a simultaneous and increasing number of socio-ecological pressures. The objectives of this study were (i) to examine and relate FPK on Arabica Coffee Pests and Diseases (CPaD) to altitude and the vegetation structure of the production systems; (ii) to contrast results with perceptions from experts and (iii) to compare results with field observations, in order to identify constraints for improving the information flow between scientists and farmers. Data were acquired by means of interviews and workshops. One hundred and fifty farmer households managing coffee either at sun exposure, under shade trees or inter-cropped with bananas and spread across an altitudinal gradient were selected. Field sampling of the two most important CPaD was conducted on a subset of 34 plots. The study revealed the following findings: (i) Perceptions on CPaD with respect to their distribution across altitudes and perceived impact are partially concordant among farmers, experts and field observations (ii) There are discrepancies among farmers and experts regarding management practices and the development of CPaD issues of the previous years. (iii) Field observations comparing CPaD in different altitudes and production systems indicate ambiguity of the role of shade trees. According to the locality-specific variability in CPaD pressure as well as in FPK, the importance of developing spatially variable and relevant CPaD control practices is proposed.

Shade Effects on the Dispersal of Airborne Hemileia vastatrix Uredospores

Hemileia vastatrix caused a severe epidemic in Central America in 2012-13. The gradual development of that epidemic on nearly a continental scale suggests that dispersal at different scales played a significant role. Shade has been proposed as a way of reducing uredospore dispersal. The effect of shade (two strata: Erythrina poeppigiana below and Chloroleucon eurycyclum above) and full sun on H. vastatrix dispersal was studied with Burkard traps in relation to meteorological records. Annual and daily patterns of dispersal were observed, with peaks of uredospore capture obtained during wet seasons and in the early afternoon. A maximum of 464 uredospores in 1 day (in 14.4 m(3) of air) was recorded in October 2014. Interactions between shade/full sun and meteorological conditions were found. Rainfall, possibly intercepted by tree cover and redistributed by raindrops of higher kinetic energy, was the main driver of uredospore dispersal under shade. Wind gusts reversed this effect, probably by inhibiting water accumulation on leaves. Wind gusts also promoted dispersal under dry conditions in full sun, whereas they had no effect under shaded conditions, probably because the canopy blocked the wind. Our results indicate the importance of managing shade cover differentially in rainy versus dry periods to control the dispersal of airborne H. vastatrix uredospores.

Local and regional drivers of the African coffee white stem borer (Monochamus leuconotus) in Uganda

The African coffee white stem borer (CWSB) Monochamus leuconotus is a destructive pest of Arabica coffee in Africa. Documentation on outbreaks, spatiotemporal development and the relationship with different environmental conditions and coffee production system is limited. To underpin effective control measures, we studied aspects of local and regional pest drivers in Eastern Uganda. At the local scale, we (i) characterized the temporal development of CWSB and explored associations with environmental and shade‐related indicators. During two growing seasons and on 84 coffee plots, we recorded CWSB incidence/infestation and microclimate on an altitudinal gradient and different shading systems. The bimodal rainfall, altitude and shade affected CWSB development through their effect on minimum temperature. At the landscape level, we (ii) analyzed the spatial pattern of CWSB. Data on CWSB were collected on 180 plots. Pest incidence showed a spatial arrangement varying by districts. A possible relationship with human movement and the landscape context contributing to pest spread is suggested. CWSB control measures should be synchronized with the bimodal rainfall patterns and an emphasis should be given to identifying and limiting pathways of pest spread from highly infested to new areas.

Tropical crop pests and diseases in a climate change setting - A few examples

Climate change alters the behaviour of pests and their distribution. There is a genuine risk that pest and disease pressure will increase as a result of environmental and agrosystem disturbances. This is a concern for all agricultural stakeholders, especially in temperate countries where introductions of new pests, diseases and weeds abound. The list of introductions in Europe is getting ever longer, with the onset of disturbing phenomena that are a real threat to food security. The impact of climate change on pest populations and their natural enemies in the tropics is even harder and more complicated to grasp—changes in pest status, introductions, dramatic development of diseases or insect populations and extension of their ranges are being observed. Based on examples of insects and diseases affecting a few tropical agrosystems, we discuss the impact of climate change on these pests and propose new agroecological protection strategies while promoting the conservation of natural regulation services to sustainably reduce pest and disease risks.