Aerial Contamination of Sugarcane in Guadeloupe by Two Strains of Xanthomonas albilineans

Two sugarcane plots were set up in Guadeloupe with disease-free tissue cultured plants in a banana growing location distant from sugarcane fields. Thirteen weeks after planting sugarcane in the field, a Xanthomonas albilineans strain belonging to serotype 3 (strain XaS3) was detected in water sampled at sunrise on the leaves in the first plot. This strain randomly invaded the sugarcane canopy. Seven weeks later, a new strain belonging to serotype 1 (strain XaS1) appeared on leaves and populations of strain XaS1 progressively increased on the leaf surface, whereas populations of strain XaS3 progressively decreased. Leaf scald symptoms were first noted 26 weeks after sugarcane planting. However, only strain XaS1 was isolated from leaves and a few sugarcane stalks showing symptoms. Both strains also colonized the second field plot, which was studied at the end of the experiment to avoid human interference of aerial contamination of sugarcane. After inoculation of three sugarcane cultivars by the decapitation technique, strain XaS1 was as virulent or more virulent than five other strains of X. albilineans isolated from diseased sugarcane plants in Guadeloupe. Although strain XaS3 colonized a few stalks, it failed to produce any symptoms and was the least virulent strain. Leaf surface colonization by X. albilineans was reproduced in a greenhouse trial by spraying the pathogen on sugarcane foliage. After 8 weeks, the pathogen was isolated from disinfected leaf blades. Although the leaf scald pathogen is thought to be mainly transmitted by infected cuttings, aerial transmission of X. albilineans is also known to occur. These results indicate the importance of sugarcane phyllosphere colonization by virulent strains in the epidemiological cycle of leaf scald disease in Guadeloupe.     

Genotypic variability of sugarcane resistance to the aphid Melanaphis sacchari,vector of the Sugarcane yellow leaf virus

The objective of this study was to analyse the genotypic variability of sugarcane resistance to the main aphid vector of the Sugarcane yellow leaf virus (ScYLV) (Polerovirus, Luteoviridae), Melanaphis sacchari. We assessed the incidence of aphids in a field trial comparing 181 sugarcane cultivars. Based on the percentage of infested leaves, aphid incidence was scored every two weeks during three cropping seasons. Analysis of variance revealed highly significant genotype, and genotype × year variance, and high broad sense heritability. Using semivariograms, we showed that the alpha lattice design used in the field trial was able to cope with spatial correlation issues caused by the patchy nature of aphid infestations. Twenty‐two aphid resistant cultivars were identified. A laboratory study of the development of M. sacchari on four of these 22 resistant cultivars confirmed the resistant status of three of them. We observed modest positive phenotypic and genetic correlations between the aphid incidence and the incidence of ScYLV. The 22 cultivars resistant to M. sacchari showed twofold lower mean virus incidence than the remaining 159 cultivars.     

Sensitivity of Podosphaera aphanis isolates to DMI fungicides: distribution and reduced cross‐sensitivity

BACKGROUND: Management of strawberry powdery mildew, Podopshaera aphanis (Wallr.), requires numerous fungicide treatments. Limiting epidemics is heavily dependent on sterol demethylation inhibitors (DMIs) such as myclobutanil or penconazole. Recently, a noticeable reduction in the efficacy of these triazole fungicides was reported by strawberry growers in France. The goal of this study was to investigate the state of DMI sensitivity of French P. aphanis and provide tools for improved pest management. RESULTS: Using leaf disc sporulation assays, sensitivity to myclobutanil and penconazole of 23 isolates of P. aphanis was monitored. Myclobutanil EC₅₀ ranged from less than 0.1 to 14.67 mg L^?1 and for penconazole from 0.04 to 4.2 mg L^?1. A cross‐analysis and a Venn diagram showed that there was reduced sensitivity and a positive correlation between the less sensitive myclobutanil and penconazole isolates; 73.9% of isolates were less sensitive to a DMI and 47.8% exhibited less sensitivity to both fungicides. CONCLUSION: The results show that sensitivity to myclobutanil and, to a lesser extent, penconazole has become less efficient in strawberry powdery mildew in France. Therefore, urgent action is required in order to document its appearance and optimise methods of control. Copyright © 2009 Society of Chemical Industry     

Contribution of cacao agroforestry versus mono-cropping systems for enhanced sustainability. A review with a focus on yield

The demand for cocoa has increased over the past years following the growth of cocoa-based products linked to the rise in living standard in highly populated countries. Cacao industry is therefore currently facing the dilemma of producing more cacao while ensuring its sustainability. Cacao monocrops and agroforestry systems (AFS) are two contrasting ways to produce cocoa, yet their impact on yields, contribution to farmer livelihood, cocoa quality remains understudied. Therefore, we reviewed existing literature comparing (1) monocrop cacao farming systems with (2) simple or (3) complex AFS. We found 19 comparisons of the cocoa yields in monocrops and simple AFS and 20 comparisons of monocrop and complex AFS. Three main research findings derive from this work. First, in about one third of cases, cacao trees yield more (or equally) in AFS than in monocrops. However, when considering only simple AFS, cacao trees yield more or equal to cacao monocrop in 52% of the cases. Second, cocoa AFS yields an average of 14% less than cacao monocrop. Yet, on average simple AFS yielded 2% less than cacao monocrops. Finally, there are too little elements to draw conclusions about the nexus between cocoa quality and cacao tree cultivation system.