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Cultivation of non-host crops after uprooting Xanthomonas campestris pv. musacearum (Xcm)-infected banana plants has been advocated for breaking Xanthomonas wilt disease (XW) cycle in fields. Knowledge on the interaction of these crops with Xcm is limited. Maize, beans and sweet potato were planted after uprooting Xcm-infected banana plants in Rwanda and eastern Democratic Republic of Congo (DR Congo). A weed fallow (mixed species) served as the control. After one, two and/or four break-crop or fallow seasons, healthy plantlets were replanted and monitored for XW for 12-24 months. XW status in adjacent fields was monitored, and diseased stems within 100-300 m radius of the two-and four-season experiments were uprooted. In Rwanda, soil and plant parts from the one-season experiments were sampled for Xcm isolation and Xcm-like colonies confirmed with Xcm-specific primers using PCR. Pathogenicity tests were performed to confirm the ability of the PCR-positive isolates to infect healthy banana plantlets. XW was observed in all the one-season experiments, with higher cumulative incidences in maize and bean plots. However, no similar trends were observed in the two-season experiments, with a 6-8% incidence observed only in bean and potato plots in DR Congo. Lengthening time under break crops to two and four seasons, respectively, reduced the incidence to 3% and zero in Rwanda and 0-8% in the two-season experiments in DR Congo. Incidence in the first-season experiments highly correlated (R = 88) to that in the adjacent fields, suggesting possible re-infections from these fields. Two season with break crop plus collective XW control are recommended in these agro-ecosystems. PCR-positive Xcm-like colonies from break crops only induced localized cell death on banana, while PCR-positive isolates from symptomatic banana plants caused full XW symptoms. Cross-infection/inoculation studies under controlled conditions are still needed to conclusively elucidate Xcm interaction with these crops. J Phytopathol 164 (2016) 659-670 Ó 2016 Blackwell Verlag GmbH 659 J Phytopathol Breaking the cycle of Xcm in banana fields A. Rutikanga et al.

Xanthomonas wilt of banana, caused by Xanthomonas campestris pv. musacearum, is a devastating bacterial disease that can cause up to 100% yield loss when appropriate control measures are not implemented. Currently, cultural practices are the only recommended means for managing Xanthomonas wilt. No cultivar is reported as resistant, nor can the disease be controlled using chemicals. On-farm experiments were established at three sites (eastern and western Rwanda, and North Kivu in Democratic Republic of Congo (DR Congo)), with the main objective of determining the most effective fallow period for eliminating Xanthomonas wilt from a highly infected banana field. At each site, three banana fields with an initial disease incidence of at least 70% were selected. All banana mats and most debris were removed before the experiment began. Thereafter, suckers that were free of Xanthomonas wilt of the cultivars 'Kamaramasenge' (Musa AAB group) and 'Injagi' (AAA-EA) in Rwanda, and 'Kamaramasenge', the plantain 'Musilongo' (AAB) and 'Vulambya' (AAA-EA) in DR Congo, were planted in experimental plots at monthly intervals, following increasingly long fallow periods, over 10 months. In Rwanda, ten plants per variety were planted each month in parallel rows in each field. In DR Congo, ten plants of each variety were randomly planted across the three experiment plots in single rows of ten plants per plot. In both countries, disease incidence was monitored for 15 months (i.e. up to 15 months after the first planting). In Rwanda, 13-15 months after planting, Xanthomonas wilt incidence in 'Injagi' was 22% for the planting in month 1, 27% for month 2 and 9% for month 3, whereas in 'Kamaramasenge' it was below 2% for the first 3 months of replanting. In 'Injagi', disease incidence declined sharply from months 4 (2.4%) and 5 (1.7%), i.e. less than 11 months from planting. Healthy suckers of the two cultivars planted from month 6 onwards did not become infected. In DR Congo, a steady decrease in Xanthomonas wilt incidence from month 1 (up to 70%) to month 10 (10%) was observed in the experimental fields. The prolonged appearance of disease symptoms (i.e. beyond replanting month 5) could be linked to the extremely high (>80%) disease incidence of, and close proximity to, diseased fields. Possible transmission of the disease into the experiment by foraging small ruminants and larger birds could have occurred. In contrast, diseased mats were continuously uprooted in adjacent farmers' fields in Rwanda. Results suggest that under Rwandan conditions, the bacterium is likely to survive for up to 5 months in soil and/or remaining plant debris. Therefore, it is proposed that replanting of previously Xanthomonas wilt-infected fields should be carried out 6 months after thorough uprooting of bananas infected with X. c. pv. musacearum. The DR Congo study also indicates that there is need for rigorous application of preventive control measures in adjacent farms to avoid possible transmission by foraging animals, large birds or runoff water. Hence, in both Rwanda and eastern DR Congo, disease management efforts should be carried out by farmers in a concerted manner.

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