Systemicity of Xanthomonas campestris pv. musacearum and time to disease expression after inflorescence infection in East African highland and Pisang Awak bananas in Uganda

Abstract

Banana xanthomonas wilt (XW) caused by Xanthomonas campestris pv. musacearum (Xcm) attacks all banana cultivars. Xcm in inflorescence-infected Pisang Awak plants with wilting male bud bracts is restricted to the upper parts of the true stem; therefore, cutting these plants at the pseudostem base has been recommended to prevent further Xcm spread. In order to fine-tune existing control strategies, this study examined the movement of Xcm into plants and mats, in relation to disease incubation period. Mature Pisang Awak and East African highland (AAA-EA) plants were inoculated with Xcm through abscission wounds of female bracts, male bud bracts, male flowers, a combination of male bud bracts and flowers, and by cutting male buds with a contaminated machete. Thirty plants per genotype and treatment were monitored for 24 months for disease symptoms. An additional 68 AAA-EA and 33 Pisang Awak plants were sampled weekly to assess the rate of Xcm spread within the plants. All floral entry points resulted in disease, with the highest incidence in combined male bract and male flower abscission wound inoculations. The study confirmed the systemicity of Xcm, with the pathogen able to live within the mat for long periods (5-16 months) without causing disease. Reliance on disease symptom expression to manage XW is therefore not sufficient. The long incubation period in lateral shoots may explain the current resurgence of the disease in locations where the disease was thought to have been successfully eradicated.

Bacterial Diseases of Bananas and Enset: Current State of Knowledge and Integrated Approaches Toward Sustainable Management

“…An internal cross-section of a floral stalk shows yellow bacterial ooze from the vascular bundles, while a cross section of a fruit shows rusty brown stains in the fruit pulp (Thwaites et al, 2000; Tushemereirwe et al, 2003; Biruma et al, 2007; Ocimati et al, 2013a,b). Late floral symptoms (when the banana bunch is physiologically mature) have also been reported due to tool-mediated infections (Ocimati et al, 2013a).…”

Symptomless banana suckers sourced from Xanthomonas wilt infected fields are a viable alternative for seed within infected banana-based landscapes lacking access to clean planting materials.

“…, showed only 3% of suckers sampled from symptomatic mats in farmers' fields (with a 70% plant disease incidence) in eastern DR Congo to test positive for Xcm using Xcm -specific PCR primers. Ocimati et al (2013aOcimati et al ( , 2014 through on station experiments also showed that not all plants in a mat got infected with Xcm when the parent banana plants were artificially inoculated with bacteria through the inflorescence. The observations in the current study do support the findings by .…”

“…The cultural practices that include rouging and destroying infected plants/mats, early removal of male buds, disinfection of farm tools and use of clean planting materials are so far the most effective ways of reducing the inoculum in infested fields (Yirgou and Bradbury, 1968;Tushemereirwe et al, 2004). The adoption of rouging and complete destruction of infected mats, early male bud removal and disinfection of farm tools has been poor because they are cumbersome and costly (Jogo et al, 2013;Ocimati et al, 2013a).…”

“…It has long been thought that due to the systemic spread of Xcm, all suckers in the banana mat would become infected once the parent plant or any other plant in the mat is infected. However, Ocimati et al (2013aOcimati et al ( , 2014 has since shown an incomplete systemic spread of Xcm in banana mats in both on station experiments and on farmer fields that is, some suckers in infected mats lack the bacteria, grow to maturity and bear healthy bunches. The on station trials were inoculated through the inflorescence whereas a combination of routes including inflorescence infections, tools and browsing ruminants could have been responsible for infections in farmers' fields.…”

“…Ocimati et al (2014) reported up to 6% deaths amongst latently infected suckers of 'Pisang Awak' (ABB genome) and 'Mbwazirume' (East African highland banana, AAA genome) from mats in which the parent plants were deliberately infected (through floral parts) in on-station studies, with the other 94% latently infected suckers growing to maturity and bearing edible bunches. Ocimati et al (2013aOcimati et al ( , 2014 noted that this phenomenon offers both a challenge and an opportunity. A challenge in that such suckers and bunches could be an avenue for XW spread, whereas an opportunity for farmers in regions with no access to clean/certified seed and who are fully reliant on the informal seed system where suckers are sourced from within the farm or from neighboring farms.…”