Persistence of Xanthomonas axonopodis pv. vignicola in weeds and crop debris and identification of Sphenostylis stenocarpa as a potential new host

Abstract

The survival of Xanthomonas axonopodis pv. vignicola, incitant of cowpea bacterial blight and pustule, in residues of infested cowpea leaves was studied in the field in the forest savanna transition zone of South Benin and under variable controlled conditions. The pathogen survived for up to 60 days when placed on the soil surface, and up to 45 days buried at depths of 10 and 20 cm. In the glasshouse, bacteria survived in residue mixed with soil for at least 2 months in dry soil and less than 2 months in moist soil. The pathogen survived at least 30 days in the field after spray-inoculation on the weed species Euphorbia heterophylla, Digitaria horizontalis and Synedrella nodiflora; 20 days on Panicum subalbidum; 10 days on Euphorbia hirta; and 5 days on Talinum triangulare. After leafinfiltration under glasshouse conditions, the pathogen was detected after 90 days in D. horizontalis; 75 days in T. triangulare, P. subalbidum and S. nodiflora; 60 days in E. hirta, and 30 days in E. heterophylla. Among 12 legume species tested as alternative hosts of X. axonopodis pv. vignicola, only Sphenostylis stenocarpa (African yam bean) showed typical symptoms of cowpea bacterial blight in a glasshouse experiment following artificial inoculation. This is the first time this legume species has been identified as a potential host of X. axonopodis pv. vignicola. Crop residue and weeds are likely sources of primary inoculum when planting two consecutive cowpea crops per year and they probably play a role in dissemination of the pathogen during the cropping season. The alternate host may form a bridge for primary inoculum between cropping seasons.

Evolutionary History of the Plant Pathogenic Bacterium Xanthomonas axonopodis

“…However, xanthomonads strains may inhabit and infect wild species [77], [78], or may occur as commensals on plants or may be found in association with seeds [79]. Part of the diversity may therefore have been neglected in our study.…”

Transmission of Plant-Pathogenic Bacteria by Nonhost Seeds without Induction of an Associated Defense Reaction at Emergence

“…A plantpathogenic bacterium can efficiently transmit from seeds to nonhost seedlings as a consequence of saprophytic multiplication, resulting in the establishment of a primary inoculum focus. It was previously demonstrated that plant-pathogenic bacteria can survive in the phyllosphere of nonhost plants (7,12,49). The next epidemiological step is the dispersal of these pathogenic bacteria from this inoculum focus situated in a nonhost crop to a neighboring susceptible host crop.…”

Identification of markers associated with bacterial blight resistance loci in cowpea [Vigna unguiculata (L.) Walp.]

“…However, symptoms were observed on non-infested leaves of the susceptible line IT84S-2246-4 confirming the systemic nature of the disease. Stem inoculation by inserting a sharp tooth-pick contaminated with bacterial suspension as suggested by Sikirou (1999) and Sikirou and Wydra (2004) using two CoBB strains did not induce canker symptoms on stems in both susceptible and resistant cowpea lines tested. It appears that most of the genotypes investigated here were indeed resistant to stem canker expression.…”

“…Pod infection appears as dark green water-soaked areas, from where the pathogen enters the seeds and causes discoloration and shriveling (Sikirou 1999). CoBB is seed-borne (Sikirou 1999) and the pathogen can be spread by wind-driven rain and insects (Zandjanakou-Tachin et al 2007), but also crop debris and weeds can play a role as inoculum sources (Sikirou and Wydra 2004). Different strategies are used to control the disease including cultural practices (Emechebe and Florini 1997), intercropping (Sikirou 1999;Sikirou and Wydra 2008), application of chemicals (Rao and Hiremath 1985;Kotchoni et al 2007), and sowing pathogenfree seeds (Emechebe and Shoyinka 1985;Soni and Thind 1991).…”

“…In cowpea, reliable assays have been established for screening for resistance to bacterial blight. The assays are based on leaf spray-infiltration with bacterial suspensions on the abaxial surface without injuring the leaves and inoculation of the stem by inserting a sharp toothpick, contaminated with bacterial suspension (Sikirou 1999;Sikirou and Wydra 2004). High variability was found among Xanthomonas axonopodis pv.…”