MECHANISMS OF SEED TRANSMISSION OF BACTERIAL BLIGHT OF COWPEA (Xanthomonas axonopodis pv. vignicola) (BURKHOLDER) DYE

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MECHANISMS OF SEED TRANSMISSION OF BACTERIAL BLIGHT OF COWPEA (Xanthomonas axonopodis pv. vignicola) (BURKHOLDER) DYE

Abstract:

Understanding transmission mechanisms of cowpea bacterial blight Xanthomonas axonopodis pv vignicola (Xav) from seed to seedling will form the bases for appropriate control measure to be initiated. In order to study the parts of seed where the pathogen usually colonize, three methods were used (seed washing and seed plating) were used to detect the external borne pathogens while the third (separation of seed into components and maceration) were used to detect the internally borne pathogen. The results showed that hilum was the most colonized part of the seeds. The embryo colonization was low and there was no Xav detected on cotyledons. In order to study the populations build up in vivo and symptom manifestation, seed samples were collected from various sources to reflect the various ways farmers obtain seeds (local markets, seed companies, research institute and treated seeds with fungicides). Fungicides had no significant effect on the pathogen survival and pathogenicity. There was significant difference between the treatments in terms of disease incidence and severity on the vegetative parts. There was steady population build up in all the treatments both in/on the plants. There was no significant difference between all the treatments in terms of Xav population build up on the floral parts. Mechanism of transmission of Xav was studied by soaking seeds of sorghum, maize, millet and cowpea in bacterial suspension adjusted to 4.7x107cfu/ml for two and four hours respectively. After 2 h of soaking seeds in bacterial suspension, cowpea seeds had higher population of Xav attached to it while after 4 h of soaking; all the seeds had Xav attached to it. The study of biofilm formation were carried out by growing bacterial in maize, millet, sorghum, cowpea extracts and extracts of cowpea + 0.5 g glucose and nutrient glucose agar (NGA) in a 96 microlitre wells. There was – 7 – statistical difference between the biofilm formation induced by the different extracts and NGA. Studies on the route of transmission of Xav to seeds indicate that the bacterium was able to transmit systemically and epiphytically from inoculated seeds to harvested seeds and also from inoculated vegetative parts into the harvested seeds. Since adhesion of bacteria to seeds was just on commensal basis, studies were conducted to evaluate the possibility of non-host seeds of cereal crops to transmit the pathogen. Maize, millet and sorghum seeds were collected from cowpea producing states for the isolation of the pathogen. The pathogen (Xav) was consistently isolated from millet samples collected from all the states except Kebbi state. In other to determine whether the Xav can directly transmit systemically from seeds to harvested seeds or indirect transmission through the floral parts to the non-host seeds (maize, millet and sorghum), seeds were inoculated before sowing and also, the floral parts were inoculated at the flowering stage. The pathogen (Xav) was able to systemically transmit from seeds to harvested seeds in millet. Maize and sorghum did not transmit CoBB pathogen systemically. However, millet and sorghum were florally transmitting Xav to seeds but maize did not.

MECHANISMS OF SEED TRANSMISSION OF BACTERIAL BLIGHT OF COWPEA (Xanthomonas axonopodis pv. vignicola) (BURKHOLDER) DYE

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