BP Singh

150 . Baleshwar Singhdiscolouration around the hilum. Pseudomonas syringae pv.phaseolicola, which causes halo blight of bean, produce a bluishwhitefluorescence in white-seeded beans when observed underultra-violet light (Wharton, 1967). Seeds showing such symptomsshould be removed from the samples. Similarly, discoloured,deformed and shrivelled seeds may carry the pathogen whichshould be sorted out by visual examination.2. Incubation testsThese methods are generally used for detection of seed bornefungal pathogens but can also be used for the detection of seedborne bacteria.(a) Blotter test: This method is used for detection of manybacterial pathogens in seeds like Xanthomonas campestris pv.campestris which causes black rot in crucifers, X. campestris pv.phaseoIi causing bacterial' blight in beans, X. campestris pv.vesicatoria causing bacterial leaf spot in chilli and tomato. To detectthe presence of X. campestris pv. campestris the crucifer seeds areplaced on moist blotter papers in plastic Petriplates and incubatedat 22±1°C. The typical black rot symptoms may be seen on theseedlings after 8 days of incubation. The bacterial association withthe symptoms may be observed by bacterial ooze test undermicroscope. Then the associated bacterium is isolated from theinfected parts on agar medium, purified and identified. Forbacterial ooze test, a piece is cut through infected tissue with sharpblade or razor and the piece is placed in a drop of water on slideunder cover glass. The slide is examined under microscope. If theinfection is due to bacteria, cloudy mass of bacterial cells is seenoozing out from the cut ends of the tissue piece. In vascularinfection bacteria ooze out forcefully at distinct pointscorresponding to the vascular strands as seen in bacterial blight ofbean caused by X. campestris pv. phaseoli whereas inparenchymatous infection the oozing of bacteria is slow, diffusedand throughout the cut ends e.g. in bacterial leaf spot of chilli andtomato caused by X. campestris pv. vesicatoria.(b) Paper towel method: The seeds are sown on moist paper'towel, then rolll7d and incubated. The symptoms of the disease canbe observed after 8 days of incubation. Xanthomonas oryzae pv.oryzae causing bacterial leaf blight disease in rice, X. campestris pv.