Seed-borne pathogens and the bean weevil (Acanthoscelides obtectus) in bean (Phaseolus vulgaris) seed and their effect on seed germination and vigour.

Abstract

Two semi-selective media, MT and MSP, were compared for their ability to recover cells of the bacteria Xanthomonas axonopodis pv. phaseoli (Xap) and Pseudomonas syringae pv. phaseolicola (Psp), the causal organisms of bean common and halo blights respectively. Isolates Sx70 and Sp75 (for Xap and Psp respectively) were plated on the media by dilution plating. Greater number of Xap colonies on MT than on MSP was obtained, but the number of Psp colonies was not significantly different on the two media. Four media, XCP1, MT, MT(new), and TSM were also compared for the number of contaminants and Xap colonies grown on them, using data obtained from ISTA/ISHI Comparative test for Xap (2002), which was conducted at the Agricultural Research Council (ARC) in Roodeplaat, north of Pretoria. In most cases, the number of colonies was similar in XCPI, MT, MT(new), but bigger in TSM. The number of contaminants was smaller in MT than in MT(new). Bean plants (Phaseolus vulgaris), variety PAN 146, were grown and inoculated with the bacteria Xap and Psp, and two virus isolates of bean common mosaic virus (BCMV). Uninoculated controls were also grown. Germination and vigour tests were conducted on the seeds harvested from these plants. Germination was significantly lower for seeds harvested from Psp inoculated plants, while seed vigour was reduced both for seeds harvested from Psp and Xap inoculated plants. Neither germination nor vigour was affected for seeds from BCMV inoculated plants. Late and early matured seeds from the BCMV inoculated plants were compared for the virus transmission and emergence. There was greater virus transmission and reduced emergence in the late matured seeds than in the early matured ones. Seeds harvested from Psp and BCMV inoculated plants and uninoculated controls were unexpectedly found to be infested with the bean weevil (Acanthoscelides obtectus (Say)). The BCMV infected batch of seeds appeared to repel weevil attack, whereas 25% of the uninfected batch of seeds were attacked.