Changes in population of Pseudomonas solanecearum at different soil depths*

Population of Pseudomonas solanacearum was monitored in the infested fields in 0–20, 20–30, 30–40, 40–50 and 50–60 cm soil depths before planting, 90 days after planting, at harvest of potatoes and after keeping the land fallow for seven months at two places in Kumaon hills of U.P. at the altitudes of 830m (Pondichore) and 1500m (Bhowali). The trend of vertical distribution of the pathogen in field soil was similar at both the places. Before potato planting, the population was nil upto 20cm, 0.6–0.7×10⁴ CFU/g in 20–30 cm, 2.2–4.8×10⁴ CFU/g in 30–50 cm and 0.9–1.8×10⁴ CFU/g in 50–60 cm. Similar population trend was found on keeping the land fallow for 7 months. During crop season, the bacterial population was the highest in top 30 cm soil (10.0–9350×10⁴ CFU/g soil) but during the seven months fallow period, it decreased drastically and became non detectable in 0–20 cm. The bacterium thus survived in 20–60 cm soil depth with a maximum population (6.5–10.6×10⁴ CFU/g soil) in 30–40 cm.