Growth and survival of root-nodule bacteria in legume inoculants stored at high temperatures

The maximum temperatures to which legume inoculants were exposed when despatched to five destinations in Australia (36–39°C) and nine in Indonesia (29–42°C) were used to define experimental storage conditions to study survival of root-nodule bacteria in peat culture. Cultures of three strains of Bradyrhizobium and two strains of Rhizobium were prepared in sterilised peat at a moisture potential of −log10 4.5 Pa and incubated at 25°C for 14 days. They were then stored at either 25, 30, 35 or 40°C for 28 days. There was no significant effect of temperature in the first 5 days. By day 7, the numbers of the clover strain declined significantly at temperatures between 30 and 40°C and those of the medic, lupin and both soybean strains at 40°C. Numbers of all strains exceeded log10 8.3 g−1 peat when stored at 35°C for 28 days; the two strains for soybeans exceeded log10 9.0 g−1. Although when stored at 40°C for 2 weeks, the numbers of the five strains declined, they all multiplied rapidly when returned to 25°C. This response was repeated when the same packets were subjected to this regimen a second time. After a third cycle, recovery when returned to 25°C was slower than before but numbers still exceeded log10 7.6 g−1 peat. Storage at 40°C for 42 days did not affect the infectivity of the strains. These results indicate that peat-based cultures may be distributed with safety in the tropics from a centralised production centre. Regional reincubation at a favourable temperature (25°C) for growth is recommended when temperatures inside the packaging are likely to have exceeded 35°C for 7 days.