Peer review

In the multiple cropping systems used for bean (Phaseolus vulgaris L.) production in Latin America, seed treatment is often needed for control of soil pathogens and pests, but can be incompatible with Rhizobium inoculation. This study looks at the interaction between Rhizobium and seed protectant. One laboratory and three field studies are reported. The survival of R. phaseoli on bean seeds treated with PCNB, thiram, or captan was measured under laboratory conditions using standard serial dilution and plate count techniques. At Popayan, Colombia (typic dristandept, pH 4.8) the nodulation and nodule development of bean seeds treated with PCNB, thiram, captan, and furadan was evaluated, and different inoculation methods, including a granular soil applied inoculant, compared. Nodulation of beans protected with PCNB or thiram, and inoculated with seed or soil-applied rhizobia was also studied at La Selva, Colombia (typic dristandept, pH 5.1). While contact with PCNB, thiram or captan on seeds reduced Rhizobium survival, with captan most toxic, PCNB-treated seeds maintained counts of l03 rhizobia/g seed until almost 48 hours after inoculation. Field results demonstrated the delicate balance between inoculant and seed protectant. At Popayan, nodule number/plant was not affected when thiram, furadan, or PCNB-treated seeds were inoculated and promptly planted, but delay between inoculation and planting did reduce the nodulation of seeds protected with PCNB or thiram. At La Selva, all fungicide treatments lowered nodule number/plant. Seed protection with PCNB enhanced nodule development at Popayan and reduced root rot incidence at La Selva. Despite the somewhat acid soil conditions the best nodulation at both Popayan and La Selva was attained with granular soil-applied inoculants. Further studies on this method of inoculation for beans are warranted.

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