Relationship of Seedbed Fertilization and Fumigation to Infection of Sour Orange Seedlings by Mycorrhizal Fungi and Phytophthora parasitica1

Abstract

Preplant fumigation with methyl bromide, sodium N-methyl dithiocarbamate, and heat treatment temporarily eliminated Phytophthora parasitica Dast. from most seedbeds, prevented damping-off and reduced root rot on sour orange ( Citrus aurantium L.) seedlings. Stunting of seedlings due to elimination of mycorrhizal fungi occurred frequently following these treatments. Seedlings in non-fumigated, captan-treated seedbeds were not affected by damping-off or lack of mycorrhizae, but growth was reduced compared to that in fumigated beds because of severe root infection by P. parasitica . Application of 500 to 750 kg P/ha to fumigated seedbeds temporarily stimulated the seedling growth, and larger seedlings resulted when mycorrhizae subsequently became reestablished in the beds. Application at these rates did not overcome the stunting problem if mycorrhizae failed to become re-established. Application of 1900 to 2000 kg P/ha stimulated early growth of seedlings in fumigated seedbeds but inhibited mycorrhizal formation and induced copper deficiency symptoms. Fumigation programs which are effective in eradicating P. parasitica also eliminate mycorrhizal fungi and induce severe stunting symptoms which are partially reversible by application of P, but completely reversible only by inoculation with mycorrhizal fungi.