AbstractPythium aphanidermatum, a previously unreported species for Puerto Rico, and P. deliense were frequently isolated from sugarcane fields in the southern coast of Puerto Rico where symptoms of ratoon stunting disease were also observed. P. acanthicum, P. butleri, P. echinocarpum, P. graminicola, P. oligandrum and P. periplocum were also isolated. The pathogenicity of these Pythium species to sugarcane variety PR 1152 was demonstrated in replicated laboratory and greenhouse tests. P. aphanidermatum alone significantly reduced dry weights of top and root growth of sugarcane variety PR 1085. Reductions in dry weights of top and root growth incited by the combination of P. deliense and P. aphanidermatum were the same as those caused by P. aphanidermatum alone. However, the reductions in dry weights of top by inoculation of sugarcane with both P. aphanidermatum and the ratoon stunting disease causal agent were greater than those caused by inoculation with either one alone.
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