The Allelopathic Potential of Pigeon peas in Puerto Rico

Abstract

In laboratory and field tests, pigeon pea leaf litter was associated with root tip necrosis, stunting, and reduced germination in seeds of soybeans, lablab beans, pigeon peas, and local weeds. Leaf litter harbored, and was substrate for, species of the fungal genera Pythium, Fusarium, and Rhizoctonia, which are often associated with similar symptoms in other plants. Extraction of leaf litter, with potassium hydroxide, eliminated fungal pathogens; however, the diluted and pH adjusted extracts (pH = 6.2) still caused typical symptoms of root necrosis, stunting and germination loss. Weed species varied in their sensitivity to pigeon pea litter. Grasses were inhibited more than dicotyledons. The germination, emergence, and survival of pigweed increased under pigeon pea leaf litter. Accumulation of pigeon pea leaf litter commenced with the closing of the crop canopy and the shading, dying, and dropping of the lower leaves in the field. In fields planted to high densities of pigeon peas on the south coast of Puerto Rico, pigweed became the dominant weed under the pigeon pea canopy at harvest. Yield of residues was greater for stems than for roots or leaves and harvest index was low (26% based on pod yield and 15% based on dry seed yield). Minimum thresholds for detecting plant inhibition by leaf litter in the laboratory was 5-10 grams of litter per square meter surface area, and approximately 25 grams of litter equivalents when using potassium hydroxide extracts. At harvest time, leaf litter in the field was greater than 150 grams per square meter which was close to the optimum levels for plant inhibition found in laboratory tests (100-200 grams of litter per square meter).