The effects of foliar applications of seaweed extracts on plant growth and pest resistance

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Virginia Polytechnic Institute and State University


A commercial extract of kelp, Ascophyllum nodosum (L.) LeJolis, was applied as a foliar spray on third and fourth cutting alfalfa (Medicago sativa L.) in fertilized and nonfertilized plots. Kelp treatments alone had no effect on alfalfa yield and quality, on tissue levels of nitrogen, phosphorus, and potassium, or on populations of potato leafhopper (PLH), Empoasca fabae (Harris). For the fourth cutting, fertilizer and kelp*fertilizer effects were observed. Yield and tissue levels of potassium were higher and PLH numbers were lower in fertilized plots. In fertilized plots, kelp temporarily reduced numbers of PLH adults one week after kelp treatment, followed by lower numbers of PLH nymphs. In nonfertilized plots, PLH adult levels were initially higher, followed by increased numbers of nymphs. The number of fungal leaf spots on the top five leaves was lower on fertilized plants than on nonfertilized plants, on kelp-treated plants in nonfertilized plots, and on plants treated with the insecticide dimethoate compared to nontreated plants.

Extracts of kelps, A. nodosum and Durvillea potatorum (Labill.) Aresch., were applied to cantaloupe (Cucumis melo L.) plants grown under three nutrient regimes. Kelp treatment lowered the tissue concentration of total nitrogen in one instance, but had no effect on nitrogen deficiency symptoms which were present in all plants. Kelp treatment slightly reduced phosphorus and increased boron and magnesium concentrations in plant tissue. Both kelp sprays increased dry root weights; A. nodosum also increased dry shoot weights and root:shoot ratio; D. potatorum decreased root rot. These kelp effects were most prominent in plants receiving the mid-level nutrient concentration.