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Temporal Changes in Forage Alfalfa Associated with Insect and Disease Stress

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The effects of insects, fungus diseases, and their combined stress on stand persistence and dry matter production in irrigated forage alfalfa (Medicago sativa L.) were evaluated over 5 yr. Plant populations in plots subjected to disease or insect and disease stress declined 36% faster than in plots maintained pest free or plots subjected to unilateral insect stress. Disease, primarily Stagonospora crown rot incited by Stagonospora meliloti (Lasch.) Petr., was the principal contributory agent to long term plant mortality. Insect-imposed stress had no unilateral effect on stand deterioration. Losses in dry matter production were dependent on nonpersistent insect and foliar disease attack and the persistent stand reduction caused by Stagonospora crown rot. Insects and foliar diseases were most common before first cutting. Although these pests were not biologically active in later cuttings, the injury they caused before the first harvest continued to reduce yields for several additional cuttings. Hypera brunneipennis (Boheman), Acyrthosiphon pisum (Harris), and A. kondoi Shinjiwere the most destructive insects. Common leaf spot and spring black stem, caused by Pseudopeziza medicaginis (Lib.) Saccoand Phoma medicaginis Malbr. & Rhoum. var medicaginis, respectively, were the most significant foliar diseases. Total forage yield over 5 yr was reduced 18 and 22% by insects and diseases respectively; their combined effect reduced yield 36%. Stress attributable to fungal diseases alone and the combined stress of insects and diseases shortened productive stand life by 1 and 2 yr, respectively, compared with plots maintained insect and disease free.

Document Type: Research Article

Publication date: August 1, 1991

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  • Journal of Economic Entomology is published bimonthly in February, April, June, August, October, and December. The journal publishes articles on the economic significance of insects and is divided into the following sections: apiculture & social insects; arthropods in relation to plant disease; forum; insecticide resistance and resistance management; ecotoxicology; biological and microbial control; ecology and behavior; sampling and biostatistics; household and structural insects; medical entomology; molecular entomology; veterinary entomology; forest entomology; horticultural entomology; field and forage crops, and small grains; stored-product; commodity treatment and quarantine entomology; and plant resistance. In addition to research papers, Journal of Economic Entomology publishes Letters to the Editor, interpretive articles in a Forum section, Short Communications, Rapid Communications, and Book Reviews.
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