Masters Theses

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Gary L. Lentz

Committee Members

Ernest C. Bernard, Craig Canaday, Charles Mullins

Abstract

The production of broccoli in the United States increased 22% from 1987 to 1992. However, broccoli production in Tennessee decreased. The number of farms producing broccoli decreased 30%, and harvested acreage decreased 89%. Part of this decline is due to bacterial soft rot, a common and potentially threatening disease in broccoli production. This disease is caused by soil-borne bacteria, primarily Pseudomonas spp. and secondarily Erwinia carotovora, after prolonged periods of rain and high humidity.

Brassica spp. are among the wide range of host plants of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois). Host plants include alfalfa, corn, cotton, legumes, mustard, strawberry, and several weed species. This research explored the relationship of the tarnished plant bug and bacterial soft rot in broccoli, and evaluated the insect fauna in broccoli fields at the West Tennessee Experiment Station (WTES), Jackson, TN, and the Plateau Experiment Station (PES), Crossville, TN. Broccoli was evaluated in late spring (WTES), summer, and early fall (PES).

Two plantings of broccoli, differing by tillage and irrigation practices, were grown in each season. One crop, the soft rot nursery (SRN), was conventionally tilled and watered with overhead irrigation to induce bacterial soft rot. In the test field, broccoli seedlings were planted into untilled soil and watered with drip irrigation. Two soft rot-susceptible cultivars. Green Comet and Premium Crop, were used for treatments, and plants of a resistant cultivar, Arcadia, served as border rows. A planting of mustard served as a tarnished plant bug reservoir.

Test treatments consisted of tarnished plant bugs placed on diseased heads in the SRN, confined to the broccoli head with organza cages, and allowed to feed for two days, then removed and placed on healthy broccoli heads in the test plot. Two control treatments were included. One control consisted of insects placed on disease-free heads in the test plot (insect control treatments), and the other consisted of disease-free heads caged without insects, also in the test plot (plant control treatments). Five and ten days after caging in the test field, disease severity was rated on a scale of 0 to 5, where 0=no symptoms, 1=1%, 2=10%, 3=30%, 4=60%, and 5=100% of head affected with necrotic or water-soaked appearing areas.

In spring, soft rot was observed in 65% of the test treatment broccoli heads. Both control groups were heavily diseased (81% for insect control treatments, 77% for plant control treatments). In the summer crop, 24% of test treatments, 35% of insect controls, and 33% of the plant control treatments developed soft rot symptoms. In the fall planting, 40% of test treatment broccoli heads, 50% of the insect controls, and 41% of the plant controls displayed disease symptoms. Disease incidence did not differ among treatments and controls in each season; thus, the presence of the tarnished plant bug did not increase the incidence of bacterial soft rot in broccoli.

Faunal surveys were performed in spring and early fall months, with a total of insect families collected. Insect numbers in the spring increased as sampling dates progressed, but declined in the late summer months. Pest insects collected included members of Chrysomelidae (leaf beetles), Miridae (tarnished plant bug), Pentatomidae (harlequin and stink bugs), Pieridae (imported cabbage worm) and Plutellidae (diamondback moth). Beneficial and predatory families included Anthocoridae (minute pirate bugs), Lygaeidae (big-eyed bugs), and Pteromalidae (pteromalid wasps). Differences were found in the number of families and of insects in each field. More insect families occurred in spring and late summer in test plots than in the SRN. Among locations, the spring season had a higher number of families than the late summer season. Further investigations of leaf damaging insects that may vector pathogens or increase disease incidence in broccoli is warranted.

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