Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Febina Mathew

Second Advisor

Adam Varenhorst


Soybean Glycine max (L.) Merr., is an important crop grown in South Dakota, with an estimated production of $2.33 billion in 2015. However, soybean production in South Dakota is compromised by four pests, the fungal pathogens Diaporthe longicolla (Hobbs) Santos, Vrandecic, and Phillips and Diaporthe caulivora (Athow and Caldwell) Santos, Vrandecic, and Phillips, the soybean cyst nematode Heterodera glycines Ichinohe (SCN), and the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae). Additionally, these pest can co-occur in fields within South Dakota and the implications associated with interactions among these pests are unknown. We hypothesized that both Diaporthe species would interact with H. glycines, and that D. longicolla would not interact with A. glycines. To test our hypotheses, both studies were set up as a completely randomized design in the greenhouse. For the Diaporthe-SCN interaction, five treatments were designed (Diaporthe alone, SCN alone, co-inoculation of SCN and Diaporthe, and Diaporthe inoculated either 15 days before or after SCN). For the D. longicolla- A. glycines interaction, eight treatments were designed that consisted of two infestation times (Inducer: V1; response: seven days later). Plants were infested with either five A. glycines, 2000 eggs of SCN, and a four mm plug infested with Diaporthe depending on the treatment and experiment. We assessed stem length, lesion length, and SCN reproduction for the Diaporthe-SCN interaction and lesion length and aphid counts for the D. longicolla- A. glycines interaction. Our results showed that SCN reproduction was reduced by 90% when either fungus preceded SCN. Additionally, when SCN preceded D. longicolla, we see an increase in lesion length of 76% or greater on soybean stem. When SCN preceded D. caulivora for experiment one, we see a decrease in lesion length of 35%. Additionally, D. longicolla and A. glycines did not interact on soybean. However, we observed a potential compensatory effect from soybean plants in the concomitant infestation of both D. longicolla and A. glycines, where aphid counts were reduced by 47% when both pests were introduced together. Interactions among pests is relatively understudied, such studies may lead to new management strategies for soybean pest and disease complexes.

Library of Congress Subject Headings

Soybean cyst nematode -- South Dakota.
Soybean -- Diseases and pests -- South Dakota.
Diaporthe -- South Dakota.
Aphids -- South Dakota.


Includes bibliographical references



Number of Pages



South Dakota State University



Rights Statement

In Copyright