Document Type

Thesis - Open Access

Award Date

2025

Degree Name

Master of Science (MS)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Madalyn Shires

Abstract

Cucurbit crops comprise the largest area in vegetable production in South Dakota (USDA 2022). Information on the prevalence, distribution, and fungicide sensitivity of their fungal pathogens are unknown. This study characterized the major foliar fungal species affecting cucurbits in South Dakota through a 2-year, state-wide survey. Between 2023 and 2024, 789 leaves were recovered from pumpkin, cucumber, watermelon, squash, and bottle gourd. Alternaria spp. (70.72% of leaves) Fusarium spp. (6.84%), Podosphaera xanthii (4.18%), both Alternaria spp. and Fusarium spp. (3.42%), and both Alternaria spp. and Podosphaera xanthii (1.90%), Stagonosporopsis spp. (1.27%) and other genera of fungi (4.94%) were identified using a combination of microcopy and sequencing of the internal transcribed spacer (ITS) region of ribosomal DNA , with 6.72% of leaves exhibiting no sporulation. A subset of Alternaria spp. isolates (n=238) recovered from cucumber and pumpkin leaves were selected for multi-locus ((translation elongation factor-1α (TEF1), glycerldehyde-3-phosphate dehydrogenase (GAPDH), and RNA polymerase II core subunit (RPB2)) sequencing to determine species. Unique haplotypes (n=46) of Alternaria spp. were identified by concatenating loci. A combined maximum likelihood and Bayesian phylogeny revealed 18 haplotypes, comprising 133 isolates, clustered with Alternaria alternata and A. tenuissima (syn. A. alternata). Haplotype visualization with minimum spanning networks identified no influence of region, production environment, or host on genetic diversity. Koch’s postulates were fulfilled by inoculating cucumber ‘Straight 8’ plants inoculated with four representative isolates of A. alternata. Morphological and phylogenetic comparison supported the identification of fungal isolates as A. alternata. Three representative isolates of the most abundant A. alternata haplotypes and haplotype 14 from the survey screened in-vitro for fungicide sensitivity to pyraclostrobin (FRAC 11) and penthiopyrad (FRAC 7). All representative isolates of A. alternata screened were sensitive. A randomized complete block cucumber ‘Straight 8’ field trial was conducted in 2024 and 2025 under natural Alternaria spp. pressure to assess efficacy of fungicide applications on disease progression or yield. Treatments A) Kocide (M01), B) Fontellis (FRAC 7), Inspire Super (FRAC 3 + FRAC 9), or Quadris (FRAC 11) rotated weekly with Bravo Weatherstik (M5), C) Fontellis (FRAC 7), Switch (FRAC 9 + FRAC 12), or Cabrio (FRAC 11), rotated weekly with Bravo Weatherstik (M5) controlled disease progression compared to treatment D) water control. Treatment C was significantly more effective at reducing foliar disease progression than other treatments. No significant differences in marketable yield or USDA grades were observed among treatments.

Library of Congress Subject Headings

Cucurbitaceae -- Diseases and pests -- South Dakota.
Fungal diseases of plants -- South Dakota. 
Foliage plants -- Diseases and pests.      

Publisher

South Dakota State University

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Rights Statement

In Copyright