Date of Award
Doctor of Philosophy (PhD)
Plant and Environmental Science
Anthony P. Keinath
W. Patrick Wechter
Sandra E. Branham
Steven N. Jeffers
Peter S. Ojiambo
Cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, is a major constraint on melon (Cucumis melo) production in the eastern United States, but P. cubensis populations infecting C. melo are not well characterized. Between 2019 and 2021, 248 P. cubensis isolates were collected from cultivars Halona and Hale’s Best Jumbo in ten states. Isolates were genotyped with nine microsatellites, and mating type and clade were determined. All isolates displayed an inverse mating type / clade relationship: 90.3% and 9.7% of the isolates were mating type A1 / Clade 2 and mating type A2 / Clade 1, respectively. Both types of isolates were recovered from South Carolina and Maryland, with both types recovered from a single plot in South Carolina. Genetic differentiation was low among years, states, and cultivars, with moderate differentiation between mating type / clade combinations. Grafting melon onto cucurbit rootstocks was evaluated as a management strategy and to determine its effect on P. cubensis populations. In controlled experiments, grafting ‘Halona’ onto ‘Carnivor’, ‘Carolina Strongback’, and ‘Pelops’ significantly reduced disease severity on the second true leaf but not on the first true leaf, suggesting a resistance mechanism in later developmental stages after grafting. However, plants in both grafted and non-grafted treatments were equally susceptible to CDM in the field. Low genetic differentiation was observed between P. cubensis populations recovered from plants in grafted and non-grafted treatments. This study demonstrated the presence of one major P. cubensis genotype infecting melon in the eastern United States, low genetic diversity across the country, and a negligible influence of grafting on pathogen populations.
Currently, no C. melo cultivars have significant levels of resistance. Additionally, little is understood about the genetic basis of resistance in C. melo. Recombinant inbred lines (RILs; n = 169) generated from a cross between the resistant melon breeding line MR-1 and susceptible cultivar Ananas Yoqne’am (AY) were phenotyped for CDM resistance against a P. cubensis isolate identified as Clade 1 / mating type A2 in both greenhouse and growth chamber studies. A high-density genetic linkage map with 5,663 binned SNPs created from the RIL population was utilized for quantitative trait loci (QTL) mapping. Nine QTLs, including two major QTLs, were associated with CDM resistance. Of the major QTLs, qPcub-10.1 was stable across growth chamber and greenhouse tests whereas qPcub-8.2 was detected only in growth chamber tests. qPcub-10.1 co-located with an MLO-like protein coding gene, which has been shown to confer resistance to powdery mildew and species of Phytophthora in other plants.
The RIL population was also screened with a Clade 2 / mating type A1 isolate of P. cubensis from the 2004 CDM epidemic in the eastern United States. Five QTL, including two major QTL, were associated with CDM resistance. qPcub-10.3-10.4 was identified only in greenhouse tests whereas qPcub-8.3 was consistently identified in both greenhouse and growth chamber tests. These two major QTL were identified on the same chromosomes (8 and 10) but in different locations as two major QTL previously identified for resistance to an isolate of Clade 1 / mating type A2. Kompetitive allele specific PCR (KASP) markers were developed for these four major QTL (two for each mating type/Clade) and validated in the RIL population followed through QTL mapping. These markers will provide melon breeders a high-throughput genotyping toolkit for breeding for resistance to P. cubensis.
A nationwide, quantitative synthesis of fungicide efficacy data on management of cucurbit downy mildew (CDM) caused by P. cubensis is needed to broadly evaluate fungicide performance. Three-level, three-level meta-regression, and network meta-analyses were conducted on data from 46 cucumber (Cucumis sativus) CDM fungicide efficacy studies conducted in the eastern United States retrieved from Plant Disease Management Reports published between 2009 and 2018. Three response variables were examined in each analysis—disease severity, marketable yield, and total yield—from which percent disease control and percent yield return compared to non-treated controls were calculated. Moderator variables used in the three-level meta-analysis or three-level meta-regression included year, disease pressure, number of fungicide applications, and cucumber type (slicing or pickling). In the network meta-analysis, fungicides were grouped by common combinations of Fungicide Resistance Action Committee (FRAC) Codes and modes of action (MOA). Overall, fungicides significantly (P < 0.001) reduced disease severity and increased marketable and total yields, resulting in a mean 54.0% disease control and 61.9% marketable and 73.3% total yield return. Subgroup differences were observed for number of fungicide applications, control plot disease severity, and cucumber type for marketable yield. Based on the meta-regression analysis for disease severity by year, fungicides have been significantly decreasing in efficacy from 2009 to 2018, potentially indicating a broad development of fungicide resistance over time. Treatments containing quinone inside inhibitors, pyridinylmethyl-benzamides, and protectants and treatments containing oxysterol binding protein inhibitors and protectants most effectively reduced disease severity. The most effective fungicide combinations for disease management did not always result in the highest yield return.
Toporek, Sean, "Cucurbit Downy Mildew of Melon: Pathogen Diversity and Host Resistance" (2022). All Dissertations. 3027.
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