Identifier

etd-07022015-144703

Degree

Master of Science (MS)

Department

Plant Pathology and Crop Physiology

Document Type

Thesis

Abstract

Soybean (Glycine max) is one of the most widely grown crops in the world. Many pathogens attack soybeans, but of particular importance to tropical and subtropical areas such as Louisiana is Cercospora leaf blight (CLB). This disease is caused by the fungus Cercospora kikuchii and favored by high temperatures. This fungus utilizes a toxin, cercosporin, as its primary pathogenicity factor. Soybean rust (SBR), caused by Phakopsora pachyrhizi, is another common disease that occurs in Louisiana under cooler environmental conditions of spring and fall. Both diseases occur regularly in Louisiana and may result in severe yield losses. Cercospora kikuchii has recently developed fungicide resistance, and there are few if any resistant soybean cultivars. For these reasons, a search for new management strategies is underway. This study analyzed the efficacy of using plant-essential micronutrients applied foliarly to suppress CLB and SBR. This was tested in field experiments on these diseases as they occurred, as well as on isolates of C. kikuchii on amended agar media; both in randomized complete block design. Disease severity and yield were measured in field experiments, while fungal growth and toxin production were measured in vitro for C. kikuchii. Results showed that copper (150 – 300 g ha-1) and high rates of aluminum (160 – 190 g ha-1) and iron (200 – 400 g ha-1) applied foliarly to soybeans were effective in suppressing CLB severity. Iron; in the forms of iron nitrate and iron sulfate, as well as low rates of aluminum (20 – 100 g ha-1) were effective in reducing SBR severity. Agar media amended with iron (> 250 mg L-1), manganese (> 250 mg L-1) and zinc (> 125 mg L-1) inhibited fungal growth at high physiological concentrations found in soybean leaf tissue. Additions of aluminum (up to 150 mg L-1) and copper (up to 35 mg L-1) resulted in increased fungal growth. Aluminum, manganese, and zinc treatments caused higher toxin production by C. kikuchii than unamended controls. Iron at lower concentrations (up to 80 mg L-1) resulted in higher fungal growth rates than unamended controls, but this effect was reversed as concentrations exceeded this value. Conversely, toxin production was enhanced as fungal growth began to decline up to the point of complete fungal inhibition. Finally, some metals applied in vitro affected pigmentation of C. kikuchii. The normally purple pigmentation of cercosporin shifted to yellow when grown on aluminum, black when grown on iron, and green when grown on zinc-amended agar media. Boron (50 g ha-1) and iron (50 g ha-1) applied as iron sulfate were effective at reducing frogeye leaf spot (Cercospora sojina) in the one year the disease occurred. While many treatments were effective in suppressing disease severity in the field and reduced fungal growth in culture, there were no significant effects on soybean yield by treatment for either CLB or SBR. Future work identifying the mechanisms of disease suppression as well as more pronounced effects from commercial formulations and technologies surrounding minor element nutrition could yield sustainable strategies for managing soybean diseases. Research is planned on the siderophore-like properties of the toxin, cercosporin.

Date

2015

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Schneider, Raymond W.

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