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Lima bean (Phaseolus lunatus L.) is one of the most important vegetables grown in the mid-Atlantic region of the US. Delaware has more acreage of land per year allocated for lima bean production, primarily used for processing, than any other state. The yield of lima bean is severely affected by white mold caused by Sclerotinia sclerotiorum. Currently, there is limited information on the population diversity of S. sclerotiorum in the mid-Atlantic region compared to other production regions, such as New York. Due to lack of research conducted in mid-Atlantic region, there are no specific fungicide application guidelines for lima beans. Improved understanding of the diversity within or among S. sclerotiorum isolates obtained from different geographical regions and various hosts will assist selecting representative isolates for use in developing improved disease management strategies including development of host resistance and effective fungicide guidelines. The main objectives of this research includes 1) studying the diversity of S. sclerotiorum isolates from lima bean and other crops in the mid-Atlantic and other regions and 2) improving disease management guidelines for white mold.

Forty-two S. sclerotiorum isolates were collected from ten crops within eight different states in the US. The diversity of the collected isolates was evaluated for, a) lesion length and oxalic acid production on nine cultivars (five lima bean, two soybean, and two common bean), b) mycelial compatibility groupings (MCGs) and molecular characterization, and c) fungicide sensitivity (in-vitro) to two concentrations of boscalid, cyprodinil, fludioxonil, fluazinam, prothioconazole, and thiophanate-methyl . A field study also evaluated six application timings of boscalid, at 20% flowering, 100% flowering, two weeks, and three weeks after 20% flowering, a double applications, and non-treated control for management of white mold in lima bean.

The collected isolates produced different lesion lengths, which were dependent on the crops and cultivars tested. Isolate 13, which was obtained from soybean, NJ, was the most aggressive in causing the longest lesions. Isolate 6, which was obtained from snap bean, DE, was the least aggressive isolate in causing the shortest lesion. Isolates were also significantly differed in oxalic acid production. Isolate 13 and isolate 4 were the highest oxalic acid producers. Seventy-five percent of the MCGs interactions were incompatible. The Shannon index (Ho) values of the MCGs were between 0 - 0.35 indicate that there is high diversity among the S. sclerotiorum isolates tested and that the isolates may reproduce sexually rather than via vegetative reproduction. The molecular characterization of the sequences examined at the ITS region and β-tubulin gene provided high sequence similarities among our isolates. The low variability did not allow us to evaluate differences among isolates. The molecular/genetic variability within the population was 1 - 2%. To evaluate the fungicide sensitivity of isolates, the percent reduction in mycelial growth (PRMG) of each isolate in presence of Dimethyl Sulfoxide (DMSO) and the fungicide was compared to the control (the isolate grown in the presence of DMSO). The collected isolates varied in PRMG to all six fungicides. The PRMG of the isolates differed at the two concentrations, except for cyprodinil and fludioxonil. There was a significant interaction between the concentrations and isolates sensitivity to all fungicides except boscalid and thiophanate-methyl. Correlations were conducted to identify associations between fungicide sensitivities, lesion length, and oxalic acid production. Isolates’ sensitivity to boscalid was negatively correlated to lesion length (r=-0.28397; P=0.0004) and oxalic acid production (r=-0.23370; P=0.0040). In addition, fungicide sensitivity to fluazinam was positively correlated to fungicide sensitivity to prothioconazole (r=0.35695; P<.0001) and thiophanate methyl (r=0.46247; P=<.0001). Likewise, fungicide sensitivity to fludioxonil was positively correlated to fungicide sensitivity to boscalid (r=0.19309; P=0.0179) and thiophanate methyl (0.28760; P=0.0004). However, fluazinam sensitivity was negatively correlated to boscalid sensitivity (r= -0.20119; P=0.0136). In the fungicide timing evaluation, the disease incidence was reduced by 6.4%, 5.4%, 3.9%, and 7.6% compared to no treatment when fungicides were applied at 20% flowering (P<0.0001), 100% flowering (P<0.0001), one week after 100% flowering (P<0.0128), or at 20% and 100% flowering (P<0.0001), respectively. These application timings also reduced the disease severity by 5.7%, 8.0%, 6.0%, and 7.0% compared to no treatment, respectively. Earlier, within 2 weeks of 20% flowering and double fungicide treatment reduced disease incidence and disease severity and improved yield of lima bean.

This research improves our understanding of the diversity of the mid-Atlantic Sclerotinia sclerotiorum population and suggests that, during selection of resistant lima bean cultivars, plants should be challenged by an array of S. sclerotiorum isolates, not just one putatively aggressive or susceptible isolate. My research also establishes guidelines for timing of fungicide management of white mold and developed baseline data on isolate sensitivity to fungicides.