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Timothy Coolong, William Randle, and Ronald Walcott

Onion (Allium cepa L.) is an economically important vegetable in the United States. Though considered a minor crop in terms of total acreage, onions have high value when compared to other crops and, nationally, their value approaches $800 million. Because harvested onions are routinely stored for long periods, disease can be a major obstacle to the industry. The primary disease reported in stored onions is botrytis neck rot caused by the fungus Botrytis allii (syn. B. aclada). Losses from neck rot can approach 35% of the stored crop. In order to accurately quantify the level of B. allii inoculum in bulbs at harvest to be able to predict potential botrytis neck rot in storage, a quantitative real-time PCR test to quantify levels of B. allii DNA present in onion bulb tissue has been developed. We have employed the TaqMan real time PCR assay and report log-linear (R 2= 0.9915) relationship between B. allii DNA concentration and cycle threshold (Ct) value with a detection limit of 5 pico gram/microliter DNA. In addition, a log-linear standard curve plotting mycelial dry weight against Ct value has been developed to allow prediction of mycelial weight in onion tissue at harvest. Currently, the ability of this test to predict botrytis neck rot during storage is being tested.

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Timothy W. Coolong, Ronald R. Walcott, and William M. Randle

A real-time polymerase chain reaction (PCR) assay has been developed for the detection and quantification of Botrytis aclada (Fresenius), a causal agent of neck rot in onion (Allium cepa L.) bulbs. The assay uses TaqMan probe-based chemistry to detect an amplicon from the L45-550 region of B. aclada while using a DNA sequence from the onion serine acetyl transferase gene (SAT1) as a control. The assay detection limits for B. aclada and onion were 10 pg·μL−1 of genomic DNA. The detection limit for lyophilized B. aclada mycelium was 1 μg. The presence of onion tissue in the samples did not affect the performance of the real-time PCR assay. The assay distinguished among different amounts of B. aclada mycelium growing on onion disks that were inoculated with 0, 102, or 104 B. aclada conidia. Visual observations during the incubation period corresponded with changes in real-time PCR results. This assay could be used to determine the amount of B. aclada mycelium in bulbs during growth, harvest, and storage, thus giving researchers an objective and efficient tool by which to quantify the growth rate and virulence of B. aclada strains in vivo.

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Juan C. Díaz-Pérez, William M. Randle, George Boyhan, Ronald W. Walcott, David Giddings, Denne Bertrand, Hunt F. Sanders, and Ronald D. Gitaitis

Sweet onions (Allium cepa L.) are typically grown on bare soil and irrigated with high-pressure systems such as sprinklers or center-pivots. The objective of this study was to determine the effects of irrigation system and mulch on bolting, bulb yield and bulb quality over 3 years. The experimental design was a split plot, where the main plot was irrigation system (drip or sprinkler) and the subplot was the type of mulch (bare soil, black plastic film or wheat straw). The results showed that individual bulb weight and bulb yields under drip irrigation were similar to those under sprinkler irrigation. Plants grown on bare soil had the highest total yield during the three seasons and among the highest marketable yield. There were no consistent differences in the bulb number or yield of plants on plastic film mulch compared to those of plants on wheat straw mulch. Plants on wheat straw mulch had reduced foliar nitrogen content. Variability in yields among mulches and seasons was partly explained by changes in seasonal root zone temperature and soil water potential. Total and marketable yields and weight of individual bulbs increased with increasing root zone temperatures up to an optimum at 15.8 °C, followed by reductions in yields and individual bulb weight at >15.8 °C. Onion bolting increased with decreasing foliage nitrogen content, with plants on wheat straw having the highest bolting incidence. Bolting also increased with decreasing root zone temperatures for the season. Total and marketable yields increased with decreasing mean seasonal soil water potential down to -30 kPa. Irrigation system and mulches had no consistent effect on the soluble solids content or pungency of onion bulbs.