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Peter L. Sholberg and Paul Randall

Stored apples and pears are subject to blue and gray mold decay incited by Penicillium expansum and Botrytis cinerea respectively. Hexanal, a C6 carbon aldehyde, used as a vapor provided effective control of both blue and gray molds in laboratory experiments on apple slices. A preliminary trial with ‘Anjou’ pears in bins showed that hexanal was not corrosive and could reduce gray mold in pears stored for 7 months. However details on the correct procedure for fumigating pome fruit were lacking, and further studies were needed to develop a reliable fumigation strategy. In trials with inoculated fruit, hexanal inactivated conidia of B. cinerea contaminating the pear surface when used at a rate of 2 mg·L−1 for 24 hours or 4 mg·L−1 for 18 hours. It was less effective on ‘Gala’ apples inoculated with conidia of P. expansum, but reduced blue mold decay to low levels at 15 ºC. On the other hand, hexanal increased gray and blue molds when used after wounds were made in inoculated fruit. The use of a preharvest treatment with cyprodinil (0.62 g·L−1) reduced both blue and gray molds in wounds with or without hexanal fumigation. Thus a strategy for controlling postharvest decay was developed by which fruit were treated 2 weeks before harvest with cyprodinil, followed by fumigation with hexanal immediately after harvest. The use of this strategy on ‘Anjou’ pears produced the highest number of mold-free fruit in 2003 and the least amount of gray and blue mold decay in 2003 and 2004 on pears stored for 4 months. Wounded apples only developed 1% rot compared with 10% in the control, indicating that hexanal fumigation of stored apples reduced contamination. Monitoring hexanal during fumigation showed that hexanal concentration declined slowly over a 24-hour period and could accurately be described by a third-order polynomial equation. Hexanal fumigation at low rates (2–3 mg·L−1) was not phytotoxic and improved aroma in ‘Anjou’ pears and ‘Gala’ apples with no harmful effects on apple or pear firmness, pH, titratable acidity, or soluble solids.

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Peter L. Sholberg, Paul Randall, and Cheryl R. Hampson

Acetic acid (AA) fumigation of rootstocks and dormant shoots was explored as a method of eliminating plant pathogens from propagation material. Dormant shoots were tested in early winter to determine the rate of AA vapor that they could tolerate before being damaged. Apricot (Prunus armeniaca), apple (Malus ×domestica), and peach (Prunus persica) shoots collected from a single site in Dec. 1999 tolerated 30, 12, or 6 mg·L–1 AA, respectively. Vineland 3 (V3) and Malling-Merton 106 (MM.106) rootstock liners fumigated with 1 mg·L–1 AA were adequately surface-sterilized although the effect on growth was not recorded. A similar experiment with Malling 9 (M9) rootstocks showed that 12 mg·L–1 AA would eliminate most surface microorganisims from roots although it delayed shoot growth when the trees were planted. The higher 15 mg·L–1 rate delayed tree growth and appeared to kill some trees. The 12 mg·L–1 rate prevented growth of Erwinia amylovora and Pseudomonas syringae pv. syringae bacteria on shoots even when an enrichment technique was used to detect them. Finally, when 96 `Jonagold' apple shoots known to be infected by Podosphaera leucotricha were fumigated with AA in 2001, none developed powdery mildew, although 99% of the control shoots did. These promising results suggest that further research should be done toward adapting AA fumigation for use by commercial nurseries.

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Paul Randall, Peter Sholberg, Gary Judd, and Joan Cossentine

Fumigation with glacial acetic acid (AA) vapor successfully kills post-harvest pathogens on tree fruits and berries and reduces their spoilage in storage. In this study, we investigated whether a similar approach could be implemented to eradicate diapausing larvae of the codling moth, Cydia pomonella (L.), from fruit harvest bins they commonly infest. In 24-h tests conducted in 0.023-m3 fumigation chambers using two concentrations of vaporized AA [117,360 and 174,823 cumulative parts per million-hours (ppm-h)], mortality of diapausing larvae was 81% and 100%, respectively. A similar 24-h exposure to a 61,940 cumulative ppm-h treatment of AA caused no mortality. A 24-h fumigation of diapausing codling moth larvae placed in scaled-down plastic fruit bins treated with 55 mL of AA evaporated into a 1-m3 chamber caused 100% mortality. The same fumigation treatment of artificially infested, scaled-down wooden fruit bins caused no significant mortality of test larvae. Atmospheric concentrations of AA vapor in 1-m3 fumigation chambers containing wooden bins could not be maintained at levels necessary to cause insect death, even after multiple injections of AA. We hypothesize that either the wood or the moisture contained therein absorbs or in some other way interacts with the AA vapor. The use of AA as a fumigant targeting codling moth larvae in wooden bins is not practical or economical at this time. Fumigation of plastic fruit bins with AA would provide an economical and environmentally friendly method of killing diapausing codling moth larvae. The successful disinfestations of plastic fruit bins of codling moth would prevent these bins from being an external source of infestation, thereby decreasing overall codling moth infestation in orchards, which in turn benefits current density-dependent management practices used for the area-wide control of codling moth. chemical name: Glacial Acetic Acid (AA).