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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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David L. Ehret, Carol Koch, Jim Menzies, Peter Sholberg, and Tim Garland

Foliar sprays of a nonswelling chlorite mica clay were applied to leaves of greenhouse-grown long English cucumber (Cucumis sativus L.) plants, either before or after an artificial inoculation with powdery mildew [Sphaerotheca fuliginea (Schlech.:Fr.) Poll.] and to field-grown wine grapes (Vitis vinifera L.) before natural inoculation with powdery mildew [Uncinula necator (Schwein.) Burrill]. In all cases, the clay sprays did not eradicate the pathogen, but resulted in significant reductions in disease severity. In cucumber, a single spray of 0.5% clay reduced colony numbers on leaves by up to 60%. Spraying after inoculation was generally more effective than spraying before inoculation. In grapes, repeated sprays of either 2% or 4% clay were applied through the season to `Reisling' and `Chancellor' vines. Four percent clay reduced the amount of leaf surface covered by mildew by 22% in `Reisling' and 51% in `Chancellor'. Both concentrations reduced the incidence of mildew on clusters and canes. No treatment effects were observed on fruit quality. Our results demonstrate that foliar sprays of clay can reduce the severity of Sphaerotheca fuliginea and Uncinula necator on cucumbers and grapes, respectively.