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T.G. McCollum

The effects of gene B on susceptibility to chilling injury (CI) in two types of summer squash (Cucurbita pepo L.) were investigated. Two pairs of near-isogenic lines with (BB) and without (B+ B+) gene B were included in the study: `Caserta' (B+ B+) and `Precocious Caserta' (BB) of the vegetable marrow type, and `Benning's Green Tint' (B+ B+) and `Benning's Yellow Tint' (BB) of the scallop type. Respiration and ethylene evolution at nonchilling temperature were consistently higher in marrows than in scallops. Gene B had no influence on respiratory rates at nonchilling temperatures; however, the presence of gene B enhanced the chilling-induced stimulation of respiration in both marrows and scallops. Temporal differences in the patterns of chilling-induced stimulation of ethylene evolution indicated a greater sensitivity to chilling in marrows than in scallops and in both types in the presence of gene B. Electrolyte leakage was decreased by storage at chilling temperature in both marrow genotypes and was not influenced by storage temperature in B+ B+ scallops, but was increased by storage at chilling temperature in BB scallops. Therefore, electrolyte leakage was not a good CI index for these summer squash.

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John R. Yeo, Jerry E. Weiland, Dan M. Sullivan, and David R. Bryla

. The plants from both experiments were washed and divided into shoots (stems and leaves) and roots, oven-dried at 70 °C, and weighed to determine the total dry biomass. Susceptibility to phytophthora root rot was expressed as the relative difference in

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M.K. Ehlenfeldt, A.W. Stretch, and J.S. Lehman

Shoot growth of six blight-resistant highbush blueberry (Vaccinium corymbosum L.) cultivars and of one susceptible cultivar was manipulated during the primary infection period of mummy berry disease to determine if some portion of the observed resistance was based on disease avoidance. In experiments across 2 years, resistant cultivars either increased continually in susceptibility or exhibited a peak and then decreased in susceptibility as shoots elongated. In a larger experiment that included both susceptible and resistant cultivars, peaks of susceptibility were identified for `Bluejay', `Darrow', and `Jersey'. In contrast, general decreases in susceptibility were identified for `Duke', `Blueray', and `Croatan' as shoots elongated. Shoot lengths associated with peak susceptibility varied among and within cultivars across experiments. The increases in susceptibility observed at longer shoot lengths were generally small. This finding suggests that cultivars identified as resistant have intrinsic levels of resistance, but maturity and general condition of the plant tissue can also affect disease levels.

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Nathalie Delhomez, Odile Carisse, Michel Lareau, and Shahrokh Khanizadeh

Seventeen strawberry (Fragaria ×ananassa Duchesne) cultivars and six selections were tested under greenhouse conditions for susceptibility to leaf spot induced by Mycosphaerella fragariae (Tul.) Lindau. The level of susceptibility was evaluated based on maximum disease severity and area under the disease progress curve (AUDPC). The 23 genotypes were ranked based on AUDPC and grouped according to their susceptibility. Cluster analysis for AUDPC gave four groups corresponding to low, moderate, high, and very high susceptibility to leaf spot. `Annapolis', `Chambly', `Glooscap', `Redcoat', and `Veestar' consistently showed a low level of susceptibility. The selections SJ89700-1 and SJ8518-11 and `Tribute' showed a very high level of susceptibility, and the remaining cultivars were grouped as either moderately or highly susceptible.

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T.M. Gradziel

Susceptibility to brown rot, caused by Monilinia fructicola (Wint.) Honey, changed with fruit ripening in the susceptible clingstone peach [Prunus persica (L.) Batsch] cultivar Corona and two moderately resistant genotypes. Nonwounded fruit were more resistant before epidermis color break from green to yellow. Susceptibility increased from color break to the period when the epidermis had acquired a uniformly yellow ground color. With continued ripening, susceptibility remained constant or decreased, depending on genotype. A ripeness index based on fruit color permitted objective within and between genotype comparisons of susceptibility. The genetic selection for precociously developing high yellow-orange flesh color has resulted in a clingstone peach selection possessing a flesh quality suitable for processing and with high levels of brown-rot resistance at the mature-green fruit development stage.

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M. Radi, M. Mahrouz, A. Jaouad, M. Tacchini, S. Aubert, M. Hugues, and M.J. Amiot

Phenolic composition and susceptibility to browning were determined for nine apricot (Prunus armeniaca L.) cultivars. Chlorogenic and neochlorogenic acids, (+)-catechin and (-)-epicatechin, and rutin (or quercetin-3-rutinoside) were the major phenolic compounds in apricots. In addition to these compounds, other quercetin-3-glycosides and procyanidins have been detected. Chlorogenic acid content decreased rapidly during enzymatic browning, but the susceptibility to browning seemed to be more strongly correlated with the initial amount of flavan-3-ols (defined as catechin monomers and procyanidins). As chlorogenic acid is certainly the best substrate for polyphenol oxidase, the development of brown pigments depended mainly on the flavan-3-ol content.

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Sarah A. Weis and William J. Bramlage

Cool preharvest temperatures and increasing fruit maturity at harvest reduce poststorage superficial scald incidence. In the absence of cool preharvest temperatures, the role of fruit maturity in determining scald susceptibility becomes greater. Larger amounts of preharvest rainfall also contribute to reduction in scald incidence. Data from `Delicious' grown in a number of locations worldwide will be used to demonstrate this.

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B.W. Wood, C.C. Reilly, and W.L. Tedders

Fungal leaf scorch, a potentially devastating disease in pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards, was influenced substantially by irrigation and genotype. Three years of evaluating 76 pecan cultivars revealed that all cultivars exhibited scorch symptoms and that at least three classes of scorch susceptibility existed. Severity of symptoms was also much greater in nonirrigated than irrigated trees, and there were substantial differences in the concentrations of free nitrogenous compounds and free sugars in leaves between irrigated and nonirrigated trees.

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Darrell Sparks and I.E. Yates

Sooty mold washed from leaves of four cultivars of pecan [Carya illinoensis (Wangenh.) C. Koch] was quantified. The amounts of sooty mold accumulation differed significantly (P ≤ 0.05) among the cultivars. Leaf surface morphology of each cultivar was examined. A higher incidence of sooty mold was associated with cultivars having a rough, granulated leaf topography than those with smoother leaf surfaces. Characteristics of leaf surface morphology may be useful in selecting germplasms with reduced susceptibility to sooty mold accumulation.

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Steven K. Whitesides and Robert A. Spotts

Conditions were established for inducing pear (Pyrus communis L.) blossom blast caused by Pseudomonas syringae (Ps) on detached shoots. Highest incidence of infection followed occurrence of a major exotherm in the presence of Ps suspended in water drops on blossom tissue. Eight pear cultivars were evaluated for susceptibility to blossom blast, with the red-fruited `Beurré d'Anjou' sports `Gebhart' and `Columbia' least susceptible and `Doyenné du Cornice', `Beurré d'Anjou', and `Beurré Bosc' most susceptible.