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Esmaeil Fallahi and S. Krishna Mohan

The influence of four rootstocks and four levels of nitrogen supply on tree growth, precocity, fruit quality (size and color), leaf mineral concentrations, and fire blight [Erwinia amylovora (Burrill) Winstow et al.] severity in `Scarlet Gala' apple (Malus domestica Borkh.) trees was studied between 3 to 4 years. Trees that received an annual ground application of 0.15 lb (68 g) actual N as urea over four growing seasons had greater trunk cross-sectional area (TCA), higher yield, better fruit color, lower leaf N and less fire blight than those which received higher amounts of N. Trees on Malling (M.9) were more precocious and had higher yields in early years while trees on Malling-Merton 106 (MM.106 EMLA) and Malling-Merton 111 (MM.111 EMLA) had higher production 4 years after planting. Trees on Malling 26 (M.26 EMLA) had higher leaf Mg than those on other rootstocks. Trees on M.9 and M.26 EMLA had more fire blight damage than those on other rootstocks.

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Michael K. Thornton and S. Krishna Mohan

Pathogen populations, disease development and onion yield were compared in solarized, fumigated and non-treated plots during 1992 and 1993. Soil solarization was accomplished by covering plots with clear plastic for six weeks beginning in mid-August, prior to the year of onion production. Solarization was also combined with metham sodium, a plied prior to covering with plastic. Soil temperatures reached a maximum of 48°C at the 10 cm depth in solarized plots, and were consistently 10 to 15°C higher than in non-solarized plots. Disease resistant (Bravo) and susceptible (Valdez) onion cultivars were planted the following spring. Only the solarization + metham sodium treatment significantly controlled pink root and plate rot in 1992. In 1993, all solarization and fumigation treatments controlled pink root. Solarization and fumigation did not significantly increase yield in comparison to the check, except for the solarization + metham sodium treatment in 1992. Bravo exhibited lower disease incidence than Valdez in both years of the study. Bravo produced 32.7 t/ha and 6.2 t/ha higher yield than Valdez in 1992 and 1993, respectively.

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Esmaeil Fallahi, S. Krishna Mohan and Brenda R. Simons

Effects of several growth regulators and mineral nutrient sprays on `Rome Beauty' apple (Malus domestica Borkh.) russetting under climatic conditions of southwest Idaho were studied in the 1990 and 1991 growing seasons. Zinc (Zn-50), Calcium as a 12% liquid nutrient (Stopit-6), and fungicide (Polyram) sprays slightly increased, while GA (Provide) decreased fruit russetting.

In 1991, GA and repeated Semperfresh (sucrose ester, carboxymethyl cellulose and mono and diglycerides) applications reduced russetting while Bayleton application increased russetting. Russetting varied from year to year. In 1991, a season of severe fruit russetting, trees which received a fungicide treatment for powdery mildew had generally higher incidence of fruit russetting regardless of treatments. It is believed that interaction between relative humidity, temperature, and systemic sprays used for powdery mildew control contribute to `Rome Beauty' russetting.

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Esmaeil Fallahi, Michael Colt, S. Krishna Mohan and John Fellman

Influence of prebloom and full bloom applications of hydrogen cyanamide on `Simka' and `Friar' plums in Southwest Idaho and `Florda Prince' peach in Southwest Arizona was studied. Prebloom application of 0.5% hydrogen cyanamide caused severe toxicity to the fruit buds in `Friar' lure, while 2% hydrogen cyanamide did not cause toxicity in `Simka' plum. `Simka' fruit was effectively thinned with 1-2% prebloom application. At full bloom, 1.5% hydrogen cyanamide caused severe flower and leaf burning in both `Friar' and `Simka' plums, while concentrations between 0.1% and 1% thinned flowers (fruits) in both of the plum cultivars. Influence of hydrogen cyanamide on final fruit set, fruit size and maturity are also studied. Prebloom or full bloom applications of 2% or 3% hydrogen cyanamide eliminated 95 to 100% of the blooms, while application of this chemical at 1% sufficiently thinned the fruit. Number of commercially packed large peaches in trees receiving 1% hydrogen cyanamide was the same as that in trees thinned by hand, suggesting hydrogen cyanamide as a potential chemical for stone fruit thinning.

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Clinton C. Shock, Erik Feibert, Lynn Jensen, S. Krishna Mohan and Lamont D. Saunders

Onion (Allium cepa) varieties for commercial production in eastern Oregon and southwestern Idaho are evaluated annually in replicated trials conducted at the Malheur Experiment Station, Oregon State University, near Ontario, Oregon. Characteristics evaluated include bulb yield, market grade, and the frequency of single centers. After the emergence of iris yellow spot virus (IYSV) as a threat to commercial onion production in the early 2000s, onion varieties at the Malheur Experiment Station have been evaluated for virus symptoms since 2004. Varieties showed differences in the severity of IYSV symptoms each year. Symptom severity increased over the years from 2004 to 2006, and variety virus ratings showed a strong negative correlation of severity with yield in 2005 and 2006. Marketable yield after 3 months of storage averaged 781, 534, and 551 cwt/acre in 2004, 2005, and 2006, respectively. Averaging over varieties, yield of bulbs larger than 4 inches in diameter was 438 cwt/acre, 56 cwt/acre, and 76 cwt/acre, and the average virus severity ratings were 1.1, 1.3, and 2.7 in 2004, 2005, and 2006, respectively. A few varieties showed a combination of high yield, large bulb size, low incidence of virus symptoms, and a predominance of single-centered bulbs. With the prevalence of IYSV, variety tolerance to IYSV has become an important production factor in the Treasure Valley.

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Nancy W. Callan, James B. Miller, Don E. Mathre and S. Krishna Mohan

Sweet corn (Zea mays L.) seed is commonly infected or infested with fungi that can impair stand establishment. Among these, Penicillium oxalicum Currie and Thorn is known to cause preemergence damping-off or postemergence seedling blight. Supersweet, or shrunken2 (sh2), sweet corn cultivars are particularly affected by seedborne fungal pathogens, although the effects of seed infection on seedling emergence and stand are variable under field conditions. This study was conducted to examine factors that could influence the impact of P. oxalicum on seedling stand, including P. oxalicum inoculum density on seed and in soil, soil moisture, soil temperature, and control of seed decay caused by soilborne Pythium ultimum Trow. Seed surface disinfestation usually had no effect on seedling stand under conditions favoring infection by P. ultimum. Inoculation of sh2 sweet corn seeds or infestation of soil with conidia of P. oxalicum resulted in increasing severity of damping-off and seedling blight as inoculum density increased. In pasteurized soil in the greenhouse, an inoculum density of 102 P. oxalicum conidia per seed reduced emergence and induced seedling blight. In the field, where P. ultimum was also a factor, 106 conidia per seed were needed to reduce emergence and 105 conidia per seed to reduce healthy seedling stand. When pythium seed decay was controlled by metalaxyl seed treatment, seedling emergence and healthy seedling stand were both reduced at 1 × 106 P. oxalicum conidia per seed. When sh2 sweet corn seed was inoculated with conidia of P. oxalicum and incubated in soil at subgermination moisture contents (4.2 to -7.8 MPa) for 2-4 weeks before planting and irrigating, P. oxalicum reduced seedling emergence at all soil moisture levels, but caused the greatest amount of injury after planting when seeds were incubated in soil above -5.1 MPa. As soil temperature increased from 9-25C, seedling emergence from seed inoculated with P. oxalicum was progressively reduced, with a decrease of nearly 50% at 25 C. Penicillium oxalicum has the greatest potential to reduce seedling stand when infected sweet corn seeds are planted in warm, dry soil, but the effects of this and other seedborne fungal pathogens may be masked under conditions favoring infection by P. ultimum.