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Keith S. Mayberry and Thomas M. Perring

In the fall of 1990, a new form of whitefly, tentatively identified as the poinsettia strain of Bemisia tabaci (Gennadius), was introduced into the agricultural regions of the desert southwest. Large densities of whitefly nymphs developed on cruciferous crops and substantial increases in pesticides for whitefly control were used. After overwintering in active stages on these crops, whiteflies moved into spring cantaloupes and developed moderate populations levels in some fields. In March, whiteflies migrated to newly-planted cotton and developed huge densities by August. At this time emerging fall cantaloupe was attacked and over 95% of this crop was destroyed by whitefly feeding. Whiteflies also developed to damaging numbers on alfalfa, grapes, citrus, crops not known to host the cotton strain of B. tabaci. Population densities remained high through the fall crucifer and lettuce seasons causing crop losses and delayed maturity. Damage estimates presently rest at roughly $122 million.

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Colleen Kennedy, Luis F. Osorio, Natalia A. Peres and Vance M. Whitaker

Powdery mildew (PM) of strawberry (Fragaria sp.) is a ubiquitous, wind-spread disease caused by the obligate parasite Podosphaera aphanis. To control PM, multiple fungicide applications are necessary each season, and none of the major cultivars in Florida have high levels of resistance. Therefore, the objectives of this study were to observe the response to selection and to estimate genetic parameters for PM and related traits in the University of Florida breeding population. In 2010, clonally replicated individuals from seven biparental crosses arising from 11 parents were included in a field trial in which clonally replicated seedlings were evaluated visually for percent coverage of PM mycelium using a modified Horsfall-Barratt scale of 0 to 6. Broad- (H2) and narrow-sense (h2) heritabilities for PM score were (mean ± se) 0.50 ± 0.08 and 0.40 ± 0.39, respectively, for the base population. After the second round of selection in the resistant population, no additive variance was detected, indicating that alleles for PM resistance had become fixed. In contrast, after two rounds of divergent selection in the susceptible population, there remained considerable additive variance (h2 = 0.42 ± 0.65). Moderate to high heritability estimates and a clear response to selection indicate that resistance to PM is genetically controlled through mostly additive effects. Selection of parents based on field trials with natural inoculum should result in good progress toward more resistant cultivars. The consistently moderate to strong genotypic and genetic correlations among PM and canopy density (CD) indicate that selection for PM resistance will result in reduced CD. Therefore, CD must be monitored over successive rounds of selection for low levels of PM to prevent CD falling below the commercially acceptable range.

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Colleen Kennedy, Tomas N. Hasing, Natalia A. Peres and Vance M. Whitaker

Many breeders have turned to wild relatives in search of beneficial traits such as disease resistance. In strawberry, the wild octoploid species Fragaria virginiana and F. chiloensis are fully interfertile with the cultivated species, F. ×ananassa, and are therefore potential sources of resistance. Powdery mildew may increase in economic importance in Florida in the near future as a result of the use of high tunnels and rowcovers for freeze protection, which limit free water and provide a favorable environment for disease development. The objective of this study was to screen an elite group of wild strawberry accessions for resistance to powdery mildew under two production systems. In 2010–11 and 2012–13, wild accessions, commercial standard cultivars, and susceptible controls were planted in open-field and high tunnel environments at the Gulf Coast Research and Education Center in Balm, FL. Although there was a significant year × genotype effect, some taxa showed high levels of resistance that were consistent across years. There was a high correlation for ratings of powdery mildew between the high tunnel and the open field for all genotypes (r = 0.89, P < 0.001). This information may be useful for breeders, because sources of resistance to powdery mildew are available within the tested genotypes. However, some accessions are highly susceptible to powdery mildew, and this must be considered when using these genotypes in breeding programs.

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Craig K. Chandler, Bielinski M. Santos, Natalia A. Peres, Celine Jouquand, Anne Plotto and Charles A. Sims

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Craig K. Chandler, Bielinski M. Santos, Natalia A. Peres, Celine Jouquand and Anne Plotto

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A. Naor, I. Klein, H. Hupert, Y. Grinblat, M. Peres and A. Kaufman

The interactions between irrigation and crop level with respect to fruit size distribution and soil and stem water potentials were investigated in a nectarine (Prunus persica (L.) Batsch. `Fairlane') orchard located in a semiarid zone. Irrigation treatments during stage III of fruit growth ranged from 0.62 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (200 to 1200 fruit/tree in the 555-tree/ha orchard). Total yield did not increase with increasing irrigation rate above 0.92 ETp in 1996 and maximum yield was found at 1.06 ETp in 1997. Fruit size distribution was shifted towards larger fruit with increasing irrigation level and with decreasing crop level. The two highest irrigation treatments had similar midday stem water potentials. Our findings indicate that highest yields and highest water use efficiency (yield/water consumption) are not always related to minimum water stress. Total yield and large fruit yield were highly and better correlated with midday stem water potential than with soil water potential. This confirms other reports that midday stem water potential is an accurate indicator of tree water stress and may have utility in irrigation scheduling.

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A. Naor, R. Stern, M. Peres, Y. Greenblat, Y. Gal and Moshe A. Flaishman

The effects of the timing and severity of postharvest water stress on the productivity and fruit quality of field-grown nectarine [Prunus persica (L.) Batsch cv. Snow Queen] were studied for two consecutive years. Three levels of postharvest water status (midday stem water potentials of -1.2, -2.0, and -2.8 MPa) were examined. They were designated as High, Med, and Low, respectively. In the second year two additional treatments were examined in which Low and Med water status were interchanged on 1 Sept. 2002, and these treatments were designated as Low/Med and Med/Low. The percentages of double fruits and of those having a deep suture increased with decreasing postharvest midday stem water potential during the previous year, and most of these defects were stimulated by water deficits that occurred prior to 1 Sept. Postharvest water stress led to decreased crop yield in the subsequent year because there were fewer fruits per tree. Flower buds with double pistils were first noticed in mid-September, and by mid-November the ranking of double pistils in the various treatments were similar to the ranking of double fruits measured a month after bloom in the subsequent season. Postharvest water stress delayed flower bud development. The percentage of double fruits increased from 10% in 2002 to 40% in 2003 and the higher percentage in 2003 was associated with higher air temperatures during the reproductive bud development stage in 2002 than in 2001. Our data and others suggest that high temperatures create a potential for the occurrence of double fruits, but that the fulfillment of that potential is highly dependent on postharvest tree water status. The occurrence of double and deep suture fruits were highly correlated with midday stem water potential in August of the previous year, i.e., during the initial stages of flower bud development. The occurrence of double fruits was observed to increase sharply as the midday stem water potentials fell below -2.0 MPa, which suggests that a midday stem water potential of -2.0 MPa could serve as a threshold for postharvest irrigation scheduling.

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A. Naor, H. Hupert, Y. Greenblat, M. Peres, A. Kaufman and I. Klein

The interactions between irrigation and crop level with respect to fruit size distribution and midday stem water potential were investigated for 3 years in a nectarine (Prunus persica L. `Fairlane') orchard located in a semi-arid zone. Wide ranges of crop loads and irrigation rates in stage III were employed, extending from practically nonlimiting to severely limiting levels. Irrigation during stage III of fruit growth ranged from 0.63 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (300 to 2000) fruit/tree in the 555-tree/ha orchard. The yields and stem water potentials from 1996, 1997 and 1998 were combined together and the interrelations among yield, crop load and stem water potential were examined. Fruit <55 mm in diameter growing at 400 fruit per tree were the only ones not affected by irrigation level. The yield of fruit of 60 to 75 mm in diameter increased with irrigation level, but only a slight increase was observed when the irrigation rate rose above 1.01 ETp. A significant decrease in the yields of 60 to 65, 65 to 70, and 70 to 75-mm size grades occurred at crop levels greater than 1000, 800, and 400 fruit per tree, respectively. Midday stem water potential decreased with increasing crop level, and it is suggested that midday stem water potential responds to crop load rather than crop level. Relative yields of the various size grades were highly correlated with midday stem water potential. It was suggested that the midday stem water potential integrates the combined effects of water stress and crop load on nectarine fruit size.

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Vance M. Whitaker, Craig K. Chandler, Natalia Peres, M. Cecilia do Nascimento Nunes, Anne Plotto and Charles A. Sims

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Vance M. Whitaker, Craig K. Chandler, Bielinski M. Santos, Natalia Peres, M. Cecilia do Nascimento Nunes, Anne Plotto and Charles A. Sims