Twenty field experiments were conducted to evaluate the response of iceberg lettuce (Lactuca sativa L.) to N and evaluate various diagnostic technologies as tools for assessing the N nutritional status of lettuce. Lettuce yields showed a curvilinear response to N in most experiments. Generally, the dry midrib nitrate-N test and the sap nitrate-N test appear to be sensitive indicators of the N nutritional status of lettuce after the folding stage of growth. The chlorophyll meter was not a sensitive indicator of the N nutritional status of lettuce. Preliminary data also show that canopy reflectance, including digital analysis of aerial photographs, is correlated to N nutritional status of lettuce. However, reflectance technologies do not readily distinguish between N deficiencies and other factors (insects, diseases, water stress, etc.) that affect plant biomass and color. Because plant tests do not appear to be sensitive indicators of N nutrition during early growth stages (before folding), a post-thinning (and pre-sidedress) soil nitrate-N test is currently being evaluated.
C.A. Sanchez, M. Wilcox, J.L. Aguiar, and K.S. Mayberry
Elsa S. Sánchez, Ermita Hernández, Mark L. Gleason, Jean C. Batzer, Mark A. Williams, Timothy Coolong, and Ricardo Bessin
The goal of this study was to develop a systems-based strategy for organic muskmelon (Cucumis melo var. reticulatus) in Pennsylvania (PA), Iowa (IA), and Kentucky (KY) to manage bacterial wilt (Erwinia tracheiphila) and nutrients while safeguarding yield and enhancing early harvest. Spunbond polypropylene rowcovers deployed for different timings during the growing season were evaluated for suppressing bacterial wilt and locally available compost was applied based on two different estimated rates of mineralization of organic nitrogen (N) to manage nutrients. In KY only, the use of rowcovers suppressed bacterial wilt incidence compared with not using rowcovers. However, the timing of rowcover removal did not impact wilt incidence. Under lower cucumber beetle [striped cucumber beetle (Acalymma vittatum) and spotted cucumber beetle (Diabrotica undecimpunctata howardi)] pressure in PA and IA, rowcovers did not consistently suppress season-long incidence of bacterial wilt. In four of five site-years in PA and IA, more marketable fruit were produced when rowcovers were removed 10 days after an action threshold (the date the first flower opened in PA; the date when ≥50% of plants in a subplot had developed perfect flowers in IA and KY) than when no 10-day delay was made or when no rowcovers were used. In addition, the no-rowcover treatment consistently had lower weight per marketable fruit. In KY, the same action threshold without the 10-day delay, followed by insecticide applications, resulted in the largest number of marketable fruit, but did not affect marketable fruit weight. In PA, marketable yield was higher using compost compared with the organic fertilizer in 1 year. No yield differences were observed by nutrient treatments in 2 years. In IA, marketable yield was lower with the low amount of compost compared with the organic fertilizer and yields with the high amount of compost were not different from the low amount or the organic fertilizer in the year it was evaluated. In KY, marketable yield was unaffected by the nutrient treatments in the year it was evaluated. Given these results, muskmelon growers in PA, IA, and KY who use compost may choose the lower compost rate to minimize production costs. Overall, these findings suggest that rowcover-based strategies for organic management of bacterial wilt need to be optimized on a regional basis, and that fertilization with compost is compatible with these strategies.
J.L. Anaya-López, I. Torres-Pacheco, M. González-Chavira, J.A. Garzon-Tiznado, J.L. Pons-Hernandez, R.G. Guevara-González, C.I. Muñoz-Sánchez, L. Guevara-Olvera, R.F. Rivera-Bustamante, and S. Hernández-Verdugo
Screening for resistance to mixed infections with pepper huasteco virus (PHV) and pepper golden mosaic virus (PepGMV) was carried out on plants representing wild pepper accessions collected in different states of México. One accession collected in Yucatán (BG-3821) corresponded to Capsicum chinense Jacq., and three collected from Michoacán (BG-3818), Tamaulipas (BG-3820), and Sinaloa (BG-3819) were identified as C. annuum L. Forty-eight plants were initially inoculated with a 1:1 mix of PHV and PepGMV DNAs by a biolistic method. Those plants that did not show typical symptoms after the biolistic method, were inoculated by grafting. Half of the plants (24) were highly susceptible, while the other half expressed different degrees of resistance. Of the resistant individuals, eight plants were asymptomatic and viral DNA of both viruses was detected in low levels. Two individuals showed delayed symptoms 34 days after symptom expression in the control plants. This delay was correlated with an increase in PHV DNA levels when plants became symptomatic. The remaining 14 plants showed symptom remission in newly developed leaves at 31 days postinoculation, and this asymptomatic effect was correlated diminished PHV DNA within the plants. Our results suggest that the resistance shown by some individuals to geminivirus mixed infections (PHV and PepGMV) is likely due to constrains in viral movement.