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R.T. Nagata

Complete (100%) hybridization in lettuce (Lactuca sativa L.) was accomplished consistently using the clip-and-wash method of emasculation. The clip-and-wash method is a combination of washing and clipping, two previously described procedures for pollen removal. The wash and the clip methods produced 98% and 95% hybridization, respectively. The method is quick and easy and eliminates inadvertent self pollination; with it one can produce the many hybrid seeds necessary for backcross and F1 genetic analysis in lettuce.

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L.E. Datnoff and R.T. Nagata

The effects of corky root (CR) disease, caused by Rhizomonas suberifaciens(van Bruggen, Jochimsen, and Brown) on fresh and marketable weights of lettuce (Lactuca sativa L.) were assessed during the 1988 and 1989 cropping seasons in several commercial lettuce fields. The resistant crisphead cultivars Raleigh and South Bay and the susceptible cultivars Ithaca and Shawnee produced similar yields in fields either planted in lettuce for the first time or in continuous lettuce production for three cropping cycles. Average yields of the resistant cultivars, from two fields cropped for six cycles naturally infested with CR, ranged from 875 to 1062 g/head fresh weight and 674 to 907 g/head marketable weight. The average yields of the susceptible cultivars in these infested fields ranged from 419.8 to 668.7 g/head fresh weight (37% to 52% yield loss) and 317.5 to 488.2 g/head marketable weight (46% to 53% yield loss). CR severity ratings were highly negatively correlated with root dry matter accumulation and whole and marketable head weights of-lettuce.

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Yu Sung, D.J. Cantliffe, and R.T. Nagata

Lettuce seeds differentially fail to germinate at temperatures above 21C according to genotype. Twenty-one lettuce lines were screened for their ability to germinate at temperatures from 24C to 36C. Four cultivars, `Dark Green Boston', `Valmaine', `Floricos 83', and `PI251245', were selected for this study because of their range of ability to germinate at temperatures above 24C. Seeds of the four cultivars were collected from mother plants grown in growth chambers at 20/10C(day/night temperature), 25/15C, 30/20C and 35/25C. Seeds were germinated on a thermogradient table from 24C to 36C under light (12 h). Seeds from `Floricos 83' produced above 30C had higher germination percentage at 33C and 36C than those produced below 30C temperatures. At 30C germination temperature seeds of `Valmaine' produced above 30C had 98% germination compared to 45% of those produced below 30C. `Dark Green Boston' seeds produced at 35C had higher germination percentage(70%) at 30C than those produced at other temperatures. Seeds collected from the mother plant grown above 30C day temperatures had greater germination than those grown below 30C.

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J. Alvarez, L.E. Datnoff, and R.T. Nagata

The severity of corky root disease (Rhizomonas suberifaciens Van Bruggen et al.) increases with continuous lettuce (Lactuca sativa L.) cropping and exerts a negative impact on the quantity and quality of the lettuce produced. Experimental data from commercial fields were used to analyze profitability outcomes resulting from various management strategies, including cultivars, locations, and field cropping history, to control corky root. Regardless of the field cropping history, net returns were not negatively affected when resistant cultivars were planted. For susceptible cultivars, even when considering land development costs, producers maximize net returns by planting lettuce following sugarcane in land not previously cropped to lettuce. After the first crop of lettuce following sugarcane, yields slowly decreased but remained profitable for three to four crop cycles.

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R.T. Nagata, C.A. Sanchez, and F.J. Coale

Four field experiments were conducted during 1988 to 1990 to evaluate the response to fertilizer P of six crisphead lettuce (Latuca sativa L.) cultivars grown on Histosols. There were season × cultivar interactions for total mass produced, marketable yield, and P uptake by lettuce. A significant yield response to fertilizer P was demonstrated during all four seasons. The performance of individual cultivars within a given season led to cultivar × P rate interactions for marketable yield. However, there were no significant P rate × cultivar interactions for total mass produced, P uptake, and marketable yield during the one season when growing conditions were near ideal. Calculated critical soil-test P values for both eastern and western lettuce types produced in Florida were all within error currently associated with P fertilizer recommendations for lettuce produced in Florida. Therefore, we conclude that no immediate change in P fertilizer recommendations is required for the new western-type lettuce cultivars produced on Histosols in Florida.

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V.L. Guzman, R.T. Nagata, L.E. Datnoff, and R.N. Raid

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R.T. Nagata, V.L. Guzman, L.E. Datnoff, and R.N. Raid