Introduction
Present crisphead lettuce varieties are not well-adapted to culture in the Willamette Valley, particularly in mid-summer. Oregon State University plant breeders N.S. Mansour and J.R. Baggett are attempting to develop well-adapted varieties based on crosses of 'Ithaca' x 'Salinas,' each of which has desirable characteristics. Qualities sought include good head size (about two pounds/head at maturity), resistance to bolting, uniform maturity, a moderately firm head with good color, and resistance to tipburn and rib blight (brown rib). The OSU release 'Summertime' has shown promise but its response to soil fertility is not well known. The purpose of these three trials was to study the yield and quality of 'Summertime," as compared to 'Salinas,' in response to various rates of N and K fertilizer.
Methods
For the first planting, 135 pounds P/acre as triple superphosphate was broadcast to the plot area. Potassium chloride was then applied to main plots (15 feet x 25 feet) at rates of 0, 83, and 167 pounds K/acre in a randomized block design with five replications of each rate. Fertilizers were then incorporated by rotary tillage. 'Salinas' and 'Summertime' were seeded with a belt planter at about five seeds/foot on 22 April, 1987, with two rows/bed on 16-inch centers, followed by an application of pronamide at 1.5 pounds/acre. The seedlings were thinned to about 12-inch in-row spacing on 13 May and the main potassium plots were split randomly by a sidedressed application of ammonium nitrate at 50, 100, or 150 pounds N/acre. The experimental design consisted of a split-split plot with K rates as main plots, N rates as subplots, and varieties as sub/subplots. Carbaryl was applied for insect control at weekly intervals at a rate of 1.0 pound/acre. Sub/subplot size was a single two-row bed of 4 x 10 feet. First harvest was on 1 July and all remaining heads were harvested on 6 July.
The methods for the second planting were similar except as follows. Seeding date was 21 May, 1987, and thinning and N application date was 12 June. The K rates were 0 and 167 pounds/acre; N rates were 30, 60, 90, and 120 pounds/acre, and there were four replications. First harvest was on 31 July. Remaining marketable heads were cut on 5 August.
Methods for the third planting were also similar except that 250 pounds/acre of ammonium sulfate was applied with the superphospate and potassium chloride. The lettuce was seeded on 15 June, 1988, and thinned on 29 June. The main K plots were then split by application of ammonium nitrate at 0, 50, 100, or 150 pounds N/acre, resulting in total N application rates of 50, 100, 150, and 200 pounds/acre. Sub/sub-plot size was a single two-row bed of 4 x 15 feet. First harvest was on 18 August, the second on 23 August.
Results and Discussion
First planting
Head size was small for the first planting even though heads were harvested when firm (Table 1). 'Summertime' matured earlier than 'Salinas' as evidenced by the higher percentage of heads cut at the first harvest. Head size 'Summertime' was unacceptably small, however, regardless of treatment. There was a trend, although not statistically significant, for greater head size of both varieties at higher rates of N. Rate of K had no consistent effect on head weight.
'Summertime' was much less susceptible to both rib blight and tipburn. Incidence of both disorders was higher at the second harvest for both varieties. Higher rates of N reduced rib blight at the second harvest, but this effect was not consistent over both harvests. Higher rates of K also reduced incidence of rib blight and tended to reduce tipburn.
All heads were cut at the second harvest in order to obtain a more accurate effect of N and K on head weight. Since the carton weights (weight of 24 heads) were unacceptably low for all treatments, essentially no treatment produced marketable size in this planting.
There were no significant two- or three-way interactions affecting yield or quality; only main effects are given in Table 1.
Second planting
Head size was greatly improved in the second planting, although most 'Salinas' heads were starting to bolt by the second harvest (Table 2). A much higher percentage of 'Summertime' than 'Salinas' heads were cut in the two harvests. Most 'Salinas' plants left unharvested were bolters. Bolting was not a problem in 'Summertime.'
Head size of 'Salinas' was again greater than for the 'Summertime.' Averaged across all fertilizer treatments, 'Summertime' carton weight was still only marginally acceptable. At the highest N rate, however, carton weight was acceptable (Table 3). Head weight increased with increasing N rate but was not affected by rate of K. 'Summertime' responded more to N rate than did 'Salinas' (Table 3). As in the first planting, 'Summertime' matured earlier than 'Salinas' and the high rate of K appeared to retard maturity (Table 2). Neither K nor N rate affected the percentage of heads cut in the two harvests.
Rib blight and tipburn were, again, less prevalent in 'Summertime' than in 'Salinas.' Rate of N had no effect on either disorder when averaged over K rates and varieties, but both were reduced with the high rate of K. This is consistent with the first planting. The interaction of variety and N rate was significant for rib blight. The greatest occurrence of blight was at low N for 'Summertime,' but at intermediate N rates for 'Salinas.'
Third planting
As with the first planting, there were no significant two-way or three-way interactions among K rate, N rate, and cultivar affecting any component of yield or quality. Thus, only main effects of treatments are presented.
Head size was again very small, particularly for 'Summertime' (Table 4). As in 1987, 'Summertime' matured earlier, as evidenced by the greater percentage of heads cut at the first harvest. Increasing rate of N tended to delay maturity of both lines and K rate had no significant effect.
Neither N nor K rate had any statistically significant effect on head weight at either harvest (Table 4). However, the trend was for applied K to reduce yield on this high potassium soil, and the 100 pound/acre N rate tended to be optimal.
As in the 1987 trials, 'Salinas' was much more susceptible to rib blight and tipburn than was 'Summertime' (Table 5). Also as in 1987, N rate had no significant effect on incidence of rib blight or tipburn, but rib blight was reduced by K application.
Observations were made on 4 August of the number of plants exhibiting a yellows disorder of the lower leaves. This disorder has been attributed to beet western yellows virus or perhaps to some other luteovirus. Neither N nor K rate had any effect on the disorder (Table 5), but the disorder occurred more often in 'Summertime.'
Results of these trials indicate that 'Summertime' is less susceptible to tipburn, rib blight, and bolting, under widely varying conditions of soil fertility, than is 'Salinas.' Rate of K had no effect on yield on this soil but high K rates appeared to reduce rib blight. The response to N was small, and 'Summertime' was marginal for head size. Rates of N up to 150 pounds/acre can be applied to 'Summertime' without reducing head quality. 'Summertime' heads may be excessively firm. A yellows disorder of the older leaves was more prominent in 'Summertime' than in 'Salinas.' This requires removing more leaves at harvest, increasing the difficulty of obtaining satisfactory head size in 'Summertime.'
Table 1. Main effects of N and K rates and cultivar on head weight, and rib blight and tipburn incidence in crisphead lettuce, first planting, 1987 Meanz carton % heads cut, Mean head wt. (g) wt. Rib blight (%) Tipburn (%) Treatment harvest 1 Harv. 1 Harv. 2 Mean (lb) Harv. 1 Harv. 2 Mean Harv. 1 Harv. 2 Mean Cultivar Salinas 22.6 703 543 577 37.2 33.3 39.7 38.4 6.6 12.3 10.5 Summertime 42.7 567 413 483 30.0 0.7 13.5 7.3 0.6 4.6 3.1 **y ** ** ** ** ** ** ** * ** ** N (lb/A) 50 31.3 615 467 519 32.5 9.9 33.1 25.4 6.0 10.7 8.0 100 33.9 637 484 532 33.7 22.7 26.1 24.2 2.1 8.7 6.8 150 32.7 643 483 538 34.0 16.2 20.5 18.9 2.6 6.1 5.5 NS NS NS NS NS NS * NS NS NS NS K2O (lb/A) 0 29.9 654 473 526 34.6 15.9 33.5 27.6 4.7 9.2 7.2 100 35.4 636 499 545 33.6 17.8 27.6 24.6 3.8 10.5 8.4 200 32.6 615 462 518 32.5 17.4 18.6 16.4 2.3 5.7 4.7 NS NS NS NS NS NS * ** NS NS NS ZFirst harvest only. Y**, *, NS: significant differences among means at 1% level, 5% level, and no significant differences, respectively. Table 2. Main effects of N and K rates and cultivar on head weight, and rib blight and tipurn incidence in crisphead lettuce, second planting, 1987 Mean % % % heads cut Mean head wt. (g) carton Rib blight z Tipburnz No. cartons/acrey Treatment Harv. 1 Total Harv. 1 Harv. 2 Mean wt.(lb) Cultivar Salinas 18.5 80.3X 845 829 831 43.9 10.3 5.9 875 Summertime 45.7 96.8 773 780 776 41.0 2.3 1.4 882 ** ** * * * * ** ** NS N rate (lb/A) 30 33.4 90.3 771 775 774 40.9 4.9 3.6 839 60 24.6 86.5 758 793 773 40.9 6.0 3.4 882 90 35.3 87.7 850 814 828 43.8 9.5 1.7 879 120 35.1 89.9 852 836 846 44.7 4.9 5.9 910 NS NS NS NS * * NS NS NS K2O rate (lb/A) 0 36.5 88.6 790 813 810 42.8 9.4 5.3 871 200 27.7 88.5 826 796 806 42.6 3.2 2.0 885 ** NS NS NS NS NS ** * NS ZSecond harvest only. No tipburn or rib blight at first harvest. y24 heads/carton. xMost heads not cut were bolting. Table 3. Interaction of nitrogen rate and cultivar on mean head weight and rib blight incidence, second harvest of second planting, 1987 Carton wt. Cultivar N rate(lb/A) Mean head wt.(g) (lb) Rib blight (%) Salinas 30 811 42.9 3.5 60 803 42.4 11.9 90 854 45.1 16.9 120 847 44.8 8.6 Summertime 30 740 39.1 6.3 60 784 41.4 0.0 90 774 40.9 2.0 120 825 43.6 1.1 LSD(0.05) NS NS 10.0 Table 4. Main effects of N and K rates and cultivar on head weight and maturity of crisphead lettuce, 1988 % heads cut Mean head wt.(g) Mean carton Treatment Harv. 1 Both Harv. 1 Harv. 2 Mean wt. (lb) Cultivar Salinas 27.7 87.9 627 556 577 30.5 Summertime 40.2 91.9 565 483 518 27.4 ** * ** ** ** ** N rate (lb/A) 50 42.0 93.2 598 510 550 29.1 100 36.7 92.8 612 528 561 29.7 150 30.2 90.1 574 525 540 28.5 200 26.9 83.4 598 515 539 28.5 * NS NS NS NS NS K20 rate (lb/A) 0 37.4 92.4 601 530 561 29.7 200 30.5 87.4 591 509 534 28.2 NS NS NS NS NS NS Table 5. Main effects of N and K rates and cultivar on disorders of crisphead lettuce, 1988 Rib blight (%) Tipburn (%) % plants Treatment Harv.1 Harv.2 Mean Harv.1 Harv. 2 Mean with yellows Cultivar Salinas 71.7 62.4 63.6 33.1 70.7 59.1 11.5 Summertime 12.2 11.3 11.8 3.5 8.1 5.8 15.6 ** ** ** ** ** ** * N rate (lb/A) 50 46.2 36.1 38.7 15.6 38.7 29.8 12.7 100 45.1 39.2 39.8 16.9 44.5 36.8 12.9 150 38.9 34.5 36.2 15.5 37.0 31.3 14.5 200 37.6 37.6 37.6 25.2 37.6 31.9 14.1 NS NS NS NS NS NS NS K2O rate (lb/A) 0 43.6 43.5 43.5 18.5 40.9 33.1 13.9 200 40.3 30.3 32.3 18.1 38.0 31.9 13.2 NS * * NS NS NS NS