Research report from OSU's North Willamette Agricultural Research and Extension Center
Delbert Hemphill
OSU Dept of Horticulture, NWREC
Mary Powelson
OSU Dept of Botany and Plant Pathology
Introduction
The objective of this project was to evaluate the effect of plant spacing and nitrogen rate on yield and disease incidence in broccoli.
Methods
'Gem' broccoli was transplanted on May 15 and July 25, 1986. In the early planting, the 18 treatments included all combinations of two N rates (200 or 300 lb/acre), three between-row spacings (12, 16, 20 inches), and three in-row spacing (8, 14, 20 inches), resulting in nine distinct plant populations ranging from 15,000 to 65,000/acre. In the late planting, the N rate was 250 lb/acre, the between-row spacing was 16 inches, and within-row spacing was 8, 10, 12, 14, 16, or 18 inches. Resulting plant populations ranged from 22,000 to 49,000/acre. Erwinia carotovora, the causal organism of soft rot of broccoli heads, was applied at weekly intervals in both plantings.
Results
In the early planting, the higher rate of N caused a yield increase of more than 800 lb/acre, sufficient to pay for the extra fertilizer. Head weight increased, but yield decreased, with increasing space between or within rows (Table 1). The highest average yield of 6.5 tons/acre was at 14-inch within-row spacing, and between-row spacing of 12 inches (37,340 plants/acre) but yield changed very little between plant populations of 25,000 to 65,000/acre.
In the late planting, head weight increased nearly linearly with increasing within-row spacing. The increased head size was not sufficient to offset the decrease in plant population. The highest gross yield was at a plant population of 39,200/acre (Table 2).
No significant downy mildew was observed in either planting and head rot did not occur in the early planting. Erwinia did not become established on plant leaves or heads in this experiment, primarily because of unusually warm, dry weather.
In the late planting, the weather turned cool and wet immediately before the first harvest on September 16. Head rot was well established by the second and third harvests. The percentage of heads affected by rot decreased with greater space between plants. However, the total number of disease-free heads was greater at higher plant density since the larger number of heads harvested more than offset the increased head rot percentage (Table 2).
It appears that reducing plant population much below 35,000 plants/acre would not be a cost-effective means to reduce head rot. Erwinia failed to become established on broccoli foliage before head formation, even under favorable moisture conditions. However, the bacterium did become established on floret and stem tissue in the late planting. This has important ramifications for spray timing as application of bacteriocides or chlorine compounds before head formation would not likely be cost effective.
Well water and seed were eliminated as likely sources of head rot inoculum, but surface water is an important source. Irrigation from deep wells rather than rivers would be preferred. Irrigation timing and duration remains one of the most important cultural methods that influence head rot development, since long periods of free moisture favor disease development.
Grower cultural practices will have little effect when the macroclimate is highly favorable or unfavorable for disease development. Only when conditions are marginal for disease development will plant populations, irrigation sources and duration, and bacteriocide application have any effect on head rot incidence.
Table 1. Main effects of N rate, between-row spacing, and within-row spacing on canopy closure, head size, and yield of broccoli, sum of all harvests; early planting, 1986 Treatment CanopyZ Mean head Mean head Yield closure wt. (lb) width (inches) (tons/A) N rate: 200 lb/acre 2.3 0.46 3.9 5.83 300 lb/acre 2.2 0.50 4.1 6.28 NSY * NS NS Between-row: 12 inches 2.8 0.44 3.9 7.16 16 inches 2.2 0.48 4.1 5.91 20 inches 1.6 0.51 4.2 5.48 * * * * Within-row: 8 inches 3.3 0.37 3.6 6.31 14 inches 2.3 0.50 4.2 5.94 20 inches 1.2 0.56 4.4 5.25 * * ** * Z5-point scale with 5 = complete coverage, 1 = ground showing within and between rows. YNS, *, **: no significant differences, differences significant at 5% and 1% levels, respectively. Table 2. Effect of within-row spacing on broccoli plant height, yield for the sum of three harvests, and head rot incidence; late planting, 1986 Within-row Plants/ Plant Mean Yield % rotten No. of spacing acre height head (tons/acre) heads usable (inches) (inches) wt. (lb) heads/plot 8 49,005 15.2 0.44 6.76 39 20 10 39,204 13.9 0.48 6.83 27 23 12 32,670 13.8 0.46 6.01 23 22 14 28,003 13.1 0.52 5.29 22 18 16 24,503 12.9 0.49 4.78 18 17 18 21,780 12.2 0.58 4.91 29 12 linear **Z * NS NS * quadratic NS NS NS NS NS ZNS, *, **: no significant differences among means, significant differences at 5% and 1% levels, respectively.