Introduction
Head rot, caused by the soft rot bacterium Erwinia carotovora, and downy mildew, caused by Peronospora parasitica, are two serious diseases of broccoli in Oregon. Head rot is characterized by water-soaked lesions on heads which rapidly expand, resulting in a soft decay of the florets. Symptoms of downy mildew include light yellow lesions on the leaf surface and yellowing of the florets. Environmental conditions can play a major role in the development of these two diseases. Head rot is most prevalent when warm, moist conditions occur at the time of head formation whereas downy mildew is favored under cool, moist growing conditions. Because temperature and moisture are environmental parameters which can be influenced by plant population and irrigation practices, these studies were designed to evaluate the effect of plant populations and irrigation frequency and amount on yield, head rot, and downy mildew of broccoli.
Methods
In the 1987 plant population experiment, 'Gem' broccoli was seeded in 1.5-inch cells on 17 March, and placed in an unheated greenhouse. Seedlings were watered daily and fertilized with a soluble 20N-8.7P-16.7K fertilizer. The seedlings were transplanted to a Willamette silt loam on 16 April, 1987. The eight treatments included two between-row spacings (16 and 20 inches) and four in-row spacings (6, 8, 10 and 12 inches). Each plot was a four-row bed, 15 feet in length. The experimental design was a randomized block with treatments replicated six times. The plot area was treated with trifluralin at 0.75 pound/acre on 10 April and with propachlor at 4.0 pound/acre on 17 April. A diazinon drench at 1.0 pounds/acre was applied twice for maggot control. Total N applied in all the experiments was 250 pounds/acre. Following the establishment period, the plot area was irrigated by overhead sprinkler. At head initiation and one week later, a cell suspension of Erwinia carotovora was applied to the plants. Measurements of yield were taken from the middle two rows of each plot. Heads were harvested on 15 June and 23 June; weight and number of heads were recorded, and disease noted.
In the 1987 irrigation experiment, the two cultivars, 'Gem,' 'Citation,' and the breeding line 'OSU 86-3' were seeded on 24 June, 1987, and placed in a nonheated greenhouse. Seedlings were transplanted on 29 July and were spaced 10 inches apart within rows on 16-inch centers. Each plot consisted of a four-row bed, 12 feet in length. Cultivars were arranged in a randomized block design and replicated six times. Pest control was as in the previous experiment except that methomyl was applied at 0.25 lb/acre immediately after transplanting and Bacillus thuringiensis was applied at 0.25 lb/acre on 3 September for cabbage looper control. All plots received the same amount of water until two weeks prior to head initiation. At that time two line irrigation sources were established. One-half of the field was irrigated once a week for 7.5 hours while the other was irrigated three times a week for 2.5 hours each time. Three moisture levels were established within each frequency: high (0.75 inches/week), medium (0.5 inches/week), and low (0.25 inches/week). At head initiation and at weekly intervals, an aqueous suspension of Erwinia carotovora was applied to the plants with a pressurized, backpack sprayer. Heads were harvested 28 September, and 6 and 12 October from the middle two rows of each plot.
In the 1988 population study, methods were similar to 1987 except that in-row spacings were 6, 9, and 12 inches, and the cultivar 'Gem' was seeded on 23 March, and 'OSU 86-3' on 9 March. Transplanting was on 26 April. The 12 combinations of between-row spacing, in-row spacing, and cultivar were replicated six times in randomized block design. Heads were harvested on 27 June, 5 July, 12 July, and 20 July.
In the 1988 irrigation study, methods were as in 1988, except that the seeding date was 23 June, transplanting date was 27 July, and harvests were on 6 October, 13 October, and 18 October. Water applied to each plot was measured with rain gauges located in each plot.
Results
In the 1987 population experiment mean head weight was significantly greater (p = 0.05) for the 20-inch between-row spacing than for the 16-inch between-row spacing. The average head weight was greatest at 10-inch in-row-spacing. Yield increased from 6.6 tons/acre with a population density of 26,000 plants/acre to 10.1 tons/acre at 65,000 plants/acre (Table 1).
With hot, dry weather at harvest, head rot failed to develop. Downy mildew did occur naturally within the test plots, but no significant differences in disease incidence occurred among the different population densities (Table 1).
Table 1. Effect of plant population on 'Gem' broccoli, spring 1987 Spacing (inches) Population Yield Mean head % Between-row In-row per acre (tons/acre) wt. (g) mildew 16 6 65340 10.0 181 6.4 20 6 52272 9.0 202 9.6 16 8 49005 8.0 211 7.5 20 (16)z 8 (10) 39204 8.3 233 10.6 16 12 32670 6.8 239 7.0 20 10 31362 8.4 301 10.5 20 12 26136 6.6 277 3.6 LSD (0.05) 2.4 58 NSy z20x8 and 16x10 spacings averaged since they provide same plant population per acre. yNS: no significant differences.
In the 1987 irrigation experiment, the amount of water applied had no effect on total yield in the high-frequency experiment, but did affect yield in the low-frequency experiment, with yields increasing with an increase in amount of water applied (Table 2). 'Gem,' the earliest maturing cultivar, had the greatest yield at the first harvest (data not shown). 'Citation,' which matured next, produced the highest yields during the second harvest. 'OSU 86-3' was late to mature and produced the highest yields at the last harvest, but total yield was still less than for the other two cultivars. However, if harvesting had been continued, yield of OSU 86-3 might have been comparable to the other two cultivars.
For all harvests, development of head rot was significantly affected by the amount of water applied in the low-frequency experiment. For both experiments, disease incidence tended to be higher in plots receiving high and medium amounts of water than in plots receiving a low amount of water (Table 3). There were no significant cultivar differences in susceptibility to head rot.
In contrast, the incidence of downy mildew was not significantly affected by the amount of water applied in either experiment, although there was a trend for an increase in amount of disease with an increase in amount of water applied. Cultivars, however, differed in their susceptibility to downy mildew. 'Gem' was the most susceptible cultivar.
The effects of amount of water on mildew and rot were most pronounced at the second harvest, the first in which significant disease occurred (Table 4). Both mildew and rot were more prevalent at the high rates and frequency of water application. In a separate experiment harvested once in late October, the percentage of rotten heads was 22.8 for the high frequency of irrigation but only 9.7 for the low frequency.
The fall weather was unusually dry and warm during this experiment. Despite unfavorable environmental conditions for extensive disease development, disease did occur. In plots receiving low amounts of water the incidence of head rot was reduced. When macroclimatic conditions are unfavorable for disease, a reduction in the amount of irrigation water applied could reduce the amount of disease. However, when the macroclimate is favorable for disease development, it is questionable whether a reduction in the amount of water applied would result in a decrease in the amount of disease.
Results of the 1988 population study were very similar to those of 1987 (Table 5). Mean head weight decreased with increasing plant population but yield increased up to 52,000 plants/acre as the greater number of heads more than offset the effect of reduced head size. Downy mildew incidence increased, and hollow stem incidence decreased, with increasing plant population. The degree of head rot did not vary significantly with plant population.
Table 2. Main effects of amount of water and cultivar on yield, head weight, and head width for two irrigation frequencies, 1987 Treament High frequency Low frequency Yield Head wt. Head width Yield Head wt. Head width (tons/acre) (oz) (inches) (tons/acre) (oz) (inches) Amount of waterz High 7.6 9.4 4.4 9.3 10.3 4.7 Medium 7.6 10.4 4.8 7.2 10.0 4.9 Low 7.8 8.6 4.2 6.1 7.6 4.4 LSD (0.05) NS 1.0 0.2 1.1 1.0 0.2 Cultivar Citation 8.9 10.4 4.8 8.5 9.8 4.8 Gem 7.7 8.8 4.4 8.2 9.1 4.6 OSU 86-3 6.4 9.1 4.3 5.9 9.0 4.5 LSD (0.05) 1.1 1.0 0.2 1.1 NS 0.2 zLow = 0.25 inches/week, medium = 0.50 inches/week and high = 0.75 inches/week. Table 3. Main effects of amount of water and cultivar on incidence of head rot and downy mildew for two irrigation frequencies, 1987 Treatment High frequency Low frequency Head rot Downy mildew Head rot Downy mildew % % % % Amount of water High 4.1 17.3 4.8 14.4 Medium 7.4 16.7 5.3 15.1 Low 2.3 13.2 0.9 8.6 LSD (0.05) NS NS 3.4 NS Cultivar Citation 2.6 15.4 2.7 9.4 Gem 6.1 22.4 4.0 22.7 OSU 86-3 5.1 9.5 4.4 6.0 LSD (0.05) NS 6.9 NS 5.9 Table 4. Main effects of amount of water applied, frequency of irrigation, and cultivar on head rot and downy mildew of broccoli, second harvest. October, 1987 Treatment Head rot (%) Downy mildew (%) Amount of water High 4.8 22.3 Medium 4.9 18.2 Low 0.8 11.6 LSD (0.05) 3.5 6.6 Frequency Low 3.1 13.8 High 4.0 20.9 Cultivar Citation 3.1 13.9 Gem 5.1 32.8 OSU 86-3 2.3 5.4 LSD (0.05) NS 6.6 Table 5. Main effect of plant population and cultivar on yield and disease of broccoli for spring planting, 1988 Treatment Yield Head wt. Downy mildew Head rot Hollow stem (tons/acre) (oz) % % % Plants/acre 65340 8.5 6.4 15.2 15.3 19.8 52272 8.6 7.3 10.0 17.7 29.9 43560 8.3 8.0 8.2 19.1 43.9 34848 6.8 8.3 12.3 26.2 37.6 32670 7.8 8.6 6.5 13.4 53.2 26136 6.8 9.7 7.3 16.9 59.9 * ** * NS ** Cultivar OSU 86-3 7.7 8.6 17.1 29.6 19.8 Gem 7.9 6.8 2.3 5.3 64.2 NS * ** ** **
Head rot and mildew incidence were much greater in the OSU line than in Gem. However, this is probably an effect of delayed maturity rather than a true difference in susceptibility. The OSU 86-3 heads were much slower to develop and were exposed to favorable conditions for disease development longer than were the Gem heads. The OSU line had far fewer heads with hollow stems.
Laboratory tests were conducted to determine if Gem and the OSU line differed in their susceptibility to head rot. Broccoli heads were inoculated with a cell suspension of E. carotovora, placed in large jars, sealed, and flushed with nitrogen to achieve an anaerobic environment. After six days, the heads were removed and the percent of rotted tissue was determined. No significant differences in susceptibility to head rot were observed. This indicates that the higher incidence of head rot in the OSU line in the field studies was due to the rot-promoting conditions at the time OSU heads were maturing.
Results of the two population experiments indicate that lowering plant density can not be used as a means of reducing disease incidence, at least with the range of populations used in this study. A previous experiment in 1986 indicated significant reduction in head rot with populations below 26,000/acre, but these are not economically reasonable populations for processing broccoli.
In the 1988 irrigation experiment, most of the 'Gem' heads were harvested during the first two harvests. 'Citation' matured later than 'Gem' with most of the heads harvested during the second and third harvests. The highest incidence of rot occurred in the second and third harvests of 'Gem' and the third and fourth harvests of 'Citation.' By the fourth harvest most of the heads of both cultivars were diseased (Table 6). Root maggot damage severely affected the OSU line but not the other cultivars. Data for the OSU line are not reported.
'Citation' had three times more head rot than 'Gem' (Table 6). In contrast, downy mildew was more severe in 'Gem' than in 'Citation.' Thus, there is not a positive correlation between head rot and downy mildew incidence, at least when compared between cultivars.
The incidence of head rot and downy mildew increased as the amount of water applied increased in the high-frequency experiment. In the low-frequency experiment, head rot did not vary significantly with the amount of water applied (Table 7).
Considerably less head rot developed in the low-frequency experiment compared with the high-frequency experiment. Downy mildew did not appear to vary with irrigation frequency. Total yield was not affected by irrigation frequency, but marketable yield was higher with the low frequency (Table 6). Because of the nature of the experimental design, the two frequencies can not be compared statistically.
Yield also increased with increased amount of applied water, although the plots closest to the irrigation lines (6 feet) tended to have the same yield as those 10 to 12 feet from the lines (Table 7). Gross yields did not vary with cultivar. The number of heads harvested was greater with 'Gem' but 'Citation' produced larger head size. Marketable yield (total yield minus the yield of diseased heads) also did not vary with cultivar or amount of applied water (Table 6).
Frequency of irrigation and amount of applied water should be minimized to avoid head rot problems. However, controlling the amount of water applied is probably not an effective management tool at this time, since the irrigation-yield relationship would have to worked out for different soil types, cultivars, and microclimates. Also, keeping the applied irrigation to the minimum required for economic yields will not protect against the effect of rain.
Table 6. Main effects of harvest date, cultivar, irrigation frequency, and amount of applied water on yield and disease incidence of broccoli, autumn, 1988 Total yield Marketable yield % Mildew Head rot (tons/acre) (tons/acre) Marketable % % Harvest 1 1.44 1.38 97 3 0 2 2.46 1.78 72 20 9 3 1.51 0.77 51 23 34 4 0.48 0.13 28 54 39 Significance ** ** ** ** ** Cultivar Gem 6.1 4.1 69 27 8 Citation 6.3 4.2 67 17 25 Significance NS NS NS * ** Frequency High 6.2 3.7 60 24 26 Low 6.1 4.6 75 19 7 Amount applied 12-13 inches 6.6 4.0 61 25 21 10-12 inches 6.3 4.5 71 20 13 6-10 inches 5.4 3.9 72 18 14 Significance * NS * * NS Table 7. Effects of irrigation frequency and amount of applied water on yield and disease incidence of broccoli, autumn 1988 Water applied Total yield Marketable yield Mildew Head rot (inches) (tons/acre) (tons/acre) % % High irrigation frequency 12.9 6.8 3.6 29 31 12.5 7.0 3.6 31 27 10.2 6.1 3.9 20 23 6.9 4.9 3.6 15 22 Significance ** NS ** ** Low irrigation frequency 12.4 6.0 4.7 15 6 11.8 6.0 4.8 15 7 11.4 6.7 4.8 25 8 8.9 5.9 4.2 21 6 Significance * * NS NS