Introduction
Willamette Valley spinach growers have complained of a yellows disorder or chlorosis affecting the crop. Possible causes include Fusarium wilt, which causes a general chlorosis and wilting of the plant as the vascular system is invaded by the fungus; various insect-vectored viruses; and suboptimal pH accompanied by Mn or Al toxicity. In this trial, Fusarium-resistant varieties of spinach as well as non-resistant varieties were seeded on plots of soil with pH from 4.9 to 6.5. Half of each soil pH plot was treated with insecticide in an attempt to pin down the likely cause of yellows/chlorosis problems observed by growers.
Methods
In 1974, lime was applied to 40- x 45-foot plots of Willamette silt loam at rates of 0, 4, and 8 tons/acre, and sulfur was applied at 1.0 ton/acre. The four soil pH treatments were replicated four times in randomized complete block design. Soil pH at time of planting in 1987 ranged from 4.9-5.0 for the sulfur treatment to 6.4-6.6 for the 8-ton lime treatment. On 26 May, 1987, 700 pounds/acre of ION-8.7P-8.3K fertilizer was applied to all plots. On 28 May, six cultivars of spinach were seeded on the pH plots with a single row of each cultivar and 3.0 feet between rows. Cultivars were randomized within each pH main plot. The main plots were split by an insecticide spray program, with half of each plot (22.5 feet) receiving biweekly applications of both carbaryl and acephate at 1.0 pounds/acre. Chlorpropham at 2.0 pounds/acre was used for weed control on all plots. Plots were irrigated and cultivated as necessary. An additional 35 pounds N/acre as ammonium nitrate and 25 pounds N/acre as calcium nitrate were sidedressed on 16 June and 9 July, respectively. Yields were not determined but all plots, except for the pH 4.9-5.0, were rated for quality characteristics and yellowing or chlorosis on 17 July.
Results
Possible viral symptoms were identified with the help of Alf Christensen Seed Co. pathologist Margaret Savage. Viruses present in plant samples were identified by Dr. Paul Koepsell. Beet western yellows virus was found in leaf tissue showing typical bright yellow marginal chlorosis. Cucumber mosaic virus was isolated from leaves showing a chlorotic mottle characterized by light green or yellow spots and small (1 mm diameter) raised areas on the leaf surface.
Fusarium wilt symptoms were confined primarily to the cultivar `Giant Nobel,' which is not resistant to this disease.
The spinach plants grew very poorly on the sulfur-treated plots. These plots were not rated for quality characteristics. Plant growth, as measured by plant width and height of the tallest leaf, increased with rate of lime application (increasing soil pH) for all varieties (Table 1). Soil pH had no consistent effect on leaf color, degree of bolting, chlorotic mottle, or marginal yellowing (beet western yellows symptom). The insecticide treatment, when averaged over lime rates and cultivars, had no effect on plant growth and overall color, but reduced the severity of chlorotic mottle, a possible symptom of cucumber mosaic virus. This indicates that the chlorotic mottle is a symptom of insect injury or virus infection. There was a significant interaction of lime rate and insecticide treatment on plant width. The insecticide improved plant growth at high soil pH but not at low pH (Table 2).
Among cultivars, the most vigorous and long-standing plants were of 'Olympia.' This cultivar also had the best overall color rating. The chlorotic mottle symptoms were most severe on 'Skookum.'
These results indicate the need for more exact descriptions of yellows disorders in spinach, as a number of factors may be involved, including low N fertility, low soil pH, several viruses, and Fusarium. The chlorotic mottle observed in this trial, and in a commercial field at the same date, may be a symptom of cucumber mosaic virus or other insect-transmitted virus or mycoplasm. The mottle may also be simply a wound response to injury by leaf hoppers or other sucking insects, the so-called "hopper burn."
Table 1. Main effects of lime, cultivar, and insecticide treatment on quality of spinach, 1987 Plant Plant width height Degree ofy Chloroticx Marginalx Treatment (cm) (cm) Colorz bolting mottle yellowing Lime rate (tons/acre) 0 17.3 14.4 2.5 3.1 2.8 2.3 4 22.9 18.4 2.7 3.2 3.0 2.6 8 24.8 18.9 2.8 3.1 2.8 2.2 LSD (0.05) 2.5 1.8 NSw NS NS 0.3 Insecticide Unsprayed 20.7 16.7 2.7 3.0 3.4 2.3 Sprayed 22.6 17.9 2.7 3.3 2.3 2.4 NS NS NS * ** NS Cultivar Baker 20.4 17.6 2.3 4.2 2.4 2.6 Giant Nobel 20.7 16.0 2.7 3.3 3.0 2.6 Hybrid 424 19.2 16.9 1.8 4.3 2.8 1.9 Jake 22.9 17.5 3.0 2.8 2.7 2.1 Olympia 24.8 17.8 3.3 1.3 3.0 2.5 Skookum 22.0 17.8 3.1 3.0 3.3 2.4 LSD (0.05) 3.6 NS 0.5 0.4 0.4 0.5 zFive point scale with 1 = palest, 5 = darkest green. yFive point scale with 1 = no elongation of seedstalk, 5 = male flowers shedding pollen. xFive point scale with 1 = no symptoms, 5 = severe symptoms. Probable symptom of beet western yellows virus. wNS, *, **: no significant differences and significant at 5% and 1% levels, respectively. Table 2. Interaction of lime and insecticide treatment on spinach plant width, averaged over cultivars, 1987 Plant width (cm) Lime rate (tons/acre) Sprayed Unsprayed 0 16.4 18.2 4 25.5 20.2 8 25.8 23.8 Interaction LSD(0.05)= 3.6