Research report from OSU's North Willamette Agricultural Research and Extension Center
Delbert Hemphill
OSU Dept of Horticulture, NWREC
Introduction
Lettuce, spinach, and other leafy greens in western Oregon suffer from a complex of yellows disorders which do not appear to be related to plant nutrition or fungal pathogens. In 1987, a spinach variety trial at the Station eliminated soil pH and Fusarium yellows as the cause of two yellows disorders observed in the planting. One disorder, characterized by a pale, chlorotic mottling of the leaf blade, was reduced by regular applications of acephate insecticide. The other disorder, characterized by a bright yellow interveinal coloring, particularly on older, lower leaves, was not affected by insecticide, soil pH, or cultivar. Affected plants, and similarly yellows-affected plants from other sites and grower fields, were determined to carry beet western yellows virus (BWYV) in 1987.
Insect-vectored viruses, particularly those spread by aphids, have been controlled by excluding the vector from the plants with row covers. The purpose of this trial was to attempt to reduce the yellows disorder in spinach and lettuce with either row covers, or a combination of row covers and acephate, an effective aphicide.
Methods
A Willamette silt loam, pH 6.2, was prepared for transplanting by rototilling in 1,000 pounds/acre of ION-8.7P-16.7K fertilizer. Half the field was treated with 3.0 pounds/acre of cycloate herbicide, which was incorporated in preparation for planting spinach. Three cultivars each of lettuce and spinach were seeded on 25 July into trays consisting of 0.75-inch-square cells and containing a commercial peatlite medium. The transplants were set out on 16 August with a single row of each cultivar per 10-foot-long bed. Between-row spacing was 2.5 feet and in-row spacing was 1.0 foot. The lettuce cultivars were 'Summertime,' 'Salinas,' and 'Waldmann's Green.' The spinach cultivars were 'Grandstand,' 'Liberty,' and 'Olympia.' Immediately following planting, the lettuce area was treated with pronamide herbicide at 2.0 pounds/acre.
On 18 August half the plots of each species were covered with Agronet row cover. The 10 foot wide covers covered a single 3-cultivar bed. On 22 August, half of both the covered and uncovered plots were treated with acephate at 1.0 pound/acre and carbaryl at 1.0 pound/acre, at 40 pounds/square inch pressure and 100 gallons per acre total volume. Thus, the treatments consisted of a factorial combination of cover (+ or -) and spray (+ or -) treatments, with the four treatment combinations in randomized complete block design with four replications for lettuce and three replications for spinach. Individual cultivars were subplots of these main plot treatments. The insecticide treatments were reapplied on 6 September. Plots were uncovered and rated for yellows disorder on 13 September.
Results
The insecticide sprays were associated with an increase in bright yellow coloring of the older leaves of lettuce while row covers decreased the number of plants showing symptoms by 50 percent (Table 1). Cultivars did not vary significantly in the percentage of plants showing symptoms. There were no significant 2- or 3-way interactions of insecticide, row cover, and variety, so only main effects are shown in the Table.
For spinach, neither insecticide, row cover, nor cultivar had any significant effect on the expression of the disorder (Table 1).
These results do not provide strong evidence that the observed yellows disorder was caused by an insect vector or that row covers will be an effective means of control. Tissue samples were not submitted for ELISA test of BWYV infection, but plants from test plots at the Vegetable Research Farm at Corvallis, showing similar symptoms, did not test positive for BWYV in 1988. However, lettuce samples from a grower's field near the North Willamette Station, also showing lower leaf yellowing, did test positive for BWYV. Obviously, further research is needed to pin down the cause and possible control measures for this yellows disorder(s), which has caused serious losses in commercial fields in both 1987 and 1988. Perhaps more than one luteovirus is involved, which would explain the lack of consistency in positive tests for BWYV.
Table 1. Main effects of row covers, insecticides, and cultivar on lower leaf yellowing symptoms in three cultivars of lettuce and spinach, 1988 Percent plants showing symptoms Treatment Lettuce Spinach - Insecticide 16.6 25.8 + Insecticide 31.7 19.2 *z NS - Cover 32.2 21.6 + Cover 16.1 23.4 * NS 'Summertime' 22.4 'Grandstand' 20.8 'Salinas' 20.1 'Olympia' 28.4 'Waldmann's' 29.9 'Liberty' 18.2 NS NS z*,NS: difference significant at the 5 % level, and nonsignificant, respectively.