Control of virus-vectoring insects, particularly aphids, is essential in production of potatoes for seed to exclude viruses such as potato virus Y, leaf roll, and net necrosis. Seed production fields are heavily treated with insecticides to prevent virus transmission, but control is often inadequate. Floating row covers may protect plants from attack by insect vectors, reducing the need for insecticides. Row covers might also increase yield through their effect on air and soil temperatures around the plants. The objective of this trial was to evaluate the effect of three row covers, both with and without insecticides, on yield and virus transmission in potatoes.
Methods
The trial was conducted on a Willamette silt loam, pH 5.8, to which was applied 1,000 pounds/acre of 10-20-10 fertilizer. Virus-free foundation stock 'Norgold' seed pieces were planted 15 inches apart in single rows with 17 feet per plot on April 18, 1986. Rows were spaced six feet apart with each experimental row bordered on both sides by rows of virus-infected seed. Alachlor (2.5 pounds active) and linuron (1.0 pounds active) were applied immediately after planting.
The factorial combination of eight treatments included Agronet, Reemay, and Agryl P-17 row covers, and an uncovered check, both with and without insecticide treatment. The plots were in randomized complete block design with six replications.
The beds were hilled and the appropriate plots were sprayed with Temik at 3.0 pounds active ingredient/acre on May 21 and row covers were applied the next day. The insecticide-treated plots also received an application of Di-Syston (2.0 pounds active), Guthion (0.35 pounds active), and Baythroid (0.1 pounds active) through the covers on June 26. An additional 60 pounds N/acre as urea was sidedressed on June 30. Covers were checked weekly for structural integrity and to re-cover edges, if necessary. The row covers were removed on August 7, foliar virus symptoms and senescence were rated on August 8, and paraquat was applied for vinekill on August 11. Tubers were harvested on August 26. A sample of 15 tubers from each plot was submitted for a winter test evaluation of potato virus Y. Blossom and stem ends were rated separately.
Results
The cover edges remained buried throughout the growing season. All three covers developed holes by the middle of June, with no consistent difference among covers. By the time of the second insecticide application on June 23, a few shoots had emerged through holes in all three covers. Very little further degradation of the covers occurred before cover removal.
Very few aphids were observed at any time during the season. Row covers provided significant protection against insects as evidenced by the reduction in cucumber beetle and flea beetle damage to covered plants. Foliar virus symptoms in early August were very limited, even on the source rows of infected seed. However, what foliar symptoms occurred, were mainly confined to unsprayed, uncovered plants (Table 1).
All three row covers significantly reduced the number of virus-symptomatic plants and the insecticide treatment also reduced virus symptoms when rated in the field. There were no significant differences among covers in their effect on virus symptoms, but only the Reemay-covered plants were completely free of virus symptoms.
When tubers collected from each plot were placed in greenhouse beds for evaluation of potato virus Y (PVY) symptoms, the plants resulting from tubers from row-covered plots were nearly virus-free. The cover treatments reduced virus expression symptoms far more than did the insecticide program (Table 1).
Insecticide sprays retarded senescence but row covers, when averaged over spray treatments, had no effect on senescence (Table 1). The interaction of covers and insecticide, however, was highly significant (Table 2). Covers promoted senescence on unsprayed plots, but retarded senescence on sprayed plots.
Sprays did not significantly affect the number of tubers harvested/plot but did increase yield, primarily because of increased tuber size. This may have been caused by the delayed senescence and increased shoot vigor on sprayed plots. All row covers reduced the number of tubers per plot and total yield. Mean tuber weight was reduced by Agryl but not by Reemay or Agronet.
Scab incidence in this trial was moderate and was not rated on a numerical scale. The incidence and severity of scab, however, were by far the most severe on the Agryl-no spray treatment.
Temperature records were not kept in these plots, but air temperatures were recorded under each row cover material in another experiment from June 11 to June 16, 1986. For this six-day period the maximum air temperature at one inch above the soil averaged 111.2°F under Agryl, 107.0° under Reemay, 98.5° under Agronet, and 91.9° over bare ground. Minimum air temperatures averaged 49.6° under Agryl, 50.2° under Reemay, 48.6° under Agronet, and 46.8° over bare ground. These temperatures may well have been excessive and heat stress may have contributed to reduced yields under row covers. The physical barrier to shoot growth may also have been responsible for reduced yields with row covers. The very unusually high temperatures in Western Oregon in June (record high heat unit accumulation) may have prevented any positive effects of row covers on plant growth.
Conclusions
Based on observation of foliar symptoms, row covers show promise as a means to reduce insect-vectored virus infection in potatoes. Excessive heat buildup under the covers or restricted shoot growth may, however, reduce yields. Row covers might be more effectively utilized on very early plantings with covers removed at the onset of hot weather. The initial insecticide application could probably be eliminated.
Table 1. Effect of insecticide sprays and row covers on yield and foliar virus symptoms, potato row cover trial, 1986 No. of virus SenescenceZ Tubers/ Total yield Mean tuber % tubers with PVY Treatment plants/plot on August 8 plot (tons/acre) wt. (g) Blossom end Stem end Main effects No spray 0.47 3.5 78.4 6.83 193.5 12.6 13.0 Sprayed 0.17 2.5 84.6 8.37 217.5 12.6 11.3 LSD(0.05) 0.30 0.4 NS 0.73 16.4 NS NS Reemay 0.00 3.0 71.3 7.04 219.6 4.0 4.0 Agronet 0.18 3.3 80.2 7.65 212.3 0.0 2.0 Agryl 0.08 3.0 79.4 6.57 181.8 0.0 3.3 No cover 1.00 2.8 95.1 9.12 208.4 42.7 39.3 LSD(0.05) 0.53 NS 12.1 1.04 23.1 9.3 11.6 Interaction No spray Reemay 0.00 3.8 68.2 6.09 203.1 4.0 6.7 Agronet 0.20 3.7 76.3 6.75 199.6 0.0 2.7 Agryl 0.00 4.0 78.0 5.69 161.6 0.0 0.0 No cover 1.67 2.5 91.0 8.72 209.9 46.7 42.7 Sprayed Reemay 0.00 2.2 74.5 7.94 236.2 1.3 1.3 Agronet 0.17 2.8 84.0 8.54 225.0 2.7 1.3 Agryl 0.17 2.0 80.8 7.46 202.1 8.0 6.7 No cover 0.33 3.2 99.2 9.52 206.8 38.7 36.0 LSD(0.05) 0.75 0.7 17.2 1.47 32.7 13.1 16.4 Z5 point scale, with 5 = foliage completely dead, 1 = healthy green foliage. Table 2. Statistical significance of main and interaction effects of insecticide sprays and row covers on yield and foliar virus systems, potato row cover trial, 1986 Treatment No. of virus Senescence No. tubers Total Mean tuber % tubers with PVY plants/plot 8/8/86 per plot yield weight Blossom end Stem end Spray vs. *Z ** NS ** ** NS NS no spray Row cover ** NS ** ** NS ** ** vs. none Type of cover NS NS NS * * NS NS Spray x covers * ** NS NS NS NS NS Z*, **, NS: significant differences among means at 5% and 1% levels, and no significant differences, respectively.