Research report from OSU's North Willamette Research and Extension Center
Delbert Hemphill and Gary Reed
Oregon State University
Introduction
Control of virus-vectoring aphids is essential in potato seed production to exclude viruses such as potato virus Y (PVY) and leaf roll. Floating row covers might protect plants from insect attack, reducing the need for insecticides.
Previous trials at the North Willamette Center in 1986 through 1988 indicated that covers were far more effective than the standard insecticide treatment in reducing virus transmission. Gross yields were often reduced, however. This ususally involved both reduced numbers of tubers recovered and smaller mean tuber size. In general, the longer the covers were in place, the greater the yield reduction. The only exception was 1987 when a short-season cultivar was grown. The yield reduction with covers might be attributed to heat stress, restricted room for vine growth, abrasion of leaves or growing points by the cover fabric, or outbreaks of aphids or spider mites occassionally observed on covered vines. In contrast, there are several published report from eastern states and Canadian provinces indicating that row covers promote early harvest and increase the yield of new potatoes. Our results also have consistently indicated that covers promote early vine development in potatoes.
In 1988, covers were removed and then reapplied at various times during the growing season. Virus infection occurred mostly when the vines were exposed during early to mid-season. The objective of the 1989 trial was to confirm results obtained in 1988. The 1990 trial was intended to determine at what point in the growing season covers cease to promote potato development and begin to reduce tuber growth and development.
Methods
The trials were conducted on a Willamette silt loam, pH 5.5, to which was applied 1,000 pounds/acre of 10N-8.7P-16.7K fertilizer. On 15 May, 1989, nuclear virus-tested 'Russet Burbank' seed pieces were planted one foot apart in rows spaced four feet apart. Every third row was planted with virus-infected seed. The row cover plots consisted of a pair of virus-tested rows, 20 feet long. The rows were hilled on 19 May after additional N as ammonium nitrate was banded over the row at 100 pounds N per acre. Alachlor at 2.5 pounds and linuron at 1.0 pounds per acre were then applied to the plot area for weed control. Ten-foot wide covers of polypropylene-polyamide (Agronet M) were applied to the appropriate plots on 22 May. Aldicarb was applied to insecticide-treated checks at 2.0 pounds per acre on 23 May. Shoot emergence was first noted on 3 June. Yellow-pan aphid traps were distributed through the planting on 5 June, with approximately one trap for every 1500 square feet.
Of the 18 treatments, replicated four times, 15 were covered on 22 May. Of the remaining three treatments, one was aldicarb-treated and covered on 12 June. The other two treatments were uncovered checks, one with and one without aldicarb application at hilling. Other treatments were uncovered at weekly intervals during the growing season. Most were recovered one week later after an application of aldicarb. Dates of uncovering and recovering for all treatments are listed in Table 1. Aphids were collected from the traps weekly, just before cover removal and replacement took place. All covers were removed and the vines were sprayed with diquat at 0.25 pounds/acre on 5 September. Paraquat at 0.38 pounds per acre was applied to finish vine kill on 14 September. Plots were harvested on 18 September. Tuber samples from each plot were sent to the Hermiston Research and Extension Center for virus evaluation. The greenhouse-forced plants were rated on 6 Feb., 1990.
On 9 May, 1990, nuclear virus-tested 'Russet Burbank' seed pieces were planted nine inches apart in rows spaced three feet apart with 13 feet between the centers of each pair of rows. Plot length was six feet (18 seed pieces) with a four foot gap between plots. Aldicarb was applied to the seed furrow at 6 g per plot immediately after planting. The furrows were then closed and were rehilled one week later after applying 100 pounds per acre of nitrogen as ammonium nitrate in a band over the row. After rehilling, alachlor was applied at 2.5 pounds per acre and linuron at 1.0 pounds per acre. Ten foot-wide polypropylene-polyamide (Agronet M) row cover was applied to the appropriate plots on 24 May.
Plots were either left uncovered for the season, covered initially and then uncovered after four weeks, uncovered after nine weeks, or covered for the entire growing season of 16 weeks. Eight-plant samples were removed from the four replicates of each treatment at 2, 4, 6, 8, 11, and 16 weeks after shoot emergence. Shoot fresh and dry weights and tuber numbers and weight were measured at each harvest. Vines were killed with paraquat (0.4 pounds per acre) after 16 weeks. Air and soil temperatures were measured on two replicates of uncovered and covered plots for the entire growing season using a Speedomax 250 recorder and thermocouples set at six inches above the soil surface and at two inches depth in the soil.
Results
In 1989, as in most previous trials, row covers reduced the yield of potato tubers compared to vines which were never covered, primarily by reducing the number of tubers harvested (Table 1). The mean weight per tuber also was reduced for many of the cover treatments. Greatest mean weight per tuber occurred with the treatment that was uncovered early in the season and never recovered (treatment 5). Gross yield for this treatment also did not differ significantly from that of the never-covered treatment. Plots uncovered one month later (treatment 10) and not recovered did not significantly outyield plots covered for the entire season.
In Table 2, the treatments in which plots were uncovered for a week and then recovered are grouped into three four-week categories, consisting of plots uncovered during the first, second, and last third of the growing season, respectively. Yields tended to be slightly higher for plots uncovered earlier in the growing season. Also, plots which were not recovered tended to outyield recovered plots for each third of the growing season. Aldicarb treatment had no effect on yield, reflecting the absence of serious foliage-devouring insects in the plots.
In 1990, plants grown under row covers had significantly greater vine fresh weight at both 2 and 11 weeks after emergence (Table 3). Although not statistically significant, the same trend was observed at 4, 6, and 8 weeks after emergence. Vine dry weight, however, was not consistently affected by cover treatment. This was because of a consistent trend toward lower percent dry matter in covered than in noncovered vines. This may be due to greater moisture stress in plants grown in the open as row covers generally cause higher relative humidity and reduced air movement in the plant canopy.
Tuber development was first noted five weeks after emergence. As early as six weeks after emergence, tuber numbers tended to be reduced for covered plants. Although not statistically significant at each harvest, this trend continued for the remainder of the season. At the final harvest, there was a trend toward decreasing tuber number per hill with the exception that tuber number increased for plants covered the entire season. Mean tuber weight increased with length of the covered period. These results are consistent with those obtained in 1986, 1988, and 1989, except that in the previous years, both tuber number and tuber weight tended to decrease as the covering period lengthened.
It is not possible to determine from these results whether the reduced tuber yield with row covers is due to heat stress or to some other cause. Mean maximum air temperature under covers was 5oF (99.2 vs. 94.1) higher for the season, while the mean maximum soil temperature was increased by more than 11oF (89.1 vs. 77.9) under covers. Mean minimum soil and air temperature were not significantly altered by covers. The increase in mean maximum soil temperature could have caused tuber resorption or increased respiration may have limited tuber formation.
These results contrast strongly with those reported from several eastern states and provinces where row covers have promoted yield and earliness of new potatoes.
Effect of treatment on virus incidence was very consistent with the pattern seen in 1988. A one-week exposure to aphids early in the growing season led to greater PVY incidence than did exposure late in the season. Plants uncovered for a week in early August had infection levels that tended to be less than for those covered for the entire season. For plants left exposed the entire season, aldicarb treatment at planting had no effect on PVY incidence. Incidence of leaf roll virus did not vary significantly with time of exposure.
Table 1. Uncovering and recovering dates, yield, and percentage virus infection for the treatments in the virus exclusion trial, NWREC, Oregon, 1989 Date Date Yield No. tubers Mean tuber % tubers with Treatment uncovered recovered (T/A) harv./plot wt. (g) PVY Leaf roll 1 6/12 6/19 18.7 290 121 35.0 18.8 2 6/19 6/26 19.6 289 129 74.4 8.1 3 6/26 7/03 21.8 331 124 26.3 23.2 4 7/03 7/10 24.1 333 137 55.3 24.3 5 7/10 Never 30.5 353 162 50.4 11.8 6 7/10 7/17 20.3 293 132 52.7 18.6 7 7/17 7/24 19.2 289 127 41.8 11.3 8 7/24 7/31 18.9 285 126 55.7 14.3 9 7/31 8/07 16.0 282 108 15.6 22.5 10 8/07 Never 20.1 295 129 12.9 2.9 11 8/07 8/14 17.9 284 118 11.0 10.8 12 8/14 8/21 16.4 280 111 4.4 42.1 13 8/21 8/28 18.3 269 130 8.7 2.9 14 8/28 Never 18.3 280 124 13.5 23.4 15 9/05 Never 19.1 290 125 12.8 2.9 16z 6/03 6/12 21.0 323 124 16.3 0.0 17y Never covered 27.6 387 132 57.4 9.9 18x Never covered 31.5 427 138 52.2 6.9 LSD (0.05) 5.4 57 22 22.0 NS zThis treatment was left uncovered at planting. Emerging shoots were exposed for nine days before covers were applied. yTreated with aldicarb at hilling. xNo insecticide applied. Table 2. Effect of early, midseason, and late season uncovering intervals on yield and virus infection of potato, NWREC, Oregon, 1989 Mean yield No. tubers Mean tuber Mean % Mean % Uncovering period (T/A) harv./plot wt. (g) PVY Leaf roll Early season 21.0 311 128 47.8 18.1 Mid-season 18.6 287 123 41.4 14.7 Late season 17.5 278 121 9.4 17.3 Never uncovered 19.1 290 125 12.8 2.9 Never covered, +aldicarb 27.6 387 132 57.4 9.9 Never covered, no aldicarb 31.5 427 138 52.2 6.9 Early, not recovered 30.4 353 162 50.4 11.8 Mid-season, not recovered 20.1 295 129 12.9 2.9 Late, not recovered 18.3 290 125 13.5 23.4 Mean, all treatments 21.1 310 127 33.1 14.2 Table 3. Effect of row covers and cover removal date on potato vine and tuber development over six harvests, NWREC, Oregon, 1990 Weeks after Treatment Vine wt. (g/plant) % dry Tubers/plant Mean tuber emergence Fresh Dry matter No. Wt. (g) wt. (g) 2 No cover 15 1.8 12.4 -- -- -- Covered 2 weeks 25 2.3 9.4 -- -- -- *z NS ** 4 No cover 164 -- -- -- -- -- Covered 4 weeks 230 -- -- -- -- -- NS 6 No cover 598 55 9.3 10.5 175 16.7 Covered 4 weeks 676 60 9.0 8.0 165 20.7 Covered 6 weeks 655 52 8.2 8.4 151 18.2 NS NS NS NS NS NS 8 No cover 818 139 17.3 10.9 445 41 Covered 4 weeks 879 137 15.6 9.6 411 43 Covered 8 weeks 939 121 13.3 10.3 428 42 NS NS * NS NS NS 11 No cover 698 85 12.3 12.0 795 67 Covered 4 weeks 1360 138 10.3 9.2 774 85 Covered 9 weeks 1051 103 9.8 9.3 675 72 Covered 11 weeks 1082 95 9.7 9.1 669 74 * NS NS * NS NS 16 No cover -- -- -- 16.8 1796 107 Covered 4 weeks -- -- -- 12.7 1553 123 Covered 9 weeks -- -- -- 9.7 1401 145 Covered 16 weeks -- -- -- 11.6 1714 148 ** NS * zNS,*,**: no significant differences, significant differences exist at the 5% and 1% levels, respectively.