The Effect of Floating Row Covers on Virus Transmission and Yield of Potato Seed Stock (1990)

Research report from OSU's North Willamette Research and Extension Center

Delbert Hemphill and Gary Reed
Oregon State University


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.


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.


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.