The Use of Floating Row Covers to Prevent Aphid Transmission of Virus to Potato Seed Stock (1988)

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

Delbert Hemphill, Gary Reed, Oscar  Gutbrod, and Fred Crowe
Oregon State University


Control of virus-vectoring insects, particularly aphids, is essential in seed potato production to exclude viruses such as potato virus Y and leaf roll. Seed production fields are heavily treated with insecticides to prevent virus transmission but control is often inadequate. Floating row covers might protect plants from insect attack, reducing the need for insecticides. A preliminary trial in 1986 with three types of single-row covers indicated that covers were far more effective than the standard insecticide treatment in reducing virus transmission. Gross yields tended to be reduced, however, because of growth restriction or excessive temperatures under the covers.

The objective of the 1987 and 1988 trials was to evaluate the effect of one row cover fabric, in both single- and multiple-row widths and for several covering intervals, on yield and potato virus Y (PVY) transmission in potato. The polypropylene-polyamide material chosen for the 1987 trials had the least effect on mean daily temperatures in 1986 and was very effective in preventing virus transmission.


The trials were conducted on a Willamette silt loam, pH 5.8, to which was applied 1,000 pounds/acre of ION-8.7P-16.7K fertilizer. In 1987, nuclear virus-tested, stem-cut,.seed stock 'Norgold Russet' pieces were planted 1 foot apart in rows spaced 4 feet apart. Every fifth row was planted with PVY-infected seed. In the experiment with wide covers, plots consisted of four rows, 18 feet long. The row covers were applied over the four rows of virus-free seed, leaving the infected rows uncovered. In a separate experiment with single-row covers, plot size was an 18-foot section of a single row. In both experiments, treatments were replicated five times in a randomized block design. Planting date for both experiments was 5 May, 1987, and an additional 50 pounds N/acre was applied as ammonium nitrate on 20 May.

Alachlor and linuron were applied at 2.5 pounds and 1.0 pounds/acre, respectively, one week after planting. The rows were hilled on 25 May and covers were applied the next day. In the wide-cover experiment, the factorial combination of six treatments consisted of uncovered check, 21-foot-wide Agronet M removed on 1 July, and Agronet M removed on 11 August, both with and without insecticide treatment. The insecticide-treated plots were sprayed at biweekly intervals with acephate at 1.0 pounds/acre. In the single-row cover experiment, 6-foot wide Agronet was applied on 26 May and removed for a two-week period starting on either 11 June, 1 July, or 15 July. In addition, a fourth treatment consisted of delaying the initial covering date to 1 July. All covers were removed again on 11 August.

The foliage was rated for viral symptoms on 12 August and diquat (0.25 pounds/acre) was applied for vinekill on 14 August. The diquat was reapplied on 19 August. All plots were harvested on 9 September. A sample of 25 tubers from each plot was submitted for a winter test evaluation of PVY and leaf roll viruses.

In 1988, cultural methods were similar except as follows. Nuclear virus-tested 'Russet Burbank' seed pieces were planted on 19 May. 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 plots were hilled at initial shoot emergence on 6 June and 100 pounds N/acre as ammonium nitrate was banded on the hills on 7 June. Ten-foot-wide covers of polypropylene-polyamide were applied to the appropriate plots on 8 June. Of 18 treatments, replicated four times, 16 were covered on 8 June. Of the remaining two treatments, one was covered on 21 June, and the other was left uncovered for the entire growing season. Dates of uncovering and recovering for all treatments are listed in Table 3. A different treatment was uncovered each week during the growing season. Immediately after recovering, plots were drenched with acephate at 1.0 pounds/acre to kill any aphids invading the plot during the period of uncovering. All plots still covered were uncovered on 13 September. All plots were sprayed with diquat on 13 September and again on 16 September. Tubers were dug on 5 October. Greenhouse evaluation of virus symptoms was completed on 6 February, 1989.

Results and Discussion

The cover edges remained buried throughout the growing season but holes and tears developed during the season, primarily from snagging by spray equipment. Small holes were not repaired but major tears were patched or covers were replaced. A few aphid colonies were found on covered plants at cover removal, particularly at the final cover removal in August. It could not be determined whether these aphids entered through holes in the covers or hatched beneath the covers.

In 1987, there was very little difference among treatments in the degree of foliar damage caused by diabrotica and flea beetle. Nevertheless, both in-field and greenhouse foliar symptoms of aphid-vectored PVY infection were much greater on plants without covers than on those covered at some time during the season (Tables 1 and 2). In the wide-cover experiment, there was no significant difference in foliar viral symptoms between plants uncovered on 1 July and those uncovered just before vinekill (Table 1). In the single-row cover experiment, PVY foliar symptoms were much more prevalent in plants uncovered during June than in those uncovered during July (Table 2). Results of both experiments indicate that the critical period for viral transmission is early in the season. Any negative effects of covers on yield due to heat stress might be avoided by removing the covers during the warmest portion of the growing season. A late-season infection, while not obvious from foliar systems, might still move to the tubers, however.

A very low incidence of possible leaf roll symptoms was observed in the field (Tables 1 and 2). No leaf roll was observed in the greenhouse test. The virus source rows of potatoes in the field also failed to exhibit leaf roll symptoms.

The Agronet M covers had only a small effect on air temperatures (Table 1). Daily maxima were increased less than 8°F, daily minima by slightly more than 1°, during the measurement period. Although not statistically significant, the wide covers tended to increase the number of tubers harvested, mean tuber weight, and gross yield (Table 1).

In contrast to 1986, the insecticide program did not increase yield, nor did it reduce foliar virus symptoms. The single-row cover experiment had no uncovered check, but yields were comparable to those in the wide-cover experiment. This indicates that, in contrast to 1986, the narrow covers had no adverse affect on tuber production.

The most surprising aspect of the 1987 trials was the lack of effect of the insecticide program on viral symptoms in the field or in the greenhouse-grown plants. Since the spray program was even more rigorous than the one normally used by growers, virus transmission was expected to be reduced significantly. Leaving half the uncovered plants as well as the virus source rows unsprayed may have insured survival of large numbers of vectors. This would not be typical of a commercial seed field situation. However, it also makes the great reduction in virus symptoms on covered plants all the more impressive.

In 1988, row covers reduced the yield of potato tubers compared to vines which were never covered, primarily by reducing the number of tubers harvested. The mean weight per tuber was not significantly affected by covers (Table 4). The highest-yielding treatment was the no-cover check. The second-highest treatment was No. 5, which was covered at emergence, but uncovered early in the season and left uncovered. Treatment 9, uncovered at midseason and left uncovered, also yielded slightly higher than the average for all treatments. These results are in contrast to results obtained in 1987 when yields tended to increase with row covers, but are in agreement with 1986 results.

In Table 5, the treatments in which plots were uncovered for a week and then recovered are grouped into three 3-week categories, consisting of plots uncovered during the first, second, and last third of the growing season, respectively. Yield of plots uncovered early in the season tended to be higher than those uncovered late in the season, or never uncovered. This may be because covers were applied more loosely at recovering, allowing more room for plant growth.

Estimates of plant senescence and aphid infestation were made on 12 September (Table 3). Senescence tended to be greater for plants uncovered early in the season. Vines uncovered early and left uncovered and those never covered had the lowest degree of senescence. There was a strong positive correlation between degree of senescence and degree of aphid infestation (Rxy=0.572, p=0.001). Plots uncovered early and recovered had a greater degree of infestation than did plots uncovered later in the season. Plots uncovered early and left uncovered or never covered had the lowest degree of aphid infestation, presumably because predator insects were not excluded from these plots. These results re-emphasize that, while row covers may exclude aphids, aphids finding their way under the covers are then protected in a favorable environment.

Senescence and yield were negatively correlated (Rxy=-0.422, p = 0.001) since the prematurely senescent plants did not produce as many tubers. However, within those treatments which were initially covered, uncovered, and recovered, the correlation between senescence and yield (or aphid infestation and yield) was positive, although not significant. In this case, the effect of cover replacement and more growing room may have been more important than the tendency for premature senescence to reduce yield.

As in previous trials, covers significantly reduced virus transmission in 1988 (Tables 4 and 5). Mosaic (mostly PVY) incidence ranged from 91 percent for tubers from plots that were never covered to the 4 to 11 percent range for plots covered the entire season. Plants uncovered during the third through the fifth weeks after shoot emergence produced tubers with a greater degree of mosaic infection than those uncovered later in the season. Leaf roll symptoms also tended to be highest for plots uncovered early in the season.

The tendency for covers to reduce yields in two out of three growing seasons at the North Willamette Station and to reduce yields at the Hermiston Station (data not shown), indicates the need for further research on the causes of the yield reduction. Assuming that the cause may be heat stress, abrasion of growing points, or lack of growing room, it will be important to establish whether early cover removal will significantly reduce virus transmission without having a deleterious effect on yields.


  Table 1. Main effects of wide row covers and insecticide on yield and virus symptoms in potato, 1987                No. ofy      No. ofy                 No. of     Mean              leaf roll       PVY     Total yield  tubers     tuber   Mean temperaturez  % tubers with   Treatment  plants/plot  plants/plot (tons/acre)  harvested  wt. (g)   Max.     Min.    PVY  Leaf roll  Row cover  No cover       0.6       23.6         10.0        360        168     95.0°     50.1°   83.4    0.0  Early removal  0.3        0.6         11.3        381        179    102.6      51.3     3.1    0.0  Late removal   0.3        0.5         11.6        389        178      -         -       1.5    0.0       LSD(0.05)  NSx        **          NS          NS         NS      *         **      **      NS  Insecticide  None           0.5        7.1         11.1        385        174      -          -      29.0   0.0  Sprayed        0.3        9.3         10.8        369        176      -          -      29.7   0.0                  NS         NS          NS          NS         NS                         NS     NS     zRecorded for the period 1 June - 10 June, 1987 at 1.0 inch above the soil surface,    Degrees Fahrenheit.     yTotal plants/plot = 72.  xNS,*,**: No significant differences, differences significant at 5% and 1% levels,   respectively. No significant row cover x insecticide interactions.      Table 2. Effects of covering intervals with single-row covers on yield and virus symptoms   in potato, 1987                                                                                        Period           No. leaf roll   No. PVYz   Total yield   No. tubers  Mean tuber  Percent tubers with  uncovered        plants/plot   plants/plot  (tons/acre)   harvested    wt. (g)    PVY      Leaf roll   Planting - 1 July      0          10.0          9.7           89          166     98.7        0.0  11 June - 26 June      0           3.7         10.6          101          159     64.1        0.0  1 July - 15 July       0           0.3         11.4          104          165     47.0        0.0  15 July - 30 July      0           0.4         11.9          106          170     30.5        0.0                         NS           **          NS            NS           NS      *           NS      zTotal plants/plot = 18.      Table 3. Uncovering and recovering dates for the treatments in the virus   exclusion trial, degree of aphid infestation and vine senescence, 1988        Treatment   Date uncovered   Date recovered   Aphid infestation   Senescence	   1               6/21             6/28               2.3z            3.3y	   2               6/28             7/05               3.0             3.5	   3               7/05             7/12               2.8             4.3	   4               7/12             7/19               2.8             3.0	   5               7/19             Never              1.0             1.3	   6               7/19             7/26               1.8             2.8	   7               7/26             8/02               1.3             2.5	   8               8/02             8/09               1.5             2.5	   9               8/09             Never              1.3             1.3	  10               8/16             8/23               1.0             2.3	  11               8/23             8/30               1.3             2.5	  12               8/30             Never              1.3             2.8	  13               9/13             Never              2.0             2.8	  14               9/13             Never              2.3             3.3	  15               9/13             Never              2.0             2.8	  16               9/13             Never              1.8             2.8	  17x              9/13x            Never              2.0             3.0	  18               Never covered                       1.0             1.8	                                      LSD (0.05)       0.8             1.2      z3 point scale; 3=most severe, 1=least severe (no aphids present).   y5 point scale; 5=most severe, 1=least severe (no senescent vines).  xThis treatment initially covered on 21 June, 13 days later than    treatments 1 through 16. Emerging shoots were exposed for approximately one    week before covers applied.      Table 4. Effect of row cover treatments on yield and virus infection of   potato tubers, 1988                                                      Treatment    Yield       No. tubers     Mean tuber     % tubers with               (tons/acre)  harvested/plot     wt. (g)    Mosaic   Leaf roll   1           16.0           405             121        39         8	   2           16.3           430             114	       32        16	   3           16.3           425             115	       51        40	   4           17.7           449             119	       58        26	   5           18.2           412             135	       63         7	   6           15.3           381             120	       49         1	   7           15.6           399             117	        9         6	   8           15.6           380             124	        5         0	   9           16.6           432             115	        9        11	  10           15.3           431             106	       14         0	  11           15.1           402             113	       16         1	  12           15.4           395             118	        5        14	  13           15.8           395             121	        9         3	  14           15.5           398             118	        5         3	  15           15.5           395             118	       10        13	  16           16.6           406             123	        4         0	  17           14.6           392             112	       11        18	  18           21.4           565             115	       91         5	  Mean         16.3           416             118	       26         9	    LSD (0.05)  1.9            58              NS        20        NS         Table 5. Effect of early, mid-, and late season uncovering intervals on yield,   aphid infestation, senescence, and virus infection of potato, 1988                Uncovering period   Mean yield   Mean aphid  Mean senescence   Mean %   Mean %                      (tons/acre)    rating        rating        Mosaic  Leaf roll	  Early season           16.2          2.7           3.7           41       21	  Mid-season             16.2          1.9           2.8           39       11	  Late season            15.3          1.3           2.4           12        3	  Never uncovered        15.8          2.0           2.9            7        5	  Never covered          21.4          1.0           1.8           91        5z	  Early, not recovered   18.2          1.0           1.3           63        7	  Mid, not recovered     16.6          1.3           1.3            9       11	  Late, not recovered    15.4          1.3           2.8            5       14	  Mean,all treatments    16.3          1.8           2.7           26        9    	  zDifficult to read leaf roll symptoms because of severity of mosaic.