SEED-POTATO CERTIFICATION.

New Zealand Journal of Agriculture, Volume 46, Issue 3, 20 March 1933, Page 165

SEED-POTATO CERTIFICATION.

SOME OBSERVATIONS MADE DURING FIVE YEARS’ OPERATIONS.

J. H. Claridge,

Assistant in Agronomy, Plant Research Station, Department of Agriculture,■ Palmerston North. • ..S.

Seed-potato certification was inaugurated by the New Zealand Department of Agriculture in 1927. Now, after five years, in reviewing the progress made, it is possible to draw certain conclusions of general interest, and such is the purpose of the present article.

In the certification of seed potatoes an attempt has been made to select those crops which attain a reasonable standard of purity and freedom from virus diseases. Uniformity of grading, freedom from tuber diseases such as scab and dry-rot, and other factors of this kind also receive consideration before a line can be finally certified. It was early realized that the dominant factor influencing yield was the presence in the crop of virus diseases, such as leafroll and mosaic. These diseases are not soil-borne. They are transmitted in the field from plant to plant, and are carried over in the seed from year to year. Their presence results in a marked decrease in the yielding ability of a line.

It has been the practice in certification to make cropping power and virus counts the most important factors in deciding upon the acceptance or rejection of crops. All cropping-power figures used in this article have been obtained from the certification trials.

In these trials samples from the different lines entered for certification are grown under uniform conditions, so that comparisons of yield may be made between the lines. The tubers of each line are dug and graded into “ table,” “ seed,” and “ small.” The weights of the table and seed potatoes are obtained. The cropping-power is then determined by adding half the weight of the seed tubers to the weight of the table tubers and converting this figure into “ tons per acre.” In order to overcome the difficulty of variation in yield from season to season, the cropping-powers of each individual season are so adjusted that the best lines give a figure of approximately io tons per acre of cropping-power. . Virus counts are also made in the samples, the figures obtained being . used as a check on the virus counts made in the field.

In this article points of interest on which observations are made are : (i) The relationship between virus and cropping-power; (2) comparisons between certified; and non-certified seed; (3) the distribution of certified seed; (4) the variation in deterioration of lines when grown under different conditions ; (5) the improvement in yield of crops entering certification ; (6) the purity of crops in relationship to certification.

Relationship of Virus to Cropping-power.

In Table 1 the results of growing 281 samples of the Dakota variety are recorded. The trials cover a period of five years, and demonstrate conclusively that the cropping-power is inversely proportionate to the degree of virus infection. There are other factors, such as size of seed and condition of seed when planted, which have some small effect on the cropping power, but the table demonstrates that the question of virus infection is by far the most important. A similar table is obtained when the figures for any other variety are analysed in like manner.

Certified versus Non-certified Seed.

From the foregoing remarks it may be inferred that the main difference between certified and non-certified seed consists in the ability of the seed to yield. While it is quite possible that occasionally a line of uncertified seed may yield equally as well as certified seed, the advantage to be gained from the use of certified seed has been demonstrated time and again under a very wide range of conditions of climate, season, and soil. Also it should be realized that such productive lines cannot be recognized by merely inspecting the seed. On the average, from the results of trials carried out by the Department of Agriculture, the grower of certified seed may expect a 25 per cent, greater yield than the grower of uncertified seed. This is taking into account the fact that some lines of uncertified seed might come up to certification standard were they placed under trial.

A series of experiments was conducted by the Department during 1929-30 to demonstrate the difference between lines of seed which were of certification standard and lines of seed below certification. Twenty-three trials were carried out in six centres in the South Island. The varieties used were Up-to-Date, Dakota, Aucklander Short Top, and Epicure. The seed for the trials was obtained from the certification plots of the previous season. The seed-size tubers of the one-third highest-yielding lines of a variety were bulked together, and the third lowest-yielding lines were similarly treated. These tubers formed the material for comparison. The average yields over the whole series resulted in an increase of 3| tons per acre from the certified seed, which represents an increase of 64 per cent. At the same time the proportion of virusdiseased plants in the seed from the rejected lines was nearly twice that in the seed from the passed lines.

Distribution of Certified Seed

Certification has performed a most valuable service in stimulating the distribution of high-quality seed to replace much of the poor seed that has been used in the past.

For the purpose of illustrating this point, use is again made of the Dakota variety. One grower entered his crop in 1927-28, and the line was considered one' of. the best in the certification trials of that season. During the last two seasons, over 50 per cent, of the Dakota crops entered can be traced back to this one crop certified in 1927-28. The figures in Table 2 (next page) record the number of entries in each season which could be traced back to the original crop, and the percentage of the total number of Dakota entries received. ■

The case in point is an extreme one so far as distribution is concerned, but is an excellent example of what is possible under a combination of certification and good marketing. As a more typical example, it may be mentioned that all the eighty-seven Dakota crops passed in the last two seasons can without exception be traced back to four original lines entered since the commencement of certification.

By certifying annually some 800 acres of potatoes it is estimated that sufficient seed could be produced by the second season following to sow the whole of the Dominion’s acreage of potatoes. Therefore if growers were to aim at planting seed as little removed as possible from certification, theoretically they should never have to plant seed which was more than two seasons removed from certified stock.

Deterioration of Lines.

It has been mentioned that 105 entries of the Dakota variety can be traced back to the one original line. This line has been entered in each of the five seasons under review, while thirty-six entries have been received of seed supplied to other growers from this line. Thus in all forty-one samples of seed produced by the original grower have been under trial, and all have reached certification standard. It may therefore be concluded that in this case there has been no deterioration over a period of five years.

Thirty-five entries were received from growers whose seed had been grown away from the original farm during one season. Of these, twenty-two reached certification standard, while thirteen were rejected. Of twenty-seven entries representing seed which had been grown away from the original farm for two seasons, twelve entries were passed and fifteen rejected. Two entries representing seed grown three times away from the original line were both rejected. Thus it would appear that the farther removed from the original crop the seed became the greater was the chance of the seed having deteriorated to below certification standard. It should be noted that in seventeen cases two or more growers had purchased seed from the same line. In sixteen of these cases lines of seed originating from identical crops in the previous season were all either passed or rejected. In only one case did both passed and rejected samples originate from the same crop in the previous season.

From an analysis of the districts where the lines have been grown away from the original farm it is indicated that deterioration has been most marked in the Temuka, Washdyke, and Seadown districts of Canterbury. Only three out of eight entries of seed once grown in this area were passed, and these three were rejected in the following season after twice growing in the locality. On the other hand, the district running from Templeton to Rakaia produced lines which on the average showed little deterioration. These

remarks should not be taken as conclusive, as they are indicative only of the particular districts mentioned, and other localities not represented in the trials may be more or less suitable to the checking of deterioration due to virus disease. It may be concluded from these results that when the original line is of a high standard, that standard may or may not be maintained, depending on the conditions under which the line is grown in subsequent years.

Improvement in Crops entering Certification.

The accompanying graph serves to illustrate the general improvement that has taken place in the cropping-powers of lines entered for certification. Comparison is made between the two seasons 1928-29 and 1929-30, and those of 1930-31 and 1931-32. The figures have been taken from the Dakota variety, as being one of the most important varieties in New Zealand at- the present time.

The following features are worth noting: (1) . In the. 1928-29 and 1929-30 seasons the majority of the crops fall about the average ; there are a few good crops, while there are a number of poor crops tailing off below the average. (2) In the 1930-31 and 1931-32 seasons the bulk of these poor crops have been eliminated, the growers of these in many cases having purchased certified seed. There is also a decided improvement in the general average of the cropping-power, associated with an increase in the number of good crops. This may be attributed directly to the effect of certification in drawing attention to the better-class crops, and so stimulating their distribution and re-entry into certification.

Purity of Crops.

The standard of purity in certification allows for not more than 2 per cent, of foreign varieties. During the first three seasons 24-6 per cent, of the crops were rejected on this account. In the last two seasons the proportion has averaged 5-2 per cent., a result indicating the direct influence of certification.

The question of purity is on the whole not a vital factor in potato certification. There is only one variety—Northern Star—which is a serious “ rogue ” in white-skinned varieties. Northern Star under normal conditions throws more seed-size tubers than any other variety. Hence, under the usual practice of selecting the smaller tubers for planting, the percentage of plants of this variety, if present at all in the line, is bound to increase. Cases have been common of a line of potatoes containing, say, 2 per cent, of Northern Star rogues in one season, and in the following season showing an increase to 4 per cent. No other variety when present as a rogue has been noticed to show any appreciable increase over two or even more seasons. Therefore, when once a reasonably pure line has been obtained, there is every chance of its remaining reasonably pure provided mixing does not occur.

A source of mixing not generally realized is the practice of planting potatoes on land which produced a crop of a different variety in a previous season. In this respect Northern Star is again the worst offender. Cases are on record on a pure line of potatoes, sown on land which had grown a crop of Northern Stars two seasons previously, showing a percentage of 5 to 10 per cent, of these rogues present. So far as coloured-skinned potatoes are concerned, no variety has shown any tendency to increase appreciably when present as a rogue in other varieties.

Conclusions in Brief.

The yield of the potato crop is reduced in proportion as the percentage of virus infection increases.

Certified seed may be expected to yield in the neighbourhood of 25 per cent, better than uncertified seed.

Certification is of great value in that it results in the rapid distribution of the better lines of seed potatoes.

A line which yields well in one season will not necessarily retain its high cropping-power indefinitely. Virus disease may or may not spread, and therefore the cropping-power may or may not be affected, depending upon the conditions under which the line is grown.

There has been a general improvement in the quality of the crops entering certification, brought about by the stimulation given to the distribution of the good lines and the suppression of the poor ones.

The question of purity is closely linked up with the Northern Star variety. Provided a grower exercises reasonable care to prevent mixing of lines, and is able to identify and remove any Northern Stars present in his crop, he should have no difficulty in maintaining a pure line of seed.

Noxious Weed Order. The Weber County Council has declared milk thistle, otherwise known as variegated thistle, to be a noxious weed within that county.

Number of Crops in . Group. Cropping-power Group. (Tons per Acre.) Percentage of Plants infected with Virus Disease. 14 Over io-o ■ 10-9 .14 9•5-9•9 n-4 16 9-o-9•4 13-0 34 S-5-8-9 ■ 12-6 ' 26 ■ . . .. 8•0-8-4 , . 15-0 . 33 7’5t7’9 15-9 - 32 7-O-7-4 17-0 - . 26 j . 6-5-6-9 20-0 29 •... 6-o—6-4 27-O 15 ■ 5'5~5'9 31-0 21 . . 5-O-5-4 33’0 6 • 4•5-4•9 41 -o 3 4-O-4-4 46-0 5 3’5-3-9 59-o I 3-O-3-4 50-0 4 •• 2-5-2-9 67-0 1 . . .. 2• 0-2•4 70-0 1 ■■ •• -5-1-9 79'0

Table 1. —Comparison between Virus Percentage and Cropping-power in 281 Lines of Dakota Potatoes.

Season. ' , . Number of Crops traced back to Original Crop. Percentage of Total Number of Dakota Entries received. x 1927-28 . . . . I 4 1928-29 . . 3 9 I929-3 0 " 25 23 1930-31 53 57 1931-32 23 46 Totals. . i°5 33

Table 2. —Illustrating Distribution of Certified Seed.