Growing techniques of potato from true seed in Finland

The suitability of three production methods for potato (Solarium tuberosum L.) from true seed (TPS) were studied in field conditions at the University of Helsinki, Department of Horticulture. The production methods were direct field sowing, planting 4-weekold seedlings raised in peat pots and planting seedlings and covering them with floating polypropylene mulch. The cultivars were Explorer and Pan American Seed Company varieties numbers 2928 and 4000. Also tested were the cultivars 2909, 2928, 3018, 4000, 4016, 4048 and 4147. Floating mulch promoted vegetative growth, but also promoted the propagation and spreading of Phytophtora infestans spores. The best yield was obtained with pot-raised seedlings without mulching (3.38 kg/m 2 ), the second best yield with mulching (2.69 kg/m 2) and the lowest yield with field-sowing (2.33 kg/m 2). More small tubers were obtained under floating mulch than without mulching. Of the seven Pan American Seed Company cultivars tested, 4047 and 4016 gave the best yield, 4.1 and 4.0 kg/m 2 . In sensory tests none of the three cultivars (3018, 4047 and Explorer) proved to be better than or equal to the taste of the common vegetativelypropagated Finnish potato variety called Pito. Index words: floating mulches, mulching, potato, pot raising, TPS, true seed potato


Introduction
Growing potatoes from true seed has some advantages over vegetative potato production.
True seed is easy to produce and store, and fewer diseases are spread by seed than by vegetative potato production (Sadik 1983).The TSP method can be used to obtain healthy seed potato.In Finland, experiments on potato production from true seed have pre- viously only been made by Finska hushäll- ningssällskapet, a local extension service unit in Southwest Finland (Anon.1983 and 1984).
Potato production from true seed is limited by our short growing period, which exceeds 175 days only in South Finland.A study was planned and carried out in 1986 at the University of Helsinki, Department of Horticul- ture, to ensure the growth of potato from true seed in our circumstances.In addition, some American cultivars were compared to find cul- tivars suitable for our climate.

Material and methods
In the experiment comparing American TPS cultivars, seedlings from seven Pan Ame- rican Seed Company varieties numbers 2909,  2928, 3018, 4016, 4047, 4048 and 4000 as well as the control cultivar, Explorer, were raised in a greenhouse until four weeks old and then planted outdoors.Temperature in the green- house was 18-20°C at night and 20-25°C by day.The peat pots used were 4 cm x 4 cm in size, and they were filled with light Sphag- num-peat.The soil type of the experimental field was fine sand, and the field was fertilized with 800 kg/ha of mixed fertilizer (N:P:K = 6:7:17).The raised seedlings were planted outdoors on 12th June, on ridges in rows, 30 cm X 70 cm apart.After planting a floating mulch was kept on the vegetation for one week, to protect from frost.
Three cultivars, 2928, 4000 and Explorer, were used in the production method experiment.The three production methods were: 1. direct field sowing; 2. planting 4-week-old seedlings as in the cultivar experiment; 3. as in method 2, but covering the planted seed- lings with floating mulch.Field sowing took place on 23rd May, and after sowing the rows were covered with a PE film (0.01 mm), which was removed after 25 days.After the emer- gence of seedlings the stand was thinned to a plant distance similar to that in the cultivar experiment.
In methods 2 and 3, the seedlings were raised as in the first experiment.The floating mulch used in method 3 was Agryl P-17, a polypropylene mulch weighing 17 g/m 2 .The mulch was removed 67 days after planting, and harvesting occurred on 23rd September.Thus the total growth time of the plants in the cultivar experiment and in the planted plots of the production method experiment was 144 days (31 + 113).It was 124 days in the field- sown ones.
A randomized block design was used in the cultivar experiment, a split-plot design in the production method experiment.There were four replicates in both of them, and each ex- perimental plot comprised 18 plants.The fol- lowing observations were made: the width of plant foliage throughout the field period; insects and plant diseases; tuber yield and its quality (colour, taste, starch contents, tex- ture).The width of plant foliage was deter- mined by measuring the shortest and longest foliage diameter of ten plants per plot and then calculating the average.Insects and plant diseases were observed visually.The tuber crop harvested was sized, and the marketable yield calculated.
The data were analysed with analyses of variance, and F-values were tested with the Tukey-test.Correlation coefficients were cal- culated between plant diameter and yield, plant diameter and plant virosy, and plant virosy and yield.
Flesh colour and taste testing of cooked potato tubers was carried out by a 12 person sen- sory panel at the Laboratory of Sensory Science, University of Helsinki, Department of Food Chemistry and Technology.The methods used were triangle test and quantitative descriptive analysis (Stone et al. 1974).The pleasantness of the taste and colour of the samples was evaluated using graphic 10-cm scale anchored verbally at both ends (unpleasant pleasant).After converting the graphic ratings to numerical values (1 -10), means were calculated for the pleasantness scores, and analysis of variance was performed.
Tuber flesh texture was evaluated visually, by fork and by tasting, and I-9 point scores were given (1 = watery, wet and loose and 9 = very mealy and dry), a method recom- mended by the European Association for Po- tato Research.Starch contents, as the percentage of fresh weight, was approximated ac- cording to the Hals-Buchholz method.

Climatic conditions
The month of June was warmer than usual (mean temperature 17°C; usual mean 14.5°C), and there was less rainfall than normal in South Finland (29 mm as opposed to the usual 47 mm).The months of July and August were exceptionally rainy, and the mean tempera-direct-seeded plants was around 10 cm less tures were lower than usual than that of pot-raised plants; by the middle of August it was only 3-5 cm less than that of covered vegetation.In the covered plots, the plants of all three varieties were equally large, and of direct-seeded plants the cul-

Growth
By the end of July, the diameter of the  tivar Explorer was the smallest in diameter (Figure 1).

Tuber yield
Pot raising increased tuber yield.In the pro- duction method experiment, the highest yield average of the three cultivars, 3.38 kg/m 2 , was obtained with planted seedlings without floating mulch and lowest, 2.33 kg/m 2 , with direct-seeding (table 1).The number of tubers per plant for seedlings planted without mulch was greater than for direct-seeding.The greatest marketable yield, tuber diameter 35-65 mm, was obtained with planted seedlings wit- hout mulch.The greatest proportion of small tubers, as kg-% of total yield, was detected for the floating mulch method.
The production method experiment resulted in a positive correlation between plant diameter and yield, the correlation coefficient being r = 0.6393 (0.1 ®7o).
In the variety experiment total yields varied between 2.85 and 4.13 kg/m 2 , and marketable yields between 2.13 and 3.10.No signifi- cant differences were found in marketable yield and number of small tubers (Table 2).

Insects and diseases
Peach-potato aphids were found for the first time in the third week of June.They were  3. Effect of the production method on the quantity of diseased tubers (kg/m 2 ).  4. Effect of the cultivar on tuber quality.Taste and colour: I-lo points (0 = least plesant and 10 = most pleasant).Starch contents as % of FW, Hals & Buchholz -method.Texture: 1 9 points (1 = watery, wet and loose and 9 = very mealy and dry 7.0 most abundant in planted seedlings without mulch, and fewest in direct-seeded vegetation. In the planted, non-covered seedlings, potato leaf roll virus (PLRV) symptoms were also common.Tubers from plants that had visibly been evaluated as strongly virotic were graded separately, and the planted, non-covered seed- lings gave the greatest amount of these tubers.Very few scabbed tubers (Streptomyces sca- bies) were found in any plot (Table 3).
In the production method experiment there was a negative correlation between virosy and yield, r = -0.1264 (5%).There was also a negative correlation between the width and virosy of plants, r = -0.1446 (5%).The tuber yield of visibly virotic plants was 29 -95 %of that of non-virotic plants.
Mulching increased leaf symptoms of Phy- tophthora infestans.Tubers from the three production methods, however, did not differ as to visible infection by Phytophthora infestans and Erwinia carotovora (Table 3).

Tuber quality
The varieties evaluated were 4047, 3018, vi- sibly healthy Explorer and visibly virotic Explorer, and Pito, a common Finnish variety used as a control.In the triangle test, all the varieties differed from each other either by colour or by taste.No differences was found in the virotic and non-virotic tubers of Explorer.
In the descriptive analysis Pito, a yellow- fleshed variety was best by colour and taste (Table 4).The red-fleshed variety 3018 was second best.There was a statistical difference in taste only between Pito and 4047.No differences were found between the members of the panel.
Pito had the greatest starch contents, 17.7, and Explorer the lowest, 11.4 %.Pito also re- ached the highest scores in texture evaluation.

Discussion
The use of floating mulch did not increase tuber yield of true seed potato.Symptoms of Phytophthora infestans were found in the middle of August, when plant growth was still continuing.Leaves were destroyed and the photosynthetising leaf area was diminished, which must have had an effect on yield.
The monthly mean maximum temperatures under mulch in June-August were 3-6°C higher than in the open.Tuber formation must have been retarded by the high tempera- tures, as claimed by Burton (1981) and Ma-  lagamba (1983).This may also explain why the proportion of small tubers was much higher for the plants under floating mulch than for the non-covered plants.
Of the three methods, direct-seeding gave the lowest mean yield, 2.3 kg/m 2 .Martin  (1983) in Washington achieved a yield of 58 tonnes per hectar with this methods.In his experiment, the growth period was much longer than in ours, 195 days versus 124 days.
The good yield results in these experiments cannot be generalized, because the climatic conditions in the beginning of the summer were more favourable than the long-term ones in Finland.
Mulching gave a good growing environment for Phytophthora infestans, but treating po- tato vegetation with fungicides (mankozebi and metalaxyle) prevented the disease from spreading into the tubers.Apparently the mulch should have been removed earlier.
PLRV is spread by the peach-potato aphid.One of the hosts of this virus is Physalis alke- kengi, the plants of which were growing near the experimental field.The mulch apparently protected the potato vegetation from the virus vector, as PLRV symptoms were much less frequent under mulch and later than in the open.According to Marco (1987), white colour, in this case a white mulch, causes aphid repellancy and disturbs aphid behavi- our, thus diminishing potato Y-virus and PLRV infection.On the other hand, the plants were taller under mulch, and thus could have stood aphid damage better than the non- covered plants.
In the official Finnish potato cultivar test of 1981 (Mattila et ai.1981), Pito was given the best points, 7, for its appearance; in our test, too, it was found to be the best as to col- our.In both experiments it was also found to be the best by taste.Both in our test, and in earlier tests performed by Finska hushäll-  ningssällskapet (Anon 1984), the varieties 3018 and 4047 proved to be equally mealy.

Conclusions
It seems possible to grow potato from true seed in Finland, at least when the summer weather is favourable.Further experiments are needed to clarify more precisely the best possible growing techniques.
Fig. I. Effect of the production method of the development of plant diameter of three TPS-cultivars.

Table I .
Effect of the production method on TPS tuber yield.

Table 2 .
Effect of the cultivar of TPS tuber yield. ).