Fungicide treatment of seed tubers infected with thiabendazole-resistantHelminthosporium solani andPolyscytalum pustulans for controlling silver scurf and skin spot on stored progeny tubers

Summary Seed tubers, cv. Désirée, derived from stocks treated annually with thiabendazole were infected with thiabendazole-resistant strains ofHelminthosporium solani andPolyscytalum pustulans. Samples of seed tubers were either untreated or immersed for 5 min in fungicide suspensions of thiabendazole, imazalil or thiabendazole plus imazalil and planted on four farms in 1988 and 1989. After harvest, tuber samples from each treatment were treated with thiabendazole and stored for 6 months. Applying imazalil or thiabendazole plus imazalil to seed tubers decreased the severity of silver scurf and skin spot on stored progeny tubers. Thiabendazole applied to seed tubers or to progeny tubers after harvest did not affect the severity of either disease, but post-harvest treatment decreased the incidence of black scurf after storage.     

The control of silver scurf and development of thiabendazole resistance inHelminthosporium solani as affected by the rate of fungicide applied to potato seed tubers

Summary Seed tubers were immersed in suspensions of thiabendazole (Storite) or thiabendazole plus imazalil (Extratect) at different concentrations and planted in a field experiment. During the following 2 years samples of the produce were planted after treatment with the same fungicide formulation used on the seed. Silver scurf on daughter tubers decreased as concentrations of fungicide increased and Extratect gave better disease control than Storite applied at the same thiabendazole concentration. Isolates ofH. solani resistant to thiabendazole were found on seed tubers 6 weeks after treatment with Storite, and also on daughter tubers after the first application of the higher rates of Storite; their incidence increased with further annual treatments. In 3 years no thiabendazole-resistant isolates were found after treatment with Extratect. Chlamydospores ofH. solani developed when sensitive isolates were subcultured onto agar containing thiabendazole. Isolates sensitive to thiabendazole were slightly more sensitive to imazalil than resistant ones.     

Reduction of silver scurf on potatoes by pre- and post-storage treatment of seed tubers with imazalil

Control of silver scurf on daughter tubers was obtained by imazalil applied to seed tubers as a pre- or post-storage treatment or as a furrow treatment (at time of planting), in three field experiments. Post-storage and pre- and post-storage combination treatments provided consistent results. The most effective treatment in the first and second experiments was the combined pre- and post-storage application of imazalil. However, separate pre- and post-storage treatment also significantly reduced the incidence compared with the non-treated control. No differences were observed between dosages of 15 and 30 g a.i. of imazalil per ton of seed tubers. Although furrow treatment at planting time (1.5 1/ha) significantly reduced silver scurf incidence on daughter tubers, it was less effective than seed treatments. However, the combined treatment of pre-storage application and furrow treatment significantly reduced the disease and was more effective than the single pre-storage seed treatment. In the third experiment, a reduction of disease incidence and severity index was obtained by post-storage treatments or with the combination of pre- and post-storage applications.     

Effect of a continuous hot water treatment of potato tubers on seed-borne fungal pathogens

Summary The viability of five pathogens was decreased by treatment with hot water when tested in vitro.Polyscytalum pustulans was most sensitive andRhizoctonia solani least sensitive. Potato tubers were exposed to 55°C for 5 min in a commercial continuous hot water treatment plant using naturally contaminated seed tubers and tubers which had been inoculated by dipping in comminuted cultures. The frequency of eyes colonised byP. pustulans, Helminthosporium solani, andR. solani was reduced to virtually zero and the effect persisted on tubers subsequently stored at 4°C and at 15°C for up to 16 wk. Results withColletotrichum coccodes were inconclusive. Treatment suppressedPenicillium spp. which, however, rapidly recolonised the eyes during storage, leading to higher contamination levels in the treated than in the untreated tubers. With tubers inoculated withPhoma foveata, good control was achieved when the incubation period before treatment was 10 d but not when the period was 42 d.     

Effects of Fungicide and Rate of Application on the Development of Isolates of <Emphasis Type="Italic">Polyscytalum pustulans</Emphasis> Resistant to Thiabendazole and on the Control of Skin Spot

Resistance to thiabendazole, sometimes of varying degrees, has occurred in a number of fungal species causing diseases of potato tubers, including skin spot caused by Polyscytalum pustulans. The effect of applying a range of concentrations of thiabendazole, imazalil or a formulated mixture of thiabendazole and imazalil on the development of isolates of P. pustulans partially or fully resistant to thiabendazole and on the control of skin spot over a number of successive generations of seed potato multiplication was examined in a series of experiments over 3 years. Seed tubers were inoculated with a sensitive, partially resistant or fully resistant isolate in the first year of an experiment and combinations of fungicide and a range of concentrations were applied to the seed tubers annually before planting over two or three successive generations. Immersing seed tubers inoculated with the partially resistant isolate in a relatively high concentration of thiabendazole, 0.7 g a.i. l^-1, did not result in the development of fully resistant isolates. Moreover, the effect of all fungicides in controlling eye contamination and skin spot did not differ between partially and fully resistant isolates. For seed tubers inoculated with the sensitive isolate, applying thiabendazole resulted in the occurrence of resistant isolates on first-generation daughter tubers, but the effect of fungicide concentration on the frequency of resistant isolates, partially or fully resistant, was inconsistent. In one experiment, the highest frequency of resistance was found at the greatest concentration, 0.4 g a.i. l^-1, but, in the second experiment, the frequency was unaffected by the concentration of thiabendazole. On second-generation daughter tubers derived from thiabendazole-treated seed tubers, virtually all isolates were resistant and the frequency of partially and fully resistant isolates was similar to that in the first generation. Applying a mixture of thiabendazole and imazalil to seed tubers inoculated with a sensitive isolate resulted in the development of some resistant isolates in the first year but the frequency did not generally increase with successive applications of the mixture. A mixture of thiabendazole and imazalil, and imazalil alone gave similar control of eye contamination when applied at equivalent concentrations. However, increasing the concentration of imazalil from 0.1 to 0.7 g imazalil l^-1 did result in an increasing impairment of plant growth, but the deposits achieved within that range of concentrations were greater than that recommended by the manufacturer. These results demonstrate that control of P. pustulans by thiabendazole, imazalil and a mixture of thiabendazole and imazalil applied at a range of concentrations was similar for isolates partially or fully resistant to thiabendazole.     

Nutritional value of potato tubers from field grown plants treated with deltamethrin

Summary Tubers showed changes in their nutritional value caused by the treatment of the plants with the insecticide deltamethrin. The level of starch was increased in comparison with control tubers; but the amylose percentage was not affected. In contrast, the total protein content decreased in tubers from treated plants, whereas free amino acids level and ascorbic acid content increased. The results showed that the changes in the tubers caused by the treatment of potato plants with deltamethrin did not adversely affect the nutritional value of the progeny tubers.     

Comparative performance of different-sized seed tubers derived from true potato seed

The growth and yield of plants from different-sized seed tubers derived from true potato seed were evaluated on a per stem, per plant, and per unit area basis using either single or multiple-sprout tubers. In single-sprout tubers, haulm dry weight per stem 47 days after planting was greater in the 40–60 g tubers when compared with that in the 5–10 g or the 10–20 g tubers. This resulted in greater tuber weight per stem in the 40–60 g tubers throughout the growing season. The number of tubers per stem was not affected by seed tuber size. In multiple-sprout seed tubers of increasing size, total tuber number and total tuber weight, as well as weight of those tubers larger than 45 mm, increased on a per plant basis but decreased on a per stem basis. At different rates of planting, 1–5 g seed tubers produced smaller tubers than 5–10 g or 10–20 g seed tubers. Increased rate of planting resulted in non-significant yield increases per unit area in plots planted with 1–5 g seed tubers. The yield increases were significant when 5–10 g and 10–20 g seed tubers were planted at higher rates. The number of main stems per unit of seed tuber weight was five times greater in 1–5 g tubers compared with that in 40–60 g tubers. This resulted in low seed weights per hectare when small tubers were planted and in a high ratio of harvested to planted tuber weight.     

Changes in seed tubers after planting

Summary The changes after planting in weight and dry matter content of seed tubers were studied. The weight was found to increase after planting owing to uptake of water from the soil. The dry matter content decreased rather rapidly. The changes in weight varied within wide limits for individual tubers, showing great differences in water uptake between the tubers. These differences were also demonstrated in a soaking experiment. The variation in water uptake between tubers could partly be explained by variations in the soil but the variation between the seed tubers seemed to be more important.     

Comparative effects of cut and treated seed tubers vs untreated whole seed tubers on seed decay, Rhizoctonia stem canker, growth, and yield of Russet Burbank Potatoes

Whole seed tubers have been reported to have performance advantages over seed tubers that have been cut, even if the cut tuber pieces are treated with a fungicide dust. For this study, whole seed tubers were compared to cut seedpieces and to cut seedpieces treated with four different commercial fungicide dusts for five consecutive seasons. A new field generation 3 Russet Burbank seed lot was used for each of the five years of this study. Both whole tubers and cut seedpieces were inoculated withFusarium spp., and the seedpieces were then treated with a fungicide dust. Inoculated and uninoculated untreated controls were also included. Emergence,Fusarium seed decay,Rhizoctonia stem canker, stem numbers, and vigor, as well as yield and grade data were obtained.Rhizoctonia stem canker in stems grown from whole seed tubers was significantly higher than in stems from seedpieces receiving fungicide treatment in several of the study years. Predicting whole seed tuber performance is difficult because performance varies more from year to year than performance of cut and treated seedpieces. Yield performance of whole seed tubers was not significantly better than cut and fungicide-treated seedpieces in any experimental year, and yield of large (over 280 g) tubers was significantly less than that recorded for two of the treatments when data from all five years were combined. This study demonstrated that untreated whole seed did not outperform cut and treated seedpieces for vigor, stem numbers, disease ratings, or yield in small plot comparisons for five consecutive seasons.     

Microtuber and minituber production and field performance compared with normal tubers

Summary Microtuber and minitubers of cv. Monalisa were produced in the laboratory and compared with normal seed tubers in a field experiment. These tubers were planted at similar plant densities (13.6 sprouts per m²) with two distances between rows (60 and 90 cm). Final ground cover was almost complete only in the plots derived from normal tubers and decreased with the size of the mother tubers. Normal seed, mini- and microtubers yielded respectively 50.8, 31.7, and 17.0 t/ha (means of two spacings). At close and wide spacing between rows, microtubers yielded respectively 27.3 and 6.7 t/ha, and minitubers 38.9 to 24.4 t/ha. Row spacing did not influence the yields from normal seed tubers. Total number of tubers per m² was also affected and, as means of the two spacings, ranged from 107.8 with microtubers, 122.1 with minitubers, to 142.9 with normal tubers. Mother tuber type also affected the yield distribution in three tuber grades (<36, 36–55, and 55–80 mm) and micro and minitubers produced many small tubers. Multiplication rates and the possible use of different propagation sources are discussed.     

The effect of water stress on the yielding capacity of potato crops and subsequent performance of seed tubers

Summary The effect of water stress on yielding capacity of potatoes was studied in 3 years using four different levels of soil water potential (0.7, 1.9, 3.4 and 7.8 atm.) in experiments in specially adapted structures that excluded uncontrolled water supply. Seed tubers from these crops were planted in the following years to investigate effects of water stress on their subsequent performance. Limited soil moisture availability decreased yield and the number and size of tubers. The growing period was shortened by 1–4 weeks and dormancy period by 2–8 weeks. In the following year, seed produced under conditions of moisture stress gave plants with 20% fewer stems. 24–33% less yield, 18–22% fewer tubers and 19–22% fewer large tubers than plants from seed produced under abundant water supply. It is concluded that the yield potential of seed tubers can be improved by careful attention to the availability of soil moisture during their production.     

Field performance of potato minitubers with different fresh weights and conventional seed tubers: Crop establishment and yield formation

Summary Field performance of five fresh weight classes of minitubers ranging from 0.13–0.25 g to 2.00–3.99 g and conventional seed tubers was studied in a short growing season (79 or 82 days) in two years. The heavier minitubers gave a more regular emergence, faster ground cover soon after emergence, higher dry-matter yields, and higher fresh tuber yields. Radiation conversion coefficient (RCC) did not differ. Higher tuber yields resulted from more radiation intercepted due to a faster ground cover, and a higher harvest index. All minitubers produced plants with one primary stem. In one experiment when heavier minitubers had long sprouts, time to 50% emergence decreased with tuber weight, whereas dry-matter concentration of progeny tubers increased. Conventional tubers appeared superior to minitubers in all characteristics mentioned except RCC, which was similar. Differences in performance between minitubers and conventional tubers were attributed to weight and age of seed tubers, presprouting method and crop husbandry.     

A comparison of variability in storage behaviour of seed tubers from true potato seed and clonal tubers

Summary The tuber-to-tuber variability in storage behaviour of seed tubers from true potato seed was compared with that in clonal seed tubers after storage in the dark, in diffused light, or in diffused light with a single desprouting. The variability was estimated by calculating standard deviations of length, number and weight of sprouts, and tuber weight loss. After dark storage, the variability of these variables was greater in seed tubers from true potato seed than in clonal seed tubers. After storage in diffused light with a single desprouting, the variability of number, length and weight of sprouts of seed tubers from true seed was not statistically different from that observed in clonal seed tubers. All storage treatments resulted in a greater variability of tuber weight loss in seed tubers from true potato seed than in clonal tubers.     

Effects of inoculum source and irrigation on black dot disease of potatoes (Colletotrichum coccodes (Wallr.) Hughes) and its development during storage

Summary In 1985 and 1986 plots were planted with seed tubers affected with black dot and with healthy seed inoculated or not with cultures ofColletotrichum coccodes at planting. Half the plots were irrigated from shoot emergence to the end of July. Seed tubers rotted more often when they were diseased than when they were not and rotting was increased by irrigation; by contrast, up to 18 weeks after planting irrigation decreased infection of stem-bases, roots and tubers. Tuber infection, first recorded in July, increased throughout the season and most rapidly from September, when the irrigation applied earlier increased disease. Soil-applied inoculum gave most disease in both years and tubers from healthy seed gave least in 1986 but as much as from diseased seed in 1985. Incidence and severity of black dot increased more during storage at 15°C than at 5°C and only on unwashed tubers.     

Relationship between silver scurf levels on seed and progeny tubers from successive generations of potato seed

The level of silver scurf on potato seed tubers on successive generations of potato seed tubers and their progeny tubers was investigated during 3 years in the field. The objective was to determine the importance of seed-borne inoculum on silver scurf development on the subsequent progeny tubers. Silver scurf incidence and severity increased with each generation. Coefficients of determination for disease levels among generations were significant and ranged from 0.89 to 0.97, indicating that seed tuber source accounted for a large proportion of silver scurf on progeny seed tubers. Incidence and severity of silver scurf also increased with decreasing time periods between potato crops in the field. In a field near Paterson, WA, where potatoes had not been previously grown, the severity of silver scurf increased on progeny tubers of cvs Russet Norkotah, Ranger Russet, and Shepody as disease severity increased on seed tubers of successive generations. Disease severity index significantly increased as disease incidence increased. The relationship between the two was best described using a curvilinear regression model.     

Control of silver scurf on potato by dusting or spraying seed tubers with fungicides before planting

Silver scurf of potato caused byHelminthosporium solanl Dur. & Mont. is a blemish disease that has become economically important in recent years and is considered primarily seedborne. The objective of the present study was to compare the effectiveness of various fungicides, alone or in combination, applied to seed tubers prior to planting, in controlling silver scurf on the daughter tubers. Disease incidence and severity were significantly reduced by all treatments in three experimental locations, with the exception of fluazinam, which was not effective. Among the fungicides tested in these experiments, the most effective were fludioxonil and prochloraz-Zn applied as a LV spray (control efficiency of 88% and 82%, respectively) and propineb and mancidan applied as a dust treatment (control efficiency of 78% and 77%, respectively). Azoxystrobin and imazalil treatments were less effective with control efficiencies of 68% and 43%, respectively. None of the various treatments affected crop yields in the three experimental sites.     

Production and use of seedling tubers from true potato seed (TPS) for potato cultivation in Italy

Summary The effect of varying plant population on the production of tubers from potato seedlings was investigated during 1992 and 1993 using three different plant densities: 35, 70, 100 plants/m². Increasing the plant population significantly increased the number of tubers produced, but no significant difference was found between the plant densities of 70 and 100 plants/m². The seedling tubers produced in nursery beds were tested in the field for ware potato production using different sized tubers. The best performances were obtained using tubers 30–40 mm. Significant differences were evident among the 7 true potato seed (TPS) families used, but data grouped by type of cross did not indicate any significant difference between 4X×4X and 4X×2X crossing groups.     

Effect of fumigation of potato seed tubers with 2-aminobutane on plant growth and yield

From 1968 to 1973, 13 field experiments were carried out to discover any effects of fumigation of seed tubers with 2-aminobutane (used for tuber disease control) on subsequent plant growth and yield. Nine experiments were done with tubers treated within 1 month of harvest, the remaining 4 with tubers stored for several months before treatment. In general, fumigation did not have any substantial effect on the growth pattern or the yield of the crop obtained from treated seed, although in most cases it was probable that there were small increases in the number of sprouting eyes, percentage emergence, stem number and in weight and number of seed tubers at harvest. However, these effects rarely reached levels which gave a statistically significant increase over crops grown from untreated material.     

Influence of water and soil salinity on emergence and early development of potato (Solanum tubersum L.) cultivars and effect of physiological age of seed tubers

Summary Applying saline solutions (EC 3.2 up to 7.0 dS/m) to seed tubers of different cultivars planted in tuff delayed shoot emergence and root and shoot development. Fresh weight of most cultivars was decreased by 60% but cv. Serrana Inta was more tolerant to salinity and fresh weight was decreased by 20%. When planted in 500 cm³ pots containing loessial soil some seed tubers rotted and plant growth was severely inhibited, possibly because soil aeration was impeded. Physiologically ageing seed tubers for 33 days at 6°C decreased sensitivity of most cultivars to moderate salinity whereas ageing for 68 days increased sensitivity to high salinity. Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 2977-E, 1990 series.     

Influence of stem canker (Rhizoctonia solani Kühn) on tuber yield,tuber size,reducing sugars and crisp colour in cv. Record

Summary In eight experiments in 4 years, inoculating seed tubers withR. solani caused stem canker and in seven experiments decreased the total yield and the yield of tubers 40–70 mm. Yields of larger tubers were usually increased. In most experiments inoculating increased the reducing sugar content of the tubers and darkened the colour of crisps, but did not consistently affect the amounts of sucrose. Treating soil with aldicarb or oxamyl before planting also slightly increased the total reducing sugars.