Effects of cultural practices, Verticillium nubilum and locality on coiled sprout and yield of early potatoes

Treatments reported to affect the incidence of ‘coiled sprout’ were studied in 1968 with the cultivar Arran Pilot in experiments near Falmouth, Cornwall (F), Rothamsted (R) and near Edinburgh (E). Most coiled stems occurred at E (37%) with similar proportions at R (19%) and F (16%). Seed tubers with the longest sprouts at planting produced most coils at all sites, those with shorter sprouts less and unsprouted seed least. When coiling was assessed on the number of plants, long- and short-sprouted seed tubers were about equally affected at R and E. Sprouts emerged within 45 days at E and 30 days at R. Longer sprouts did not emerge more quickly, but emergence was more than 1 wk later from unsprouted seed at R and only a few days later at E. Harrowing ridges after planting to decrease the depth of soil covering seed tubers hastened emergence by about 1 wk at R and decreased the incidence of coiling; it had little effect at E but tended to increase coiling at F. Seed tubers inoculated with Verticillium nubilum produced more coiled sprouts but this increase was not significant. The highest final yields were achieved at R (43-2 t/ha) where short-sprouted seed outyielded long-sprouted and unsprouted seed, but cultivation treatments and fungus inoculation had no effect. At E (21-2 t/ha) only herbicide treated plots gave a significant effect, outyielding harrowed plots by 8 t/ha. At F (18-2 t/ha) short-sprouted seed outyielded long-sprouted and unsprouted seed, and herbicide treated plots outyielded harrowed plots. When ware yields from individual plants with and without coils from long-sprouted seed were compared at R on 18 June, coiled plants gave about 30% and on 2 July 60% of the yield of non-coiled, but by 16 July 120% yield (20% more); similar trends were shown by short- and by long-sprouted seed at E.     

Healthier seed potatoes. II. Effects of sprouting and benomyl treatment of stem cutting and commercial stocks on growth, yield and disease

Potato seed tubers of seven cultivars derived from stem cuttings in 1965 (healthier seed) were compared with samples from commercial stocks in 1969–72 at two sites, one clay with flints soil (Rothamsted) and the other sandy loam soil (Woburn). Plant emergence, usually slower from non-sprouted seed, was unaffected by seed source or seed treatment with benomyl. Sprouted healthier seed produced more stems/plant than sprouted commercial seed. Healthier seed yielded 6% (Rothamsted 5%, Woburn 8%) more than commercial when seed was sprouted and 7% (Rothamsted 6%, Woburn 9%) when not sprouted. The increased yields of cvs Record (10% sprouted, 23% not sprouted), Majestic (8% sprouted, 11% not sprouted) and King Edward (5% not sprouted) mostly comprised small ware (44–57 cm) and chats (>44 cm), indicating an increase in tuber numbers. Commercial Pentland Crown seed yielded as much as healthier but the healthier produced less large ware (57–83 cm). Benomyl treatment of commercial seed, especially when not sprouted, and sometimes of healthier seed also decreased tuber size. Infection of stem bases and tuber eyes by Polyscytalum pustulans was less from healthier than commercial seed and was decreased by benomyl. Stem canker (Rhizoctonia solani) was also decreased by benomyl but fungicide treatment of seed did not greatly decrease the high incidence of R. solani hyphae on eyes of tubers at Woburn. Helminthosporium solani, equally prevalent on the produce of commercial and healthier seed, was decreased by benomyl. There was slightly more gangrene (Phoma exigua) on the produce of commercial than healthier seed.     

Healthier seed potatoes. I. Effects of inoculating stem cutting stocks with Polyscytalum pustulans and Rhizoctonia solani on growth, yield and disease

Potato seed tubers of seven cultivars derived from stem cuttings in 1965 (healthier seed) were grown in 1969–72 at two sites, one clay with flints soil (Rothamsted) and the other sandy loam soil (Woburn). Inoculating sprouted tubers at planting with Polyscytalum pustulans did not affect the number of stems/plant or total yield but increased stem base and tuber infection. The yields of large tubers (57–83 cm) were increased and small tubers (>57 cm) decreased, indicating a decrease in tuber numbers. Rhizoctonia solani inoculated at planting decreased numbers of stems/plant and yield by up to 14% at Woburn but not at Rothamsted. At both sites, yields of large tubers were increased with cvs Majestic and Record and decreased with Pentland Crown. Stem canker and tuber infection were increased but infection was also prevalent on tubers from non-inoculated seed at Woburn. When both pathogens were inoculated together yields of large tubers were increased in cvs King Edward, Majestic and Record and decreased in Pentland Crown. Infection of stem bases and tubers was sometimes less than when either pathogen was inoculated singly.     

Coiled-sprout in the potato: the effect of various storage and planting conditions

The incidence of coiled-sprout was determined in Scotch and local-grown Arran Pilot and Duke of York seed tubers which had been stored at 10°, 4° and 15° C. and in a farm store with no temperature control. All four tuber types were planted in the field and, in addition, the two types of Duke of York were planted in Perlite at 7°, 10° and 15° C. In the field, and when maintained at 7° and 10° C, the percentage of sprouts coiling and the intensity of coiling was greater in tubers stored at 10° and 15° C. than at 4° C. There was no coiling when the Duke of York tubers were planted at 15° C. In a further experiment tubers were stored at 20° C. in the light and dark and samples were planted monthly for 3 months at temperatures of 7°, 10° and 15° C. During the following 3-month period only light-stored were planted because of the excessive amount of tip-death in the tubers stored in the dark. There was very little coiling in the dark-stored tubers. In the first two plantings of the light-stored tubers there was virtually no coiling of those planted at 15° C. There was some, however, at 7° and 10° C. In subsequent plantings there was more coiling and no effect of planting temperature. Attempts to isolate Verticiculum nubilum from sprouts were successful in only a small percentage of attempts and it was not possible to demonstrate any difference between its distribution on coiled and normal sprouts. It was not possible to induce coiling by infection of sprouts with spores of V. nubilum. Over a wide range of sprout sizes the amount of coiling was a function of sprout size at planting. However, the parts which coiled were those in the apical bud of the sprout at the time of planting and hence contributed only a small amount to the total sprout size. It is likely, therefore, that the correlation between coiling and sprout size reflects the changing metabolism of the elongating regions of sprouts with their increase in length, these regions developing in such a way as to produce a greater tendency to coiling. The internal reactions concerned in these changes, however, are not known.     

Fungus diseases on potato seed tubers planted in England and Wales, 1963-76

During 1963-76 samples of potato tubers from commercial seed stocks of cvs King Edward (14 yr), Pentland Crown (9 yr), Majestic (7 yr), Pentland Dell (3 yr), Record and Arran Pilot (2 yr) were received from farms in England and Wales. Fifty tubers from each sample were examined macroscopically for fungus diseases and eyes were excised from a 20-tuber sub-sample, incubated and examined for pathogenic fungi; 50 tubers were stored on trays to sprout and examined for diseases and sprouting in May and in most years samples of 50 tubers were wounded by dropping onto expanded metal, stored at 5° C and examined for gangrene and dry rot after 12 wk. Amounts of disease varied between years and during 14 yr black scurf and powdery scab on King Edward tended to increase and skin spot and late blight decrease. On average 44% of King Edward tubers were affected with skin spot, 25% with black scurf and 16% with powdery scab. Gangrene affected 5% of tubers and 97% of the isolates from rots were identified as Phoma exigua var. foveata. Wounding tubers increased the incidence of gangrene three-fold. During 1963-69 late blight affected 2% of King Edward tubers but fewer in later years and in other cultivars. Majestic had most common scab (44% tubers) and Arran Pilot most dry rot (9% tubers) and this disease was increased by wounding tubers. Conidiophores of Helminthosporium solani (silver scurf) were more common on excised eyes of Pentland Crown, Record and Arran Pilot than of other cultivars, and isolations from verticillate conidiophores that developed on the side of incubated eye plugs of King Edward and Majestic stocks gave pure cultures of Verticillium tricorpus (78%), V. nigrescens (9%) and V. nubilum (3%). Proportions of tubers with different diseases were affected by their country of origin; Scottish seed had most skin spot and gangrene, Irish seed most powdery scab and English seed most common scab, late blight and H. solani. There was also evidence of differing disease incidence in seed from different geographical areas in Scotland and England. Up to half the King Edward and Pentland Crown stocks examined in 1975 and 1976 were derived from stem cuttings and average amounts of diseased tubers were similar to those in stocks not derived from stem cuttings. Annual and cultivar differences in disease incidence and effects of date of receipt of seed on farms are discussed.     

Stem canker (Rhizoctonia solani) on five early and seven maincrop potato cultivars I. Infection of shoots, stolons and tubers

In 1983 and 1984, potato seed tubers of five early and seven maincrop cultivars were inoculated with cultures of Rhizoctonia soluni during planting to simulate severe seed infection. Shoot and stolon infection was assessed in June-August and black scurf on tubers recorded after harvest in October. Almost all shoots of all cultivars had stem canker in both years and disease on shoots, stolons and tubers was more severe in 1984 than in 1983. In 1983 similar amounts of disease developed on all early cultivars and between 11% (Ulster Sceptre) and 32% (Maris Peer) shoots were pruned off. Maris Peer had a stem canker score lower than other cultivars in 1984 but more than half the shoots were pruned off. Shoot pruning on Estima, Ulster Prince and Ulster Sceptre was more common on plants from sprouted than non-sprouted seed. Between 30 and 50% of stolons were pruned off. After harvest in 1985, black scurf was least prevalent on Arran Comet and Maris Peer tubers and in 1984 on Arran Comet and Estima tubers from non-sprouted seed. Of the maincrop cultivars, King Edward plants from sprouted seed had many shoots pruned off in both years. Shoot pruning was also prevalent on Maris Piper and Pentland Squire plants from non-sprouted seed. Record had fewest pruned shoots and stolons and the lowest stem canker score. The disease was more severe on Pentland Crown and Maris Piper plants from non-sprouted than sprouted seed. Black scurf was most common on Cara and King Edward tubers in 1983 and on King Edward and Record tubers in 1984. In both years few shoots but many tubers were infected on plants from non-inoculated seed and the significance of this is discussed.     

Stem canker (Rhizoctonia solani) of maincrop potatoes.

In two years, potato plants were sampled at 1- or 2- weekly intervals from plots planted with seed tubers bearing sclerotia of Rhizoctonia solani (black scurf) and with seed without sclerotia either infested or not with cultures of R. solani at planting. Sprouted King Edward seed was used in 1981 and sprouted and non-sprouted King Edward and Pentland Crown seed in 1982. In both years 60–80% of shoots from seed with sclerotia and 90% of shoots from seed inoculated at planting were affected with stem canker. Most disease developed before shoots emerged although it gradually increased later when new shoots arising both from seed tubers or as branches on shoots with damaged apices (pruned shoots) became infected before they emerged. Sprouting seed tubers bearing sclerotia decreased the disease on both cultivars but with soil-applied inoculum the disease was more severe on plants from sprouted than non-sprouted seed. Some stolons were infected by R. solani soon after they developed and incidence of infection later increased. Thirty to 50% of stolons were infected on plants from infected seed tubers and 60% on plants with soil-applied inoculum. With both cultivars and sources of inoculum about 70% of the infected stolons had their apices killed (pruned).     

Sprouting and Growth of Purple Nutsedge, Cyperus rotundus, in Relation to pH and Aeration

The results of experiments on the sprouting and growth of purple nutsedge (Cyperus rotundus L.) in relation to pH and aeration are described. All pH levels between 2.2 to 9.0 allowed high sprouting percentage but the sprouted tubers did not survive at pH below 3 and survival beyond pH 7 was very poor. Post-sprouting growth of nutsedge was optimal at pH 4. Pre-sprouting aeration of tubers stimulated early sprouting and produced vigorous shoots, whereas submersion of the tubers was inhibitory. Continuous aeration of the submerged tubers substantially removed the inhibitory influence on sprouting but the shoots remained weak. Seedlings grown in constantly aerated nutrient solution showed significantly better growth than in a non-aerated one.     

Effects of common scab (Streptomyces scabies) on potatoes

Seed tubers with severe (71% cover) or slight (12% cover) common scab selected from a badly infected crop of cv. Maris Piper were boxed for sprouting in November and February and planted in field experiments at Woburn and Rothamsted in 1980. The severely scabbed seed lost more moisture than the slightly scabbed seed during sprouting and sprouted from a larger number of eyes but total stem numbers were greater on plants from the slightly scabbed seed. Plant growth during the first 6–9 wk after planting was less from the severely scabbed than from the slightly scabbed seed. This was shown by records of ground cover, leaf area index, fresh weights of stems and leaves, total tuber yield (18% lower at 9 wk) and tuber number. Subsequently these differences became non-significant. Seed with slight scab boxed in November produced smaller numbers of progeny tubers than that boxed in February but there were no other consistent effects of boxing date on growth and yield. Scab incidence on progeny tubers was very slight and unrelated to the severity of disease on the seed tubers. It seems unlikely that severe scab infection of seed tubers would significantly decrease tuber yields except perhaps in early potato production.     

Stem canker (Rhizoctoniu solani) on three early and three maincrop potato cultivars: effects of seed tuber size on growth and yield

Large seed tubers (mean 134 g) and small seed tubers (50 g) of three early and three maincrop potato cultivars, spaced respectively 48 and 30 cm apart within rows, were inoculated with Rhizoctonia solani at planting in 1985 and 1986. All seed of early cultivars was sprouted and maincrop seed was either sprouted or not sprouted. In all cultivars, plant emergence was slower from small than large seed and with both was delayed by inoculation. From 11 wk after planting numbers of stems and tubers and weights of foliage and tubers/m² were usually similar from small and large seed when not inoculated, but inoculating delayed plant growth and decreased tuber numbers and yield more from small than large seed. Inoculating decreased mean yields from large and small seed of early cultivars at 11 wk by respectively 24% and 31% (Arran Comet), 12% and 18% (Estima) and by 10% and 28% (Wilja) and losses were greatest with saleable sized tubers (3–4 cm). When grown to maturity inoculating decreased yields by 7% (Arran Comet), 5% (Estima) and 14% (Wilja). With maincrop cultivars, yields in October from large and small seed were decreased by respectively 4% and 10% (DCsirke), 9% and 12% (Maris Piper) and by 14% and 22% (Pentland Squire). In all cultivars yields of tubers < 44 mm and 44–70 mm were decreased and, with Pentland Squire, tubers > 82 mm were increased. The incidence of stem canker and of black scurf on progeny tubers was not affected by seed size but in all cultivars the percentage of greened tubers was slightly increased by inoculation.     

Effects of treating seed potatoes from commercial and stem cutting stocks with benomyl, thiabendazole and 2-aminobutane on yield and disease

Potato seed tubers of six cultivars from commercial stocks and from stocks derived from stem cuttings (healthier seed) were fumigated with 2-aminobutane 2 wk after lifting or treated with benomyl or thiabendazole in January. 2-aminobutane prevented skin spot and gangrene developing on treated tubers. Experiments were planted at Rothamsted (clay with flints soil) and at Woburn (sandy loam soil) in 1973–75. Healthier seed produced more stems/plant than commercial stocks and yielded on average 8% more at Rothamsted in 1973 and 1974 and respectively 5 and 10% more at Woburn in 1973 and 1975. Seed treatments did not consistently affect stem numbers or increase yield although all treatments tended to decrease tuber size. Infection of stem bases and tubers by Polyscytalum pustulans and Rhizoctonia solani was usually less from healthier than from commercial seed and was decreased by benomyl and thiabendazole in 1973 and 1974. Infection by Helminthosporium solani of the skin around tuber eyes was greater from healthier than from commercial seed but was decreased by benomyl and thiabendazole. 2-aminobutane sometimes decreased infection of tubers by P. pustulans and R. solani but neither of stem bases nor of tubers by H. solani. Gangrene on tubers uniformly wounded at lifting was not consistently affected by seed source or seed treatment. Treating seed with benomyl or thiabendazole in 1975 decreased skin spot and black scurf in tubers stored until March 1976. These treatments also decreased silver scurf on the produce of commercial seed at Rothamsted but gangrene was not consistently affected by seed treatments.     

Stem canker (Rhizoctonia solani) on five early and seven maincrop potato cultivars. II Effects on growth and yield

In 1983 and 1984, potato seed tubers of five early and seven maincrop cultivars were inoculated with cultures of Rhizoctonia solani during planting in field experiments to simulate severe seed infection. The size of foliage was assessed during June-August and tuber yields recorded during growth and at harvest in October. Stem canker delayed shoot emergence, decreased the number and length of stems and caused increased variation in stem length; these effects were greatest with Maris Peer and Arran Comet (early cultivars) and King Edward and Pentland Squire (maincrop cultivars). Total weight of foliage was decreased, especially with earlies, dry matter of stems increased and the proportion of foliage on lateral stems increased. With the early cultivars, tuber yield from sprouted Maris Peer seed 11 wk after planting in 1983 was decreased by 24%, and 13 wk after planting in 1984 yields were decreased by 42% (Maris Peer), 40% (Ulster Sceptre), 34% (Estima), 30% (Arran Comet) and 17% (Ulster Prince) with sprouted seed and by 20, 29, 53, 39 and 28% respectively with non-sprouted seed. Decrease in total yield at harvest in October averaged 13% with sprouted seed and 10% with non-sprouted seed. In all cultivars the weight of small tubers was decreased and with Estima the weight of large tubers was increased. Tuber bulking was also delayed with all maincrop cultivars and at harvest yields from sprouted King Edward seed were decreased by 13% in 1983 and by 16% (sprouted seed) and 23% (non-sprouted seed) in 1984; yields of Pentland Squire were decreased by 5, 16 and 21% respectively. Yield losses with other cultivars ranged from 5–13% with sprouted seed and 0–16% with non-sprouted seed. The yields of small tubers were decreased with all cultivars and yields of large tubers were increased with Pentland Squire, Pentland Crown and Cara.     

Treatment of seed and ware potato tubers with thiabendazole for control of storage diseases

In March 1977 and 1978 King Edward seed potatoes from three commercial stocks and one stock derived from stem cuttings (healthier seed) were treated with thiabendazole at 320 g a.i./t and 46% of the material applied was deposited on tubers. Seed was stored on trays to sprout and in April samples were planted in replicated plots on two farms in Lincolnshire. Treatment did not consistently affect numbers of stemslplant or total yields but decreased infection of underground stems by Polyscytalum pustulans and Rhizoctonia solani and of tubers at harvest by P. pustulans, R. solani and Helminthosporium solani. In samples stored for 20 wk at Sutton Bridge Experimental Station seed treatment decreased the incidence of skin spot from 25 to 4%, of black scurf from 36 to 20% but had less effect on silver scurf (untreated 52% tubers, treated 47%) and did not affect the incidence of black dot (Colletotrichum coccodes) on stored tubers. In another series of experiments 1 t samples of seed from six King Edward stocks (1977) and four King Edward and one Maris Piper stocks (1978), collected from different farms in Lincolnshire in March, were treated with thiabendazole at 40 g a.i./t. Tubers retained 48% of the material applied. The treated seed was planted on the respective farms in a single large plot adjacent to untreated seed similarly stored and sprouted. Seed treatment significantly decreased the incidence of skin spot, black scurf, silver scurf and gangrene in tubers stored at Sutton Bridge in 1977–78. After lifting the experiment in 1978 samples from treated and untreated seed were treated with thiabendazole (40 g a.i./t) before storage (ware treatment). Skin spot was decreased more by seed treatment than ware treatment whereas silver scurf was controlled best by ware treatment. Incidence of black dot was not consistently affected by seed or ware treatment.     

Clonal Variation in the Sprouting Pattern of the Tubers in Eleocharis kuroguwai, a Cyperaceous Weed, with Special Reference to its Perennation Strategy

Abstract Clonal variation in the sprouting pattern of tubers in Eleocharis kuroguwai Ohwi was studied, using 39 clones collected within a longitudinally narrow belt from Wakasa Bay to the Kii Peninsula of Japan and one of "Ohkuroguwai", a cultivated form.
The sprouting pattern of tubers and the relationship between sprouting time and other tuber characteristics varied noticeably among clones. Based on the polygonal graphs of × (mean), (standard deviation), g₁ (skewness) and á (kurtosis) of the sprouting time, the sprouting pattern of 40 clones could be divided into the following three groups. Group A: and α were small, but g₁ and a' were generally great; almost all tubers sprouted early and simultaneously. Group B: and α were greater, but g₁ and a' were smaller than Group A; the majority of tubers sprouted quickly, but the rest of the tubers sprouted gradually. Group C: and α were very large, g₁ was small, but a' showed a comparatively large negative value; all tubers sprouted gradually over a long period.
Groups A and B were represented by the irrigation pond group from the Banshu Plain and by the Ohshima Island group in the paddy fields of the southern extremity of the Kii Peninsula. Clones from the paddy field group and from the plain group, both referable to typical rstrategists, exhibited various sprouting patterns of tubers.     

The role of water in the spread of Phytophthora infestans in the potato crop

During falls of rain before 5% of the haulm was destroyed by blight, most rain water was deposited on soil at the bottom of the ridge, and least at the top in crops of King Edward, Majestic, and Up-to-Date potatoes. Afterwards less water was trapped half way up the side of the ridge than above or below. Water was also channelled down the stems and, while these were upright, was deposited in the channel often formed around the stem bases by wind movement. Fewer than 500 sporangia per ml. were seen in water collected when blight was present in the crop. Growing tubers of Ulster Ensign and King Edward, inoculated with P. infestans, infected healthy tubers less than 1–3 cm. away, when the soil water content was greater than 20%, and sporulation on the inoculated tubers was maximal. Similarly inoculated Up-to-Date and Majestic tubers sporulated less abundantly and failed to infect healthy tubers. Sporulation also occurred on inoculated seed tubers, although infection of the haulm from these tubers could not be confirmed. Water movement from the seed tuber region to the furrows may occur under suitable conditions.     

Effects of treating seed potatoes with trace elements on sprouting and microbial spoilage during storage under tropical conditions

Attempts were made to extend the storage life of seed potatoes by treating them with trace elements as antimicrobial and antisprouting agents. In tests to measure the growth of Fusarium oxysporum, and Erwinia carotovora pv. carotovora it was found to be decreased more by iron, copper and zinc than by boron, manganese and molybdenum. Dipping seed tubers in aqueous solutions of trace element salts decreased both sprouting and microbial spoilage during storage at ambient conditions. Field testing at the end of storage proved that treating seed potatoes with ferrous sulphate did not decrease the yield of tubers or increase their iron content. It is suggested that trace elements may be used to extend the storage life of seed potato tubers, in tropical conditions.     

The Influence of the Inoculum Source of Rhizoctonia solani on Development of Black Scurf on Potato

Rhizoctonia solani, the causal agent of stem canker and black scurf on potato, survives as sclerotia on tubers, in soil and in plant residues. The objective of the present study was to evaluate the importance of inoculum source on disease development. Disease‐free minitubers and seed tubers contaminated with low levels of R. solani were planted in fumigated or artificially inoculated growth mixture in greenhouse experiments. Black scurf incidence and severity were significantly higher when the inoculum was present in both seed tubers and soil, compared with either of them separately. The severity of disease symptoms on the subterranean parts of the plant also were significantly higher in plots where both seed tubers and soil were contaminated, compared with plots where the inoculum source was either the seed tubers or the soil. Thus, both major sources of inoculum, seed tubers and soil, are important in disease development. However, when both sources are present, black scurf incidence and severity are increased, leading to economical damage to tuber yield and quality. Additional results from field trials support these findings. Disease incidence and severity on daughter tubers were correlated with levels of contamination in seed tubers and soil. When seed tubers and soil were heavily infested, the levels of black scurf incidence and severity on daughter tubers were very high; when seed tuber and soil infestation were very low, black scurf incidence and severity on progeny were also lower. Disease levels were reduced by in‐furrow fungicide treatment, but were less effective when the initial levels of the fungus on the seed tubers and in the soil were high.     

The phenology of skin spot (Oospora pustulans Owen &Wakef.) and other fungal diseases of potato tubers

Underground stems and roots of the potato varieties King Edward and Majestic became more severely infected by Oospora pustulans (skin spot) as the growing season advanced. Tubers became infected at their initiation in June and July but the proportion of infected eyes usually increased during both the growth of the crop and bulk storage. Some buds on tubers in bulk stores died in December, but few were killed until after January, when attacks increased until by March about a quarter of Majestic eyes were usually dead. Boxing (chitting) not only prevented most eyes from dying but also prevented much skin spotting on tubers. At High Mowthorpe, Majestic seed unboxed or boxed in March yielded, respectively, 3 and 1 ton/acre less than seed boxed by January. Early boxing increased the yield of seed–sized tubers but did not affect the infection of progeny tubers. Dipping Majestic seed tubers in organo–mercurial fungicide and then boxing soon after lifting decreased their infection by O. pustulans; it also lessened infection on plants they produced and increased the yield of progeny seed–sized tubers. However, dipping, even in successive years, had little effect on the infection of progeny tubers by O. pustulans. Rhizoctonia solani and Helminihosporum atrovirens became increasingly prevalent on tubers during growth after July and during bulk storage. H. atrovirens was decreased by early boxing or dipping tubers in fungicide. Verticillate conidiophores often grew on incubated tuber plugs but their prevalence was affected by past storage. The results suggest that some of the benefit for chitting may be from disease control, it was not possible to measure the extent to which benefits from chitting depend on effects on disease or on the physiology of the tuber.     

Variations in resistance against Phthorimaea operculella in wild potato tubers

Tuber resistance can contribute to current management strategies against the potato tuber moth, Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae), in field and stored potatoes. Wild potatoes represent a potential source of novel resistance traits against the moth. We assessed resistance in three wild potato species, Solanum multiinterruptum Bitt., Solanum sparsipilum (Bitt.) Juz. & Buk., and Solanum wittmackii Bitt. against neonate and developing tuber moth larvae. All three species had high levels of resistance but accessions of S. sparsipilum and S. wittmackii were significantly more resistant. Resistance in S. multiinterruptum was generally concentrated in the tuber periderm, whereas in S. sparsipilum and S. wittmackii resistance was mainly cortex‐based. Unidentified cortex‐resistance factors in all three species reduced survival and increased larval and pupal development times, but had no apparent effects on the pupal weights of survivors. A high proportion of larvae abandoned or died within tubers of S. wittmackii, which has particularly high levels of unidentified cortex‐based defenses. Resistance decreased in S. multiinterruptum and S. sparsipilum as tubers sprouted but was more stable in S. wittmackii. Periderm‐based resistance was more stable than cortex‐based resistance in S. multiinterruptum during sprouting. In contrast, cortex‐based resistance was stable in tubers of S. wittmackii as these sprouted, and resistance may have increased on some older sprouting tubers. Solanum multiinterruptum and S. sparsipilum are proposed as potential sources of resistance against the potato tuber moth.     

Effects of reducing sugar concentration on in vitro tuber formation and sprouting in yam (<Emphasis Type="Italic">Dioscorea cayenensis–D. rotundata</Emphasis> complex)

The effects of reducing sucrose level on tuber formation (% of cultures with microtubers), development (length and fresh weight of microtubers) and sprouting in yam Dioscorea cayenensis–D. rotundata complex in vitro were investigated. Only 29% of the explants showed tuber formation after 3 weeks in the presence of 1% sucrose in contrast to 100% with 3%. After 120 days of culture, the length and the weight of the tubers obtained in the presence of 1% sucrose were less than with 3% sucrose. Addition of sorbitol to keep osmolarity at the same level did not restore normal rate of tuber formation. Similar results were obtained with the use of reduced fructose or glucose level. Microtuber sprouting was also affected by sucrose level incorporated into the tuberisation medium. Tubers obtained on reduced sucrose level sprouted later and the increase of osmolarity with sorbitol did not restore normal sprouting. The bigger tubers obtained on high sucrose media could contain more carbohydrate reserves that could partially explain a higher sprouting rate. These results can be used for optimising in vitro conditions for mass production of microtubers in yam and especially in Dioscorea cayenensis–D. rotundata complex, a very important species in West Africa. They specially showed the importance of tuberisation conditions on precocity of tuberisation, on tuber length and weight and on their further sprouting.