Optimisation of sample size for the detection of latent infection byRalstonia solanacearum in potato seed tubers in the highlands of Peru

Summary A sampling strategy was evaluated in the Andean highlands of Peru to optimise the detection ofRalstonia solanacearum in seed tubers harvested from symptomless crops. A sensitive and specific serological method developed at CIP was used to detect the pathogen in latently infected tubers. Optimum sample size was evaluated for symptomless crops after analysing various numbers of composite samples and using a binomial distribution model to calculate the detection probabilities.R. solanacearum was detected in all lots from fields with visible symptoms, so validating the detection technique. About half of the seed lots from apparently healthy fields at altitudes of up to 3,100 m were found positive for the pathogen.R. solanacearum was detected with 99% probability in samples of 350 tubers from seed lots from symptomless crops. This number of seed tubers could feasibly be processed in a seed-health test without incurring too high a cost for labour and materials.     

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.     

Variation in pathogenicity on potato tubers and sensitivity to thiabendazole of the dry rot fungusFusarium avenaceum

Summary The majority of isolates ofFusarium avenaceum caused dry rot on potato tubers; many were as pathogenic asF. coeruleum. Pathogenicity ofF. avenaceum was not related to the plant species from which the isolates originated. Tubers of potato cv. Cara were more susceptible than those of cvs Romano or Maris Piper. Temperature (5–15° C) had no effect on lesion depth but lesions tended to be slightly wider at the lowest temperature. Two isolates (of 61) from wheat and white lupin had a moderate level of resistance to thiabendazole. The results are discussed in relation to the control of dry rot in stored potato tubers.     

A comparative study in five laboratories on detection ofClavibacter michiganensis subsp.sepedonicus in potato stems and tubers

Summary Potato stems and tubers grown in the field from seed tubers inoculated withClavibacter michiganensis subsp.sepedonicus, which causes bacterial ring rot, were tested by indirect, sandwich enzyme-linked immunosorbent assay (ELISA) in five laboratories. Correlation between values for each experimental treatment from the five laboratories was greater (r=0.86) than correlation between values for individual samples (r=0.71). When three or more laboratories obtained ELISA values of ≥0.200 for a sample, that sample was presumed to be positive. Conversely, when three or more laboratories obtained ELISA values <0.200, the consensus determination was regarded as negative. The percentage of stem and tuber samples that were in agreement with the consensus ELISA determination varied from 65.5 to 96.7%. Indirect immunofluorescence tests, conducted on the same samples in two laboratories, were consistent with 83.4–91.9% of the consensus ELISA determinations. Presence or absence ofC.m. sepedonicus was confirmed in some samples by an eggplant bioassay and direct isolatiion of the bacterium. The ELISA procedure was well suited for screening large numbers of samples and this study confirms it to be a promising procedure in routine indexing of seed potatoes forC.m. sepedonicus.     

Field performance of potato minitubers with different fresh weights and conventional seed tubers: Multiplication factors and progeny yield variation

Summary Multiplication factors and progeny yield variation in crops from minitubers of five weight classes (ranging from 0.13–0.25 g to 2.00–3.99 g) and conventional seed tubers were studied in field experiments in three years. Multiplication factors were calculated as the number and weight of progeny tubers produced per planted tuber or per unit planted tuber weight. They were lower for the lighter minitubers when calculated per tuber and higher when calculated per weight. Yield variation was described by coefficients of variation for the number and weight of progeny tubers produced. Variation over individual plants of a crop was higher in stands from the lighter minitubers. Variation over plots within a field was sometimes higher for the lighter minitubers, but variation over years was similar for all minituber classes. Variation over plots in progeny tuber weight was higher for minitubers than for conventional tubers.     

Detection ofClavibacter michiganensis subsp.sepedonicus in potato stems and tubers grown from seed pieces with various levels of inoculum

Summary An indirect sandwich enzyme-linked immunosorbent assay (ELISA) was used to monitor potato stems of three cultivars for the presence of the ring rot pathogen,Clavibacter michiganensis subsp.sepedonicus, during the growing season and in progeny tubers after harvest. The highest ELISA values were obtained with the highest concentration of bacteria used to inoculate seed pieces in all cultivars tested. Low ELISA values were obtained for stems and progeny tubers selected from plants grown from seed inoculated at lower bacterial concentrations. Estimates of bacterial densities in stems and progeny tubers by immunofluorescence indicated that low ELISA values were most probably caused by low bacterial numbers. It is suggested that the sensitivity of ELISA for detecting the ring rot pathogen in potato stems and progeny tubers is a function of the concentration of bacteria in individual seed pieces.     

A comparative study of phenylpropanoid metabolism in gamma irradiated and unirradiated potato tubers

Summary Phenylpropanoid metabolism in gamma irradiated potato tubers was studied by examining the pattern of incorporation of radioactivity from U-¹⁴C phenylalanine into caffeic acid. chlorogenic acid and the coniferyl and sinapyl moieties of lignin. During a post-irradiation period of 21 days a depletion in chlorogenic acid was observed. This is a result of its impaired synthesis as well as an accelerated conversion of chlorogenic acid to ferulic and sinapic acids and their deposition in lignin. The results offer an explanation for the lower level of chlorogenic acid invariably observed in irradiated potato tubers.     

Progeny testing for resistance to potato virus Y: a comparison of susceptible potato cultivars for use in testcrosses with resistant parents

Summary For potato breeding or genetical research purposes, the number of copies of a dominant major gene for resistance to potato virus Y in a parent clone can be determined by test-crossing with a susceptible parent and observing the segregation ratio of resistant to susceptible seedlings in the progeny. In a comparison of different susceptible cultivars for use in test-crosses for this purpose, their progenies differed in the clarity of symptoms and parental phenotype proved an unreliable guide. Cvs Maris Piper, Arran Peak and Dr MacIntosh were found to be suitable tester parents, Pentland Squire less so and cv. Désirée was confirmed as having a major gene conferring incomplete resistance. It was found advisable to test-cross with more than one susceptible parent.     

A glasshouse progeny test for resistance to gangrene (Phoma foveata)

Summary Tubers were obtained from 80 glasshouse-grown seedlings of each of twelve progenies, inoculated withPhoma foveata Foister, and the mean percentage of infected tubers recorded. There was a good correlation (r=0.798) between this assessment and that made previously on field-grown tubers of eleven of the progenies. A gangrene test on glasshouse-grown tubers is suggested as a more rapid means of identifying progenies with useful levels of resistance.     

The effect of condensation on sporulation ofHelminthosporium solani on potato tubers infected with silver scurf and held in simulated store conditions

Summary Chambers were designed to simulate environmental conditions present in commercial potato stores and were used to introduce condensation to tubers infected with silver scurf (Helminthosporium solani). The electrical resistance measured across the skin of tubers from the top of the chambers was consistently lower than that from the bottom of the chamber, and this was reflected in an increased number ofH. solani spores produced by the top tubers. An increase in the length of the condensation period resulted in the skin resistance remaining lower for a longer period and also resulted in higher number of spores. Tubers held at relatively high ambient temperatures (15 °C) required a shorter period of condensation to cause an increase in spore count. This investigation highlighted the need for close monitoring of store environments so that condensation events can be quickly identified and the tubers dried.     

A microplate reader assay for rapid enzymatic quantification of sugars in potato tubers

Summary A sensitive, rapid and inexpensive method has been developed for quantifying the glucose, fructose and sucrose content of potato tubers. The method, based on selective enzyme-coupled reaction systems and the reduction of NAD to NADH, uses a microplate reader fitted with a 340 nm filter. In one microtiter plate 96 samples can be analysed for all three sugars in less than two hours. Several plates can be processed in parallel.     

Management of bacterial wilt in tomatoes with thymol and acibenzolar-S-methyl

The combination of thymol, a monoterpene phenol compound originating from thyme, and acibenzolar-S-methyl (ASM; Actigard 50 WG), a systemic acquired resistance (SAR) inducer, was applied to tomato plants in field conditions to evaluate the effectiveness of both chemicals to control bacterial wilt. Thymol was applied as a soil fumigant at 9.43 kg per ha 24 h after soil infestation and seven days before transplanting. ASM was applied as a foliar spray at 3.59-8.98 ml per ha, once in the greenhouse and five times in the field. The field was inoculated by applying 50 ml of pathogen suspension (10⁷ cfu/ml) into each transplanting hole eight days prior to transplanting. The experiment was performed in 2006 and repeated in 2008 at the North Florida Research and Education Center in Quincy, FL. In 2006, the combination of ASM and thymol significantly reduced disease in the bacterial wilt tolerant genotype 7514 compared to thymol alone. In 2008, the combination of ASM and thymol significantly reduced disease and increased yield compared to the control, whereas ASM or thymol alone did not significantly reduce disease or increase yield compared to the control. This is the first report of the use of both thymol and ASM to control bacterial wilt on moderately resistant tomato cultivars. Based on this study, control of the pathogen can be achieved by using both chemicals and moderately resistant cultivars.     

A comparison of laboratory and field reactions of a range of potato cultivars to infection withSynchytrium endobioticum (Schilb.) Perc.

Summary A comparison of reactions of potato cultivars toS. endobioticum indicated that the degree of susceptibility observed in laboratory tests correlated well with the reaction of cultivars in field tests. Some cultivars found to be less resistant (Resistance Grade 2) in laboratory tests produced winter spores in the field test, and 10% of those derived from cv. Ausonia germinated in distilled water. Compost from the field plot was estimated to contain 14–18 apparently viable spores/g. and this resulted in 100% infection of the highly susceptible cv. Arran Chief. The implications of these findings for descheduling sites previously scheduled due to wart infestation are discussed.     

Subcellular localisation of calcium in potato tubers

Summary Deposits, believed to contain calcium, were located in different cells of the potato tuber after using an antimonate procedure. In the starch-storage parenchyma of the medulla and cortex antimonate deposits were abundant within vacuoles, on the tonoplast, and to a lesser extent in cell walls. Phloem tissues contained a high concentration of deposits, mainly within companion cells and phloem parenchyma, consistent sites of deposition being mitochondria, plastids and tonoplasts. Deposits were abundant within vacuoles of cells of the xylem parenchyma. Within vessel elements deposits were limited to the surface of secondary wall thickenings. Antimonate deposits associated with the middle lamella of xylem parenchyma were rarely found. The results are discussed in relation to previous findings for potato tubers and for higher plant cells in general.     

Demonstration and measurement of inorganic pyrophosphate in potato tubers

Summary Tissue from developing tubers, mature tubers, and mature tubers that had been stored at 4°C, was killed and extracted with trichloroacetic acid in diethylether. Inorganic pyrophosphate was detected in the aqueous phase of the extracts with pyrophosphate fructose-6-phosphate-1-phosphotransferase. No pyrophosphate could be detected in extracts that had been pretreated with pyrophosphatase. Pyrophosphate contents of about 3–12 nmol/g fresh weight were found. It is suggested that potato tubers contain sufficient pyrophosphate to allow the enzyme UDPglucose pyrophosphorylase to convert UDPglucose to glucose-1-phosphate during cucrose breakdown.     

The role of stem inoculum in the transmission ofFusarium sulphureum to potato tubers

Summary Over 2 years, seed tubers or stems were inoculated withFusarium sulphureum Schlecht. (F. sambucinum Fuckel f. 6 Wollenw.) and the incidence of dry rot on daughter tubers assessed by standardised wound tests. The amount of stem and soil inoculum was also measured in one year. Inoculating stem bases resulted in greater concentrations of spores on the stem bases, greater soil populations at harvest and more dry rot on tubers than inoculating the seed tuber. Overall, the incidence of dry rot and the amount of stem and soil inoculum was similar for the three methods of haulm destruction: cutting and removing the stems, applying sulphuric acid or applying diquat dibromide to the growing plants.     

Correlation between resistance to late blight in foliage and tubers in potato clones from parents of contrasting resistance

Summary The relationship between resistance to late blight in foliage and tuber was examined on 50 clones of each of five progenies from crosses where one parent was resistant in both foliage and tuber and the other susceptible. Foliage resistance was assessed in a field trial and tuber resistance in a laboratory test on glasshouse-grown tubers. The genetical and environmental components of variance and the phenotypic and genetical correlations between foliage and tuber blight scores were determined for each progeny. Foliage and tuber resistance were correlated, indicating either that both are determined by the same genes or by different linked genes. The environmental component of variation was greater for tuber blight than foliage blight. We suggest that the most effective way of selecting for resistance to both aspects of the disease in a breeding programme is to select those resistant in the foliage and then screen them for tuber resistance.     

The effect of temperature on the development of wilting and on progeny tuber infection of potatoes inoculated with South African strains of biovar 2 and 3 ofPseudomonas solanacearum

Summary Potato plants, after inoculation with a South African strain of biovar 2 or a strain of biovar 3 ofPseudomonas solanacearum were grown in growth chambers at temperatures ranging from 14/16°C (dark/light) up to 25/30°C. When grown at 14/16°C or higher, plants inoculated with the biovar 2 strain developed wilting symptoms and internal symptoms on progeny tubers and the disease was transmitted by the progeny tubers. When the plants were inoculated with the biovar 3 strain, wilting symptoms developed from 18/20°C, internal symptoms on progeny tubers from 20/22°C and the disease was transmitted by progeny tubers from plants grown at 16/18°C or higher.     

Response of potato tubers to hypoxia followed by re-aeration

Summary Potato tubers kept under hypoxia (1%) showed improved viability in comparison to anoxia, which was associated with the maintenance of intermediate adenylate energy charge values (A.E.C.=0.6) and stable adenylate pools at 50% of the initial levels. Re-admission of oxygen to the tuber resulted in an almost full recovery of adenylate energy charge and total adenylates after up to 3 days of hypoxic pretreatment. Tubers exhibited a mixed fermentation. The high lactate, ethanol and acetaldehyde levels proved to be non toxic. Ethanol was degraded to acetaldehyde during re-aeration. Posthypoxic lipid peroxidation was indicated by malondialdehyde and ethane formation. Both products occurred with a temporary delay and in lower amounts compared to post-anoxia. Ethylene release was also considerably smaller. Severe hypoxia and posthypoxia postponed tissue death compared to anoxia. Survival was correlated with an improved energy supply which stabilized membranes.     

Production of phytoalexins, glycoalkaloids and phenolics in leaves and tubers of potato cultivars with different degrees of field resistance after infection withPhytophthora infestans

Summary The kinetics of accumulation of phytoalexins, glycoalkaloids and phenolics was studied in two potato cultivars differing in their degrees of field resistance when infected withPhytophthora infestans. Tuber slices and leaves of cvs Pampeana INTA (high degree of field resistance, free of R genes) and Bintje (susceptible) were infected with race C (complex race 1, 3, 5, 7, 11) ofPhytophthora infestans. Phytoalexins and phenolics accumulated in tuber and leaf tissues which had been inoculated. The levels of these compounds in the susceptible cv. Bintje were relatively low and similar to those found before inoculation. Leaves of cv. Pampeana INTA had a very high glycoalkaloid content, suggesting that glycoalkaloids may play a role in protection of leaves against the fungus. However, we could find no correlation between resistance and glycoalkaloid content of tubers. Our results suggest a major role of phytoalexins, phenolics and glycoalkaloids in the complex mechanisms of field resistance.