Fungicidal control ofPhytophthora erythroseptica: The cause of pink rot on potato

Two shade house and six field experiments were undertaken to evaluate fungicides and timing of application for the control of pink rot of potatoes caused byPhytophthora erythroseptica. Pink rot developed in up to 60% of tubers grown in artificially inoculated soil and 21% of the tubers in naturally infected soil. Ridomil (metalaxyl) and Ridomil Gold (mefenoxan) granules applied to the soil at planting completely or nearly completely controlled pink rot throughout the season in both pot and field experiments. Foliar applications of Ridomil Gold MZ (mefenoxan plus mancozeb) applied when tubers were around 20 mm diameter and another one or two sprays 12 to 28 days later also provided more than 80% control of pink rot in most experiments. Foliar applications of Shirlan (fluazanim) or Acrobat MZ (dimethomorph plus mancozeb) showed some activity against pink rot, but were significantly less effective than Ridomil Gold MZ. Foli-R-Fos 200 (phosphonic acid) or Dithane M45 (mancozeb) applied to the canopy showed little activity against pink rot in most experiments. Yield increases of between 6 and 17 tons ha^?1 resulted from the application of either Ridomil granules at planting or two to three Ridomil Gold MZ foliar sprays around “tuber set.”     

Chemical control of pink rot of potato (Phytophthora erythroseptica PethyB.)

Field trials have been carried out for seven years to control pink rot (Phytophthora erythroseptica) at the International Potato Center (CIP). This disease has been a serious problem in the maintenance of a germplasm collection at the CIP’s Experimental Station at Huancayo, Peru (3300 m.a. s.l.). Pink rot annually causes serious loss of germplasm accessions, particularly diploid cultivars. Protective and systemic soil fungicides and soil fumigation with Basamid (Dazomet 98%) did not diminish the incidence of pink rot. Ridomil 5G (Metalaxyl 5%) applied at planting time with a preplanting fumigant reduced disease incidence from 31% of infected tubers to around 1%. Optimum control and yield increase were achieved by using Basamid at 300 kg/ha + Ridomil 5G at 30 kg/ha.     

Inheritance of blackspot bruise resistance in potato

Inheritance of blackspot bruise reaction in tetraploid potato was studied using a five-parent half-diallel, including parental selfs. The parents were chosen to represent the full range of blackspot reactions. Four 12-plant replications of each family were grown at Aberdeen, Idaho and at Prosser, Washington. The abrasive-peel method was used to evaluate the blackspot reaction. Highly significant general combining ability (GCA) effects and specific combining ability (SCA) effects were observed for this trait. The GCA and SCA variances represented 59% and 9%, respectively, of the total variance. GCA effects are presented for the parents. The narrow sense heritability was 0.85. These results indicate that breeding for resistance to blackspot should be successful if resistant parents are used; backcrossing may be needed.     

Inheritance of extreme resistance to potato virus Y in potato

Summary Segregation for extreme resistance to PVY was evaluated in progenies derived from crossing two extremely resistant potato clones with parents differing in resistance. Resistance was evaluated after mechanical inoculation with PVY^O and PVY^N, and after graft inoculation with PVY^O. Biological and serological tests (ELISA) were used for virus detection. The extreme resistance is governed by a single dominant gene, but observed segregations deviated from the expected ratios. Considerable modifying effects were detectable, depending on the potato genotype and virus isolate, for a significant excess of susceptible genotypes was observed in some progenies. Moreover, genotypes with non-parental types of resistance to PVY were observed.     

Identification of stem soft rot (Erwinia carotovora subspeciesatroseptica) resistance in potato

Development of potato cultivars resistant to Erwinia species (Erwinia carotovora subspeciesatroseptica, E. carotovora subspeciescarotovora, andErwinia chrysanthemi) causing soft rot of stems and tubers involves identification and transfer of the resistance to breeding lines possessing desirable agronomic traits. In total, 363 accessions of six wildSolanum species, three somatic hybrids and nine sexual progeny of somatic hybrids were screened for stem soft rot resistance. Thirty-one, 50, and 67 percent of accessions ofSolanum boliviense, Solanum chacoense, and Solanumsanctarosae, respectively, were resistant or highly resistant, whereas, 100, 97, and 95 percent of accessions ofSolanum canasense, Solanum tarijense, andSolanum spegazzinii were susceptible or highly susceptible, respectively. Two tuber soft-rot resistant somatic hybrids, A937 and T355-11 produced earlier by the fusion ofSolanum brevidens (PI 218228) andSolanum tuberosum (PI203900) orS. tuberosum cv. Russet Burbank, respectively, and the sexual progeny of A937 andS. tuberosum cv. Katahdin were also highly resistant. Transfer of resistance fromS. brevidens to somatic hybrid (A937) and subsequently to the progeny suggests that the resistance is simply inherited. In addition to the identification of 65 resistant or highly resistant clones of wildSolanum species, this study has identified three tuber soft rot-resistant clones that are also resistant to stem soft rot. Potato cultivars showed a range of responses to the pathogen.     

Application of a PCR-based test for detection of potato late blight and pink rot in tubers

A polymerase chain reaction (PCR)-based test for potato late blight (Phytophthora infestans) and pink rot (P.erythroseptica, P. nicotianae) diseases has been developed for use with potato tuber tissue. Primers based on sequence analysis of the ITS2 region of ribosomal DNA of late blight and pink rot pathogens were utilized in PCR assays of inoculated tubers and tubers harvested from plots known to have late blight and/or pink rot. Assays of artificially inoculated Kennebec and Russet Burbank tubers revealed thatP. infestans was detected by PCR as early as 72 h after inoculation and in the absence of visible symptoms. Much higher detection frequencies were obtained by PCR compared with plating on selective medium or placement of tissue in moist chambers. Tubers from plots known to have late blight and/or pink rot were tested using the PCR assay. Assay of late blight lesions showed ca. 80% recovery for late blight-infected tubers from the field. Results indicate that the PCR assay provides a rapid and accurate test for diagnosis of late blight and pink rot in potato tubers.     

Inheritance of the resistance toGlobodera rostochiensis pathotype Ro2 in potato

Nine advanced tetraploid potato clones withS. tuberosum ssp.tuberosum, S. tuberosum ssp.andigena, andS. vernei background were found to be highly resistant toGlobodera rostochiensis pathotype Ro2. In order to characterize the inheritance of the resistance to Ro2 and determine the genotype of these Ro2-resistant clones, a series of testcrosses to susceptibleS. tuberosum cultivars, as well as crosses between the resistant clones, were made. A total of 1081 seedlings corresponding to 18 families were grown, inoculated with 3000G. rostochiensis juveniles (J2), and evaluated by the root-ball method. To determine which genetic model best fit the data, goodness-of-fit-tests were performed. The analysis assumed tetrasomic inheritance under the hypothesis that resistance is due to one locus with complete dominance. From the analysis of the data it is concluded that there is no consistent evidence against the hypothesis that a single dominant gene is associated with the resistance to Ro2 and that the Ro2-resistant clones are simplex. In general, the number of cysts observed on the root balls was low compared with the results of other studies, the results of the different inoculations were variable, and there was an excess of resistant plants. Possible explanations for these observations are discussed.     

Inheritance and stability of resistance to fusarium tuber rot in tetraploid potatoes

Fusarium tuber rot of potato (Solanum tuberosum L.) is one of the most economically important diseases of stored potatoes. Dry rot is caused by several species of Fusaria, particularlyFusarium sambucinum in North America. The objective of this study was to determine the inheritance of resistance to Fusarium tuber rot. A highly resistant (B0172-22) and a highly susceptible (B0178-34) potato clone were crossed as female parents with two male parents. Tubers from varying numbers of progeny (28–37) from these four crosses were inoculated withFusarium sambucinum in the laboratory. Three tubers from each progeny were inoculated at approximately monthly intervals six times in each of 2 years. The depth and diameter of the lesion were measured 40 days after inoculation. Estimates of broadsense heritability (H) for lesion depth and a 95% confidence interval about these estimates were similar in 2000 and 2001: 0.84 (0.84, 0.91) and 0.77 (0.77, 0.87), respectively, as were the estimates for lesion diameter: 0.83 (0.83, 0.90) and 0.80 (0.80, 0.89), respectively. The differences in lesion depth for the female parents were significant the second year, but not the first year. The differences in lesion diameter for the female and male parents were significant both years. The female x male interactions were not significant for either lesion depth or lesion diameter. There were significant differences between clones in each of these four families for both traits both years. In 2000 and 2001, 24% and 38%, respectively, of the clones were unstable for resistance. In only one of the four families did instability increase as susceptibility increased, suggesting that there sometimes may be a genetic component to instability per se. These results indicate that resistance to Fusarium dry rot has a large genetic component. They also indicate that lesion diameter is more informative than lesion depth for evaluating resistance.     

Inheritance of resistance toFusarium spp. in tuber progenies of potato

Summary Glasshouse-grown seedling tubers of 22 progenies from parents differing in resistance toFusarium coeruleum andF. sulphureum (Gibberella cyanogena) were wound-inoculated with a cornmeal + sand culture of one or other of these dry rot pathogens. WithF. coeruleum, differences between progenies were due entirely to differences in the general combining abilities (gca) of the parents. Parental and gca values were highly correlated, as were the mean resistance of a progeny and that of its parents. WithG. cyanogena the agreement between replicates was poor and differences between progenies were less clear. It is suggested that different resistance mechanisms operate against these two pathogens.     

Inheritance of resistance to sclerotinia stem rot (Sclerotinia trifoliorum) in faba beans (Vicia faba L.)

Stem rot, a fungal disease caused by Sclerotinia trifoliorum Eriks., is often a serious problem in many important forage legumes including faba beans (Vicia faba L.). Understanding the inheritance of resistance to the disease is essential for effective breeding of resistant cultivars. Experiments were conducted to study the inheritance of resistance to stem rot of faba beans. The F₁, F₂, and the backcross generations of five crosses between four resistant and four susceptible populations (Alto × Polycarpe, A-90 × Polycarpe, ILB-1814 × A-247, A-90 × A-244, VT × Tanagra) were used. The eight populations were crossed properly in the field, and progenies of F₁ and F₂, as well as backcross progenies of F₁ with each of their parents, were evaluated for resistance to stem rot disease under controlled conditions after artificial inoculation of the plants with carrot root pieces colonized by the fungus. On the assumption that inheritance of stem rot resistance is governed by a single dominant gene, no significant differences were found between the observed and the expected frequencies of resistance for progenies, except for one cross. As the expression of resistance to the disease fits the expected ratios for a single dominant gene model, it is concluded that the inheritance of resistance to sclerotinia stem rot in the evaluated faba bean populations is controlled by a single dominant gene.     

The involvement of phenolics and phytoalexins in resistance of potato to soft rot

Summary Phenolics, for example chlorogenic, caffeic and ferulic acid, and phytoalexins, such as rishitin and phytuberin, were identified in potato tubers cv. Kufri Chandramukhi. The tissue of healthy tubers contained no detectable phytoalexins but did contain phenolics. The levels of these compounds were correlated with soft rot development. The rotting tissue either was free of these groups of compounds or had low concentrations. The wound periderm formed as a result of recovery from injury and infection contained high levels of the compounds. Much higher concentrations were detected at lower storage temperatures when oxygen supply was adequate. Antibacterial properties of the phenolics identified were tested againstErwinia carotovora which was inhibited by chlorogenic, caffeic and ferulic acids. The three phenolics were more effective together, in proportions in which they occurred in wound periderm, than individually. It was observed that none of these phenolics could inhibit pectolytic enzymes ofE. carotovora.     

Evaluation of potato varieties and selections for resistance to bacterial soft rot

Potato varieties and selections were evaluated for resistance to bacterial soft rot caused byErwinia carotovora subsp. carotovora and Erwinia carotovora subsp. atroseptica in mist chamber. A randomized complete block design with a single treatment and 3 replications was used in this study. Potato lines were inoculated with bacterial suspension (10 tubers / line) using the puncture inoculation method and incubated for 4 days in a mist chamber at 21 C and 100 % relative humidity in the dark. Although there were differences in incidence of soft rot and in amount of weight reduction, most of the potato lines were susceptible to tuber soft rot. Mean incidence (%) of soft rot ranged from 10 to 100%. Mean reduction of tuber weight (%) ranged from 0.13 to 16.43 %. Among the varieties and selections tested, AF 522-5 was the most resistant to tuber weight loss, while AF 1424-7 was most susceptible. There were also significant differences between years, and a variety by year interaction. Testing over a period of years is recommended. A significant correlation coefficient of soft rot incidence (%) and reduction of tuber weight (%) was obtained (r=0.65; P=0.05), indicating that incidence was a fair measure of soft rot potential. However, the greater variability in results for incidence make weight loss a better measure of soft rot reaction. There was a significant difference among varieties in the reduction of tuber weight when potato lines were inoculated either 1 month or 2 months after harvest, but the interaction of time by year prohibits a conclusion as to which time gives a better test reaction. Late maturing varieties had slightly less weight loss than earlier maturing varieties.     

Assessing fungicide resistance in populations of Alternaria in Idaho potato fields

Early blight, caused by the fungus Alternaria solani and brown leaf spot, caused by Alternaria alternata, are important diseases of potato crops in Idaho. In recent years growers have reported a reduction in efficacy of fungicides traditionally used in the past decade to control early blight. In 2009, a collection of A. solani 39 isolates were screened for resistance to azoxystrobin, pyraclostrobin, boscalid and famoxadone. Fungicide sensitivity testing was done using spiral plate dilution gradients. Results showed that of 39 isolates screened, all were resistant to azoxystrobin and three were resistant to boscalid. None were resistant to pyraclostrobin or famoxadone. In summer 2010, more isolates were collected (9 A. alternata and 26 A. solani) and the survey was expanded to include more fungicides with four different modes of action that targeted succinate dehydrogenase (SDH), methionine biosynthesis, mitochondrial respiration and multi-site contact activity. New isolates of A. solani and A. alternata were also collected from two additional sites. The results showed that 57% of the isolates were resistant to boscalid as well as an average of 63% of the isolates being resistant to the strobilurin fungicides. Seven and 15% of isolates were resistant to penthiopyrad (an SDH inhibitor), and pyrimethanil (a methionine biosynthesis inhibitor), respectively. However, none of the isolates were resistant to fluopyram (an SDH inhibitor) or a mixture of fluopyram and pyrimethanil. Although there appears to be cross resistance developing in Alternaria spp. to some of the new SDH inhibitors like penthiopyrad, others such as fluopyram are still showing limited to no resistance development in Alternaria spp. in Idaho.     

The inheritance of resistance to soft rot (Erwinia carotovora subsp.atroseptica) in diploid potato families

Soft rot of potato tubers, caused byErwinia (Pectobacterium), is a serious disease affecting potato crops during storage. Studies on the inheritance of resistance to tuber soft rot were undertaken on diploid potato hybrids. A total of 480 clones, derived from 12 families, were examined for resistance to soft rot in laboratory tests over 3 years and for basic agronomic traits in field experiments over 2 years. General (GCA) and specific (SCA) combining abilities were significant in the inheritance of resistance to soft rot; however, GCA for female parents were significant in 1 or 2 years during 3 years of evaluation and SCA was significant for eight, two, and six families out of 10 tested in 1994, 1995, and 1996, respectively. There were also significant differences between years. Moreover, marked interactions for GCA_females × year and SCA × year in the variation of soft rot resistance were found. Broad-sense and narrowsense heritability of resistance, measured as diameter of rotten tissue, was estimated as 0.92 and 0.89, respectively. The maternal effect, evaluated in two sets of reciprocal crosses, was not significant for the genetic determination of resistance to soft rot. No significant relationships were found between resistance to soft rot and the main agronomic traits in the tested diploid families. These results suggest that diploid potato clones with resistance to soft rot can be selected after being evaluated over a few seasons. It is also possible to select clones combining good resistance to soft rot with high yield, superior tuber characteristics, and acceptable starch content. These resistant diploids can be used as male parents in 4x-2x crosses in breeding tetraploid potato resistant to soft rot.     

Tuber yield,soft rot resistance,bruising resistance and processing quality in a population of potato (cv. crystal) somaclones

Several important postharvest characteristics were evaluated in protoplast-derived somaclones after two consecutive growing seasons (1987, 1988). None of the somaclones had tuber yields significantly greater than the protoplast donor Crystal mother clone (MC), however, 8 of 37 clones had significantly lower yields. Of the 25 somaclones selected forErwinia soft rot resistance testing, 1 clone demonstrated significantly elevated resistance to soft rot while 12 clones were found to be significantly more susceptible relative to MC. Over half (20) of the 33 somaclones tested showed significant improvement in resistance to tuber bruising while only a single clone was more susceptible than MC. Based upon Agtron color analysis, 5 somaclones showed significantly enhanced chip color when processed directly out of cold storage (6 C). Six clones had significantly improved chip color after a reconditioning period (20 C). Four of the clones with significantly better cold processing characteristics were also among the 6 with significantly enhanced processing quality after reconditioning. Some clones showed improvement in two or more characteristics. Clone 72–93 had significantly elevated resistance to both bacterial soft rot and tuber bruising while the bruise resistant clones 74-14, 71-41, 71-18 and 71-2 demonstrated significant improvement in both cold and reconditioned processing characteristics.     

Effect of calcium soil amendments on phenolic compounds and soft rot resistance in potato tubers

Nutrients such as boron, nitrogen and calcium stimulate the production of phenols. This research focuses on the role of calcium in increasing phenol metabolism in potato peels and the ensuing tuber resistance to soft rot pathogens was investigated. Two field experiments were conducted at the University of Zimbabwe campus plots in 2008 and 2009 summer seasons. Sprouted tubers of cv. BP1 were planted in plots treated with different fertilizer combinations. The treatments were: 1) compound S (7N: 21P: 8K) + ammonium nitrate (34 %N) 2) compound D (7N: 14P: 7K) + calcium nitrate (19 Ca: 15.5N) 3) compound S + calcium nitrate and 4) compound D + ammonium nitrate. The harvested progeny tubers were inoculated with Pectobacterium carotovorum subsp. brasiliense. Calcium amendment increased the activities of enzymes (phenylalanine ammonia-lyase, polyphenol oxidase and peroxidases) involved in the metabolism of phenolics and total soluble phenols. Calcium amendment significantly reduced (P < 0.05) the maceration effect of P. carotovorum subsp. brasiliense in tuber tissues. Chlorogenic acid, caffeic acid and calcium, but not ferulic acid content were significantly higher (P < 0.05) in plants grown in calcium treated plots. Calcium positively and significantly correlated with polyphenol oxidase (PPO), phenylalanine (PAL) and peroxidase (POD), while chlorogenic and caffeic acid showed a positive relationship with POD and PPO. Calcium amendment significantly reduced maceration symptom caused by the bacteria (P < 0.05), resulting in smaller decayed zone diameters on inoculated tubers from calcium-treated plots. This study shows that soil amendments of calcium increase concentration of calcium, caffeic and chlorogenic acid in tuber peels and also reduces maceration effect of pectinolytic pathogens. Reduced maceration could be due to increased levels of caffeic and chlorogenic acid which have antimicrobial properties.     

Inheritance of a pectate lyase mediated soft rot resistance in crosses between transgenic potato lines of cv. désirée andS. tuberosum cultivars

Summary Expression of the pectate lyase (PL) isoenzyme 3 in transgenic potato lines of cv. Désirée mediates an enhanced resistance of tuber tissue toErwinia carotovora (Ec) soft rot due to a pre-activation of plant defence mechanisms. Therefore, theSolanum tuberosum cvs Agave and Adretta with a moderate level of soft rot resistance were crossed with such PL-expressing potato lines. The resulting progenies were assessed with respect to plant/tuber characteristics over a period of four years and then tested for PL3-expression as well as for soft rot resistance. 71% of the selected progeny lines exhibited a stable production of the PL3 enzyme. Statistical analysis revealed differences between the transgenic and the non-transgenic progeny concerning the soft rot resistance of tuber tissue. Compared with the PL-inactive progeny, extension of Ec-rotting on the wound surface of PL-transgenic potatoes was diminished on average by 51.8%. Similarly, the degree of cell lysis caused by bacterial maceration was significantly (P<0.05) reduced in tuber tissue of PL-expressing progeny lines. The latter also revealed an enhanced PPO and PAL activity in their tuber tissue indicating an active plant defence. It is concluded therefore that the PL-mediated soft rot resistance introduced into potatoes by means of molecular techniques is heritable.