Evaluating Incidence of Helminthosporium solani and Colletotrichum coccodes on Asymptomatic Organic Potatoes and Screening Potato Lines for Resistance to Silver Scurf

Silver scurf, caused by Helminthosporium solani, and black dot, caused by Colletotrichum coccodes, cause tuber blemishes on potato (Solanum tuberosum L.) which affect processing and fresh market trade. Tubers from ten cultivars were collected at harvest from three organic farms in Wisconsin and categorized as symptomatic or asymptomatic based on visual symptoms of silver scurf and black dot and/or signs of H. solani and C. coccodes. Tuber incubation and PCR assays were performed on asymptomatic tubers to detect H. solani and C. coccodes. Tuber incubation and PCR assays were in slight to fair agreement (kappa coefficient <0.4) for detecting both pathogens. Most asymptomatic tubers tested were positive by one or both assays for H. solani (75 %) or C. coccodes (94 %). Minituber inoculation assays were also performed to screen potato lines for resistance to silver scurf. Of the 14 lines tested, a diploid interspecific hybrid, C287, had consistently low sporulation, suggesting it has partial resistance to silver scurf. Since the majority of tubers harvested are already infected with one or both pathogens further research should focus on organically acceptable management practices that may inhibit disease development in field and in storage.     

The influence of storage factors on endogenous ethylene production by potato tubers

Throughout a 7-month storage period, Russet Burbank tubers continuously ventilated at 32 F (0 C) and 45 F (7.2 C) with atmospheres of 2% O₂, air, 4% CO₂, and intermittently ventilated with air, evolved ethylene at a rate no greater than 0.008 μ 1 Kg^?1 hr^?1. Tubers stored in 80% O₂ and 12% CO₂ produced ethylene at much higher rates. In all cases where sprouting occurred, the rate of ethylene production increased. Inoculation withFusarium roseum greatly stimulated ethylene production but inoculation withAlternaria solani did not.     

Evaluation of wildSolanum species for resistance to the US-8 genotype ofPhytophthora infestans utilizing a fine-screening technique

Greenhouse assays were carried out to characterizeSolanum accessions previously reported to be late blight resistant and to identify individuals within the accessions with high levels of resistance to late blight. Foliage of wildSolanum species accessions or hybrid cultivated potato x wild species (diploid or tetraploid) from Mexico, Russia, or South America was inoculated withPhytophthora infestans (US-8) and was scored for severity of foliar symptoms at 7 and 14 days after inoculation (DAI). Mexican accessions (S. brachycarpum,S. pinnatisectum,S. guerreroense, andS.fendleri) were significantly more resistant (1.1% infection at 14 DAI) than either Russian (59.1% infection at 14 DAI) or South American (53.4% infection at 14 DAI) accessions. Moreover, the genotypes within the Mexican accessions tended to be more uniformly resistant whereas genotypes within the Russian and South American accessions tended to segregate for resistance. The more resistant genotypes of Russian and South American accessions (176/546) were retested, and 56 genotypes were identified as having potential for use in potato breeding programs for resistance to late blight. Fifty percent of the selected genotypes were fromS. microdontum accessions PI498124, PI595510, and PI595511. The potential of these species for incorporation into breeding programs for late blight resistance is discussed.     

Biological control of fusarium dry rot of potato tubers under commercial storage conditions

Lack of effective chemicals impedes control of Fusarium dry rot of stored potato tubers destined for processed and table stock use. Biological control of dry rot incited byGibberella pulicaris (anamorph=Fusarium sambucinum) has been demonstrated in laboratory studies but not in commercial storage environments. Several Gram-negative bacterial strains that were efficacious and amenable to production in liquid culture in laboratory studies were selected for pilot studies in Idaho and for bin trials at commercial storage houses in Idaho and North Dakota. In the first year of pilot studies,Pseudomonas fluorescens S22:T:04 (≈1 x 10⁸ cfu/ml) decreased dry rot caused byG. pulicaris by 19% when coinoculated with the pathogen compared to controls and to the fungicide thiabendazole (TBZ) applied at label rates. In second year pilot studies,P. fluorescens P22:Y:05 andEnterobacter cloacae S11:T:07 (≈4 x 10⁸ cfu/ml) reduced the severity of disease incited byG. pulicaris (25% and 17% average disease decrease, respectively) when antagonists were applied after pathogen inoculum. Treatment effects on dry rot that developed from inoculation withNectria haematococca (anamorph=Fusariumsolani var.coeruleum) were variable and influenced by interactions between antagonists, a wetting agent, and TBZ. In commercial storage bin trials,E. cloacae S11:T:07 reduced naturally occurring levels of dry rot by an average of 21% compared to 14% for TBZ, demonstrating that this antagonist was effective when produced using a liquid culture medium and methodology that approximates commercial practices for producing biomass.     

Effect ofUlocladium consortiale,a nonpathogen,on the severity of tuber blight caused byAlternar1a solani

Inoculation withAlternaria solani andUlocladium consortiale caused significantly more severe infection of tubers than inoculations ofA. solani alone.U. consortiale, when inoculated alone, caused no significant tuber infection. Infection resulting from a plug-type inoculation was significantly greater than that from a flap-type inoculation.     

The effect of temperature and relative humidity on wound healing and infection of potato tubers byAlternaria solani

Under environmental conditions favorable for wound healing, potato tubers became significantly (P = 0.05) more resistant toA. solani infection during a two-day interval between wounding and inoculation. This period coincided with the time required for wounded tubers to completely seal wounded surfaces by suberization at 15.6°C/98% RH. Suberization of the wounded surfaces provided tubers significant (P = 0.05) protection againstA. solani invasion but wound periderm development was also essential for total exclusion of the pathogen. Wound healing takes place faster at high temperatures and relative humidities. The effect of temperature is more critical in the wound healing process.     

Response of six wild potato species to chloride and sulfate salinity

Eleven accessions belonging to six species ofSolanum (S. bulbocastanum,S. chacoense, S. gourlayi, S. microdontum, S. papita, and S.sparsipilum) were tested for resistance to salt. Resistance was measured by survival and growth (dry weight of tops) of plants treated with solutions of NaCl and Na₂SO₄. The six species could be grouped into four classes based on good to poor resistance to salt solutions (1.S. chacoense, 2.S. gourlayi, 3.S. microdontum, S. sparsipilum, S. bulbocastanum and 4.S. papita). There was a close relationship (r=0.94**) between the survival of plants and growth of surviving plants treated with salt solutions. There was no relationship between known heat or drought resistance of various accessions and salt resistance. Results indicate there is a large genetic base available to improve the salt resistance of the cultivated potato.     

Comparison of the effectiveness of different methods of screening for bacterial soft rot resistance of potato tubers

Tubers from eight potato cultivars (cvs) grown at two different locations in Wisconsin were tested for bacterial soft rot resistance using different inoculation techniques. The procedures included 1) point inoculations of tubers with different inoculum levels followed by incubation in ambient or low oxygen condtions, 2) inoculation of mechanically bruised tubers followed by incubation in a mist chamber, and 3) a standard slice inoculation method. The point titration test followed by incubation in dew chamber and the mist chamber-bruise test showed similar patterns of resistance for cultivars that were used in these experiments. These two methods are considered to be effective for screening potato tubers for bacterial soft rot resistance. Point titration methods are very useful if only limited numbers of tubers are available. The mist chamber-bruise test is simpler than the other procedures; however, to obtain reproducible results large numbers of tubers are required. Because of the great variability of the results obtained in inoculation of slices, the reliability of this approach can be questioned as a standardized method for evaluation of resistance. Tubers of somatic hybrids ofS. brevidens andS. tuberosum and their sexual progeny were significantly more resistant to bacterial soft rot than tubers of moderately resistant cultivars when evaluated by each of the assay procedures.     

Co-current introgression of economically important traits in a potato-breeding program

Wild potato species contain many traits of economic importance. Late blight (LB) resistance and cold chipping are traits desired in potato cultivars. These traits could be co-currently introgressed if they occurred together in wild potato species. Our research objectives were (1) to determine if variation for cold chipping exists between potato species, accessions within species, and plants within accessions all having foliar LB resistance, and (2) to identify wild potato genotypes combining LB resistance and cold chipping. Materials include 665 genotypes from 43 LB-resistant accessions of 12 potato species having Endosperm Balance Numbers (EBN) of 1, 2, and 4, and 59 LB-resistant genotypes retained from these accessions for breeding. Potato chips were made from greenhouse-grown tubers following storage at 4 C for 6 months. Chip color was scored 1–10, ≤ 4 is acceptable by industry standards. Most of the variation for chip color was due to differences between species. Species ranged in the percentage of acceptably chipping genotypes (0% – 67%) with nine of 12 species having cold-chipping genotypes. Appreciable variation was present within accessions as well. The best chipping accessions wereS. verrucosum plant introduction (PI) 161173 – 4.33 / 0.67 (mean / proportion acceptable genotypes),S. stoloniferum PI 250510 -4.36 / 0.64,S. pinnatisectum PI 347766 -4.65 / 0.35 and 275233 -4.73 / 0.44, andS. megistacrolobum PI 195210 -5.14 / 0.29. Eleven 1EBN genotypes fromS. pinnatisectum andS. trifidum and five 2EBN genotypes fromS. verrucosum, S. fendleri,S. stoloniferum, andS. microdontum were identified that combined LB resistance and cold chipping. Co-current introgression would require fewer breeding cycles than other breeding methods to identify hybrid genotypes possessing both traits.     

Evaluation ofSolarium fendleri as a source of resistance toMeloidogyne chitwoodi

Twelve accessions ofSolarium fendleri collected in the United States and Mexico were inoculated with 5,000 eggs of host-races 1 and 2 ofMeloidogyne chitwoodi, Columbia root-knot nematode. The test seedlings that were derived from plant introduction true seed lots were maintained for 55 days before harvest and egg count. The results revealed that two accessions, PI 275162 and PI 275165, were non-hosts (final egg count/initial egg inoculation < 0.1) forM. chitwoodi race 1. The results were uniformly confirmed in the second experiment. No resistance was found to race 2. The tetraploidS. fendleri accessions were crossed to a nematode-susceptible cultivated diploid potato clone from aS. phurejastenotomum population. The triploid hybrids expressed resistance to race 1 at the non-host level. After somatic doubling, the resulting hexaploids also expressed non-host-level resistance to race 1. The two resistant accessions had been collected in southeastern Arizona, one each from the Huachuca and Chiricahua Mountains. Each range is an isolated island of high-elevation mesic flora surrounded by typical lowerelevation Sonora-type desert habitat. These accessions are the only known sources of resistance toM. chitwoodi from wildSolanum species in the USA.     

The effect of storage environment on the infection of potato tubers byAlternaria solani

Manipulation of storage environment to provide favorable conditions for wound healing immediately after harvest reducedAlternaria solani tuber infection. Prestorage of tubers at 15.6°C for three weeks before final storage at 10°C normally maintained for commercial storage of potato chip stocks resulted in fewer and smaller lesions than constant storage at 10°C or 4.4°C or prestorage for varying periods at these temperatures. Prestorage of potatoes at a low temperature (4.4°C) for 1–3 months resulted in moreA. solani infection suggesting that low temperatures may inhibit the wound healing process more than they reduce rates ofA. solani infection or disease development. Variations in relative humidities above 75% did not have an appreciable effect on the wound healing process nor onA. solani infection.     

Variation in resistance toPhytophthora infestans among 21Solanum verrucosum plant introductions

Detached leaflets of 21Solanum verrucosum plant introductions (PIs) were inoculated with two races ofPhytophthora infestans to evaluate levels of resistance present within this species. Inoculated leaflets were visually evaluated for the percentage area colonized byP. infestans and the intensity of sporulation over the colonized area. Disease reactions ranged from moderately susceptible to highly resistant (immune). Significant plant-to-plant heterogeneity for resistance was observed within many of the PIs, indicating that they may represent mixtures of different genotypes. Hypersensitive flecks were observed on some inoculated leaflets of all PIs tested. Adult plant reactions showed moderate agreement with those obtained by other workers on seedling populations of several PIs. These results support the hypothesis that R-genes and polygenes for resistance toP. infestans exist withinS. verrucosum accessions. This species thus may provide a useful source of novel late blight resistance genes.     

Etiology and characterization of cucumber vine decline in Oman

A long-term study was conducted between 2000 and 2009 to characterize the incidence, progress and causal agents of cucumber vine decline in Oman. A survey in 175 different greenhouses showed that disease incidence levels range from 0 to 50%, with the highest levels of mortality being in the hotter seasons. Detailed temporal disease increase data from 24 different greenhouses showed mortality progress consists of two phases. The first phase is characterized by attack of young seedlings (<3 weeks old), resulting in damping-off disease. The second phase was characterized by re-appearance of symptoms and plant death (vine decline) during the fruit setting period, 35-50 days after transplanting. Isolations from 148 declining adult cucumber plants yielded Pythium aphanidermatum (80%), Pythium spinosum (13%), Fusarium equiseti (12%), Fusarium solani (8%), Rhizoctonia solani (5%) and one isolate each of Trichoderma hamatum and Bionectria sp. P. aphanidermatum, P. spinosum, R. solani and F. solani were found to be pathogenic on cucumber, with P. aphanidermatum being the most aggressive. This appears to be the first report of association of P. spinosum with vine decline in greenhouse cucumbers and the first report of the high susceptibility of adult cucumber plants to vine decline during the initial period of fruit setting. In addition, this is the first report of association of R. solani and F. solani with declining adult cucumber plants in Oman and the first report of the occurrence of T. hamatum and Bionectria species in the country.     

A Stringent and Broad Screen of Solanum spp. tolerance Against Erwinia Bacteria Using a Petiole Test

Blackleg and stem rot caused by coliform bacteria belonging to Dickeya spp. and Pectobacterium spp. (both referred to as Erwinia in this paper) are a problem for potato growers worldwide and no sources of high tolerance are currently present in the cultivated S. tuberosum gene pool. To find sources of tolerance, 532 genotypes from 340 accessions, covering most of the known potato species, were assayed with P. wasabiae, P. carotovorum and D. ‘solani’ species in a petiole test. This petiole test was optimized later on using well responding genotypes from the broad screen. Based on the obtained data, the best developmental stage for cell wall degradation tests was identified to be the 4th-6th youngest leaf. Under the stringent biotic and climatic screening conditions used, only three genotypes were regarded as tolerant against all tested Erwinia species. These genotypes all belonged to the series Yungasensa, this series can be readily crossed with cultivated potato and is considered as a genetic source to upgrade the Erwinia tolerance level of cultivated potato.     

Occurrence,development, and losses associated with silver scurf and black dot on Colorado potatoes

Silver scurf, caused byHelminthosporium solani, and black dot, caused byColletotrichum atramentarium, are pathogens of tuber periderm whose presence in Colorado was only recently reported. A field survey conducted in September 1977 revealed thatC. atramentarium was more prevalent (21.8% tuber infection) and had a wider distribution thanH. solani (5.4% tuber infection). A greater incidence of both pathogens was observed on thin skinned tubers of chipping cultivars (49.0% infection) than on thicker skinned tubers of table stock cultivars (9.1% infection). Conidial development ofH. solani is tretic, pleurogenous, and requires 17–21 hours per conidium. Conidial septations appear while conidial elongate, and require 3–5 hours per septum. Light microscopy revealed that at least 11 conidia per conidiophore are produced in culture in 54 hours at 20–25 C (68–77 F) and humidity >90%. Scanning electron microscopy showed that fructifications ofH. solani (conidiophores-conidia) arise from beneath infected tuber periderm. Histological studies indicate some peridermal loosening and sloughing. Heavy deposition of unidentified compounds was observed in infected periderm, and hyphae were restricted to periderm cells. Fresh weight loss of tubers naturally infected withC. atramentarium was significantly greater than fresh weight loss of nearly noninfected (< 1% surface area infected) control tubers. Periderm infected with eitherH. solani orC. atramentarium appeared similar, i.e. shriveled, suggesting infections from either pathogen may result in increased fresh weight loss through alteration of the periderm.     

Activities of essential oils from Asarum heterotropoides var. mandshuricum against five phytopathogens

Some secondary metabolites of plants function as antimicrobial products against phytopathogens and constitute an increasingly important class of pesticides. In the present study, the essential oil of Asarum heterotropoides var. mandshuricum was analyzed by GC/MS and its antimicrobial activity was evaluated against five phytopathogenic fungi. Major components of the oil were methyleugenol (59.42%), eucarvone (24.10%), 5-allyl-1,2,3-trimethoxybenzene (5.72%), and 3,7,7-trimethylbicyclo(4.1.0)hept-3-ene (4.93%). The essential oil and the most abundant component, methyleugenol, were separately assayed for inhibition of 5 pathogens: Alternaria humicola, Colletotrichum gloeosporioides, Rhizoctonia solani, Phytophthora cactorum and Fusarium solani. Both the oil and methyleugenol strongly inhibited the growth of the test pathogens (IC₅₀ values <0.42 μg ml⁻¹) except F. solani, with the best activity against P. cactorum (IC₅₀ values = 0.073 and 0.052 μg ml⁻¹, respectively). It is concluded that the essential oil of A. heterotropoides var. mandshuricum has a broad antiphytopathogenic spectrum, and that methyleugenol is largely responsible for the bioactivity of the oil. The mode of action of methyleugenol against P. cactorum is discussed based on changes in the mycelial ultrastructure.     

Cultivar response to Fusarium storage rot as affected by two methods of seed origin propagation; Clonal selection andin vitro culture

Progeny tubers from seed potatoes originating from either a traditional clonal selection method or the more modernin vitro tissue culture method of propagation were tested for storage rot response following inoculation of three tuber sites with twoFusarium species. Significant differences were found among disease responses for the twoFusarium species and for the four cultivars tested. Disease symptoms were less severe forF. solani var.coeruleum than forF. sambucinum. Fusarium sambucinum caused less disease in Superior than Kennebec, Russet Burbank and Sebago. Seed propagation method did not significantly affect disease response except for one case;in vitro culture of Kennebec had less disease due toF. sambucinum at the tuber side inoculation site than clonal selection. Kennebec and Superior inoculatedF. sambucinum had significant differences with respect to the number of years of field propagation for the tuber side inoculation site and for the tuber mean rot index. Significant differences in number of years of field propagation were also found for eye-ends of Superior tubers and stem ends of Russet Burbank tubers inoculated withF. solani var.coeruleum.     

Differential response of pea (Pisum sativum) to Orobanche crenata, Orobanche foetida and Phelipanche aegyptiaca

Several broomrape species including Orobanche crenata, Orobanche foetida and Phelipanche aegyptiaca are reported to infect various grain and forage legumes in the Mediterranean and West Asia. Pea (Pisum sativum) is severely damaged by O. crenata, but there are no reports on O. foetida or P. aegyptiaca infection. We report here that pea can induce high germination of seeds of O. crenata, O. foetida and P. aegyptiaca but only O. crenata success infecting pea roots and developing further. Some differences in levels of infection by O. crenata were observed among pea accessions what can be exploited in pea resistance breeding programmes. On the contrary, all pea accessions studied were highly resistant to infection by both O. foetida and P. aegyptiaca, preventing any tubercle attachment and development. This makes pea a promising candidate as trap crop for O. foetida and P. aegyptiaca seed bank demise in infested soils.     

Influence of temperature and length of storage on resistance of potato to tuber rot induced byErwinia chrysanthemi

Tubers of 11 clones ofSolanum tuberosum subsp.andigena (andigena) and 12 cultivars ofS. tuberosum subsp.tuberosum (tuberosum) were inoculated with water suspensions (5.5 × 10⁴ 5.5 × 10⁵, 5.5 × 10⁶ CFU/ml) ofErwinia chrysanthemi. Tubers were inoculated immediately after harvest and after 6 and 16 wk of storage at 4 and 23°C. Tuber rot incidence in andigena and tuberosum increased as inoculum concentration increased. Based on tuber rot severity, clones of andigena were classified as resistant, intermediate, or susceptible and cultivars of tuberosum were classified as intermediate or susceptible. Rot severity increased in all tubers stored at 4°C and in tubers of tuberosum stored at 23°C; rot did not increase in tubers of andigena stored at 23°C. Electrolyte leakage (EL), total sugars (TS), reducing sugars (RS), non-reducing sugars (NRS) and dry matter (DM) were determined in non-inoculated tubers. There was a significant positive correlation between the rate of EL, concentration of RS and tuber rot in andigena and tuberosum. Tuber rot and DM were negatively correlated in tubers of andigena; but they were not correlated in tuberosum. Clones of andigena with low EL, TS, RS, and high DM were resistant to tuber rot, and the incidence of tuber rot in these clones was much less influenced by temperature and length of storage. The influence of temperature and length of storage on susceptibility toE. chrysanthemi may be explained by increased cell membrane permeability; increased leakage of accumulated sugars in potatoes stored at 4°C could favor bacterial proliferation resulting in more disease.     

Screening of exotic potato germplasm for nitrogen uptake and biomass production

Exotic potato germplasm may contain useful traits for improving nitrogen (N) use efficiency in cultivated potatoes (Solanum tuberosum L.). The objective of this study was to evaluate a “mini” core collection of wild germplasm for biomass production, N uptake, and N use efficiency. A field study was conducted during the 1993 growing season on a loamy sand soil at Becker, Minnesota. Uniform transplants for 39 wild accessions and 3 varieties were grown under greenhouse conditions, and after 40 days, they were transplanted to the field. Plants were subjected to two N treatments, 0 and 225 kg N ha¹, replicated three times in a split-plot design. Plant parts were collected separately 111 days after transplanting, dried, weighed, and analyzed for N concentration. Nitrogen rate and potato species had significant effects on total dry weight, N content, and tissue N concentration. The regression coefficient of N concentration on total dry weight was very low (r=0.22, NS), whereas the regression coefficient of total N content on total dry weight was high (r=0.94, P>0.001). At both low and high N environments, Russet Burbank had greater dry weight than all the accessions. Several wild accessions, especiallyS. chacoense, S. commersonii, S. kurtzianum, S. microdontum, andS.phureja, had equal to or greater dry weights than Russet Norkotah or Red Norland. For N uptake efficiency, 2,7, and 20 accessions were ranked better than Russet Burbank, Russet Norkotah, and Red Norland, respectively. Recovery of soil applied N by the varieties ranged from 16 to 36%; the top seven wild accessions recovered between 27 and 49%. Based on plant growth without added N fertilizer and relative response to N fertilizer, the species were categorized into four relative N use efficiency groups: good foragers with good response to N, poor foragers with good response to N, good foragers with poor response to N, and poor foragers with poor response to N.