Soil survival and thiabendazole sensitivity ofHelminthosporium solani isolates from Alberta,Canada

Summary Survival of the pathogen causing silver scurf of potato (Helminthosporium solani) in Alberta soils was studied by evaluatingH. solani infection in the progeny ofH. solani-free nuclear seed potato tubers planted in fields where potatoes had never been grown or were grown 1, 2, 3, or 4 years previously. Daughter tubers from all the fields developed silver scurf lesions, andH. solani was isolated from infected tubers. This is the first report of survival ofH. solani in Alberta soils. Soil-borne inoculum appears to have a role in the epidemiology of the disease and in the introduction of the pathogen into silver scurf-free potato seed stock. Of 31 plant species tested, only potato was found to be a host ofH. solani. Most of theH. solani isolates from north central Alberta were more sensitive to thiabendazole than those from southern Alberta, where thiabendazole is much more commonly used.     

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.     

Evaluation of immunofluorescence with monoclonal antibodies for detecting latent bacterial ring rot infections

Corynebacterium sepedonicum was detected in symptomless potato stems and tubers with immunofluorescence using monoclonal antibodies specific for the bacterial ring rot pathogen. The concentration of bacterial cells in potato tissue preparations ranged from >500 cells/microscope field to 1 cell per preparation. Symptomless tubers containing ring rot bacteria planted in field plots yielded plants with ring rot symptoms, plants with latent ring rot infections, or plants with no detectable levels of ring rot bacteria. Tubers with the greatest number of bacteria were most likely to develop plants expressing ring rot symptoms, but even some seed tubers with a low number of bacteria developed into plants with symptoms. Some seed tubers with high levels of ring rot bacteria produced plants with only low numbers ofC. sepedonicum.     

Influence of seed tuber infestation,chemical seed treatment,and pre-harvest climate on incidence of gangrene and dry rot of potato (Solanum tuberosum L.)

Summary In each of the five years 1978 to 1982, seed lots of cv. Bintje, variously infested byPhoma foveata Foister andFusarium solani var.coeruleum (Sacc.) Booth, as found by wound and cut tests, respectively, were planted in a series of field experiments at six sites in Sweden. A statistically significant relationship between seed and progeny infestation was found forP. foveata each year, and in three years forF. solani. Benomyl and thiabendazole mist treatments of seed tubers at grading in spring controlled the development of gangrene and dry rot in treated seed tubers, but did not consistently reduce the levels ofP. foveata andF. solani infestation in progeny from treated seed tubers. The site of cultivation markedly influenced mean levels ofP. foveata infestation, which were found to be related to temperature and rainfall during a 40 day pre-harvest period at the trail sites; however, mean levels ofF. solani infestation were not so related.     

Assessment of latent infection frequency in progeny tubers of advanced potatoclones resistant to bacterial wilt: A new selection criterion

Summary This paper reports results of a 3-year evaluation of CIP advanced potato clones in a bacterial wilt-infested field (race 3) in Peru. Clones resistant or moderately resistant to wilt were selected and all tubers harvested from each clone were tested for latent infection byRalstonia solanacearum using a sensitive serological technique developed at CIP. A sampling strategy to estimate accurately the frequency of infected tubers in the clones has been evaluated. This method will allow consideration of tuber latent infection as a new selection criterion in breeding for resistance to bacterial wilt. Thirteen clones were found resistant to wilt in all three evaluations (i.e.≤6% wilt), from which five had no wilt in all trials. However, all clones harboured latent infection in tubers averaging 30%. Analysing 30 tubers/clone provides an accurate estimation of the proportion of infected tubers with a high precision level.     

Survival ofErwinia carotovora in Wisconsin soils

Erwinia carotovora var.atroseptica (van Hall) Dye andE. carotovora var.carotovora (Jones) Dye were detected in agricultural soils in Wisconsin using baiting and enrichment techniques. These soft rot bacteria could not be detected in soils of potato fields prior to planting of crops in the spring period using standard soil dilution plating techniques on a crystal violet pectate medium, however. A procedure involving incubation of samples in pectate enrichment broth followed by preparation of smears on slides and treatment with a fluorescent antibody stain specific forE. carotovora var.atroseptica was the most sensitive of the methods tested for detecting the blackleg pathogen. Erwinia carotovora was isolated more frequently during the spring from fields in which potatoes had been grown the previous year than from fields in which other crops had been grown. It was also isolated from potato tubers and stems that had overwintered in the field. The presence ofE. carotovora could not be detected in root zone samples of weed plants using the dilution plating method.     

Plant-to-plant transmission of the bacterial ring rot pathogenClavibacter michiganensis subsp.sepedonicus

Summary Plant-to-plant transmission of the bacterial ring rot (BRR) pathogen,Clavibacter michiganensis subsp.sepedonicus (Spieck. et Kott.) Skapt. et Burkh. of potato (Solanum tuberosum L.), was studied in field trials over a three year period. Healthy and infected seed tubers were planted 35 cm apart. In one treatment, a subsurface barrier was placed between the healthy and infected seed tubers separating the root systems of neighbouring plants. In this treatment, none of 216 plants grown from healthy seed tubers was infected at harvest, as determined by indirect immunofluorescence antibody staining (IFAS) with monoclonal antibodies. In the other treatment, no subsurface barrier was used. In this treatment, two of 368 plants (0.5%) grown from healthy seed tubers were infected at harvest. It is concluded that plant-to-plant transmission may occur but at very low frequency, and is unlikely to play a significant role compared with the potential of transmission by shared potato handling equipment.     

Emerging and Re-emerging Potato Diseases in the Andes

Outbreaks of a new mating type (A2) of late blight (Phythopthora infestans) and the spread of bacterial wilt (Ralstonia solanacearum) were the most notable events among fungi and bacteria during the last 10 to 15 years in the Andes. The emergence of the A2 mating type of P. infestans was probably a result of seed movement from the Northern hemisphere. Black scurf caused by the fungus Rhizoctonia solani spread in Peru and Bolivia causing economic losses estimated at 5 to 10%. A high incidence of common scab on potato tubers, caused by the bacteria Streptomyces scabies was found in some localities in Bolivia. Among the emerging viruses, the NTN strains of PVY have been found in some countries including Peru. Recently, two other viruses have been found spreading in potatoes. One is coded SB26/29 and is transmitted by Russelliana solanicola, the first psyllid vector found for a plant virus. The other emerging virus is coded SB41 and is apparently also new to potatoes. It causes a severe mosaic on the affected plants and the virus is transmitted experimentally only by grafting. Some other viruses of minor importance have also been detected in potatoes.     

On-farm evaluation of true potato seed in the Philippines

Potato production using true potato seed (TPS) was evaluated with a cooperative group of farmers. The cooperative grew TPS or seed tubers (1 to 3 g) produced from TPS in beds in screenhouses to avoidPseudomonas solanacearum (BW). From 250 to 400 tubers/m² were obtained. Tubers > 3 g were replanted by farmers at high elevation for further multiplication. Yields always exceeded the control (clonally produced cultivar). Seed tubers from TPS of 3 to 5 g produced up to 32 t/ha resulting in a multiplication rate of 145:1. Farmers are in various stages of adopting this technology. An early maturing progeny with resistance to BW is needed to permit a more rapid spread of this technology.     

Survival and possibilities for extinction ofPseudomonas solanacearum (Smith) Smith in cool climates

Summary The importance of biovar 2 ofPseudomonas solanacearum (syn.Burkholderia solanacearum; syn.Ralstonia solanacearum) in cool temperate climates has been recently emphasised by outbreaks of potato brown rot disease in some European countries. Dissemination of the pathogen in latently infected potato tubers and in contaminated irrigation water, and overwintering in aquatic roots of the weed hostSolanum dulcamara have been implicated. Evidence from the literature suggests that in cool climates the organism is unlikely to persist long term in soil in the absence of a susceptible host. Effective control strategies rely on the enforcement of quarantine legislation at both national and European Community levels. This requires knowledge of the biology of the pathogen in cool environments, methods for accurate monitoring of its survival and distribution at and around disease outbreak sites and application of effective measures for pathogen exclusion, containment and eventual eradication. This paper reviews current knowledge on the above.     

Serogroups ofErwinia carotovora involved in systemic infection of potato plants and infestation of progeny tubers

Enrichment techniques and serological strain typing methods were used to determine the source of stem and progeny tuber populations ofErwinia carotovora (Ec) in 1980 and 1981. Plots were established in a field that had been planted with potatoes since 1972. Seed tubers of cultivars Russet Burbank and Norgold Russet were assayed forErwinia carotovora pv.carotovora (Ecc) andErwinia carotovora pv.atroseptica (Eca) by tuber peeling and direct plating and plating following enrichment in a pectate-based broth. Levels of seed tuber infestation ranged from undetectable to 2.4 × 10⁴ colony-forming units per g peel tissue. Prior to planting Ecc was detected in soil samples from field plots by enrichment in a pectate-based broth. Ec was isolated from symptomless plants with increasing frequencies from 8 weeks following planting regardless of the level of Ec populations in seed tubers. At harvest progeny tuber recontamination was low and not related to levels of seed tuber Ec populations. In 1980, serological typing of representative Ecc isolates indicated serogroups III, XXIX, and unknown strains isolated from the soil but not the seed tubers prior to planting were isolated from stems and progeny tubers. Serogroup XXIX was isolated from soil, seed, stems and progeny tubers in 1981. Throughout both years, Ecc was isolated more frequently than Eca from seed, soil, stems, and progeny tubers. These data emphasize the potential for reinfestation of seed stocks and infection of plants by populations of Ecc that overwinter in soil under Wisconsin conditions.     

Foliar Symptoms Caused by Potato mop-top virus on Potato Plants During Vegetative Propagation in Scotland and Their Association With Tuber Yield, Spraing and Tuber Infection

The effect of foliar symptomatic infection by Potato mop-top virus (PMTV) on yield of tubers, spraing and infection in daughter tubers, and foliar symptom development and tuber infection in the following generations of propagation was investigated in commercial seed potato crops in Scotland. Six crops covering cvs Atlantic, Hermes, Nicola and Cara were studied between 2000 and 2006 by labelling paired replicates of plants with foliar symptoms and plants with no symptoms. Tubers from plants with no symptoms rarely produced plants with foliar symptoms in the following generation. Plants with no symptoms produced more infected tubers if they had been derived from plants with foliar symptoms the previous year than from plants with no symptoms. The proportion of daughter plants with foliar symptoms produced by tubers from plants with foliar symptoms in year 1 ranged from 19–41% and seemed to be associated with the severity of foliar symptoms. The detection of PMTV by ELISA in samples of leaves from plants with foliar symptoms ranged from 13% for cv. Cara to 59% for a crop of cv. Atlantic in 2004. The amounts of spraing were generally low but tended to be greater for tubers from plants with foliar symptoms than those from plants with no symptoms. These results indicate that roguing plants with foliar symptoms in seed potato crops could achieve an improvement in crop health but might be impractical when diseased plants are too prevalent.     

Evidence for persistent, symptomless infection of some potato cultivars with tobacco rattle virus

Summary Infection with an M-type (particle producing) isolate of tobacco rattle virus (TRV) was detected in leaves and/or roots of some plants from 11 of 13 potato cultivars grown in soil containing viruliferous trichodorid nematodes. Virus was detected in tubers of 8 of 13 cultivars, although only two (Pentland Dell and Maris Bard) developed spraing symptoms. Six cultivars (Arran Consul, Home Guard, King Edward, Romano, Santé and Wilja) were infected with TRV without developing spraing symptoms. Plants grown from virus-containing, symptomless tubers became systemically infected with M-type TRV and produced symptomless infected daughter tubers. Virus was maintained through three generations of vegetative propagation, and the plants were sources for acquisition of the virus by trichodorid nematodes. Distribution of virus in both spraing-affected and symptomless tubers was erratic. Movement of symptomlessly infected seed tubers may be a means of dissemination of the virus and of its introduction to previously unaffected sites.     

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

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

The effect of seed-and soilborne inoculum ofColletotrichum coccodes on the incidence of black dot on potatoes

Summary Soil in a potato field naturally infested with black dot (Colletotrichum coccodes) was fumigated with methyl bromide at 126 g m⁻² or left unfumigated. Potato seed tubers (cv. BP1) uninfected, lightly infected (1–25% surface affected), severely infected (26–100% surface affected) and severely infected withC. coccodes but dusted with prochloraz manganese chloride as Octave 2.5% DP at 750 g per 100 kg seed were planted in fumigated and unfumigated soil. When harvested, the incidence of black dot on the progeny of infected seed planted in unfumigated soil was twice that of progeny in fumigated soil, with progeny of uninfected seed having a 68.5 times higher disease incidence in unfumigated soil. Black dot on progeny tubers was reduced by pre-treatment of seed with prochloraz in fumigated soil only. With black dot infested fields, planting disease-free seed or treating seed with fungicides would not decrease disease on progeny tubers.     

Evaluation of procedures for detection of pectolyticErwinia spp. on potato tubers

The effectiveness of a tuber incubation method for detection ofErwinia carotovora var.atroseptica andE. carotovora var.carotovora in potato tubers was compared with a lenticel sampling procedure. In the first method, tubers were injured by puncturing lenticels with sterile toothpicks, then wrapped in moist paper towels and polyvinylidene film, and placed in closed chambers flushed with N₂. In later experiments, wrapping tubers in two layers of polyvinylidene film and incubation in air was found to be as effective as the single layer of polyvinylidene and incubation in chambers flushed with N₂. Isolations were made on a selective crystal violet pectate (CVP) medium from homogenized samples of tissue removed from soft rot lesions developing around injured lenticels. In the second method, 10 lenticels/tuber were aseptically removed with a scalpel and homogenized in distilled water; the suspension was plated on CVP. The first method was less tedious and slightly more effective than the lenticel sampling method. In a preliminary survey, these methods were used to detectErwinia infestations in small samples of certified seed potato tubers from Maine, Minnesota, Montana, New York, North Dakota, and Wisconsin. PectolyticErwinia spp. were detected in at least one sample from each state except Montana. The percentage of tubers withErwinia infestations varied from 0–100% among samples. Characterization ofErwinia isolates showed that bothE. carotovora var.carotovora andE. carotovora var.atroseptica were present. PectolyticErwinia spp. on symptomless potato seed tubers may serve as inoculum sources for blackleg and soft rot diseases.     

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

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

Recovery ofVerticillium dahliae from North American certified seed potatoes and characterization of strains by vegetative compatibility and aggressiveness

Samples of tubers collected from commercial potato seed lots produced across North America were assayed forVerticillium dahliae. V. dahliae was successfully isolated from 65 of 224 seed lots tested, a successful isolation rate of nearly 30%. Vegetative compatibility of the isolates was assessed through complementation tests using nitrate non-utilizing mutants. AllV. dahliae isolates belonged to vegetative compatibility group (VCG) 4. Of the 162 isolates recovered, 64% belonged to VCG 4A, 33% to VCG 4B, and 3% to VCG 4AB. All 39 of the isolates tested in the greenhouse on potato cv. Superior were pathogenic to potato. Disease symptoms developed earlier, were more severe, and plants died earlier when inoculated with VCG 4A compared to 4B isolates. As a group, AUSPC values were significantly higher (p=0.05) for VCG 4A than for 4B isolates. These data suggest that (1) commercial certified seed tubers from diverse locations are commonly infected withV. dahliae and thus may serve as primary sources of the pathogen; (2) potato isolates ofV. dahliae in North America belong to VCG 4A and 4B and these strains are widely distributed via seed tubers; and (3) VCG 4A and 4B are distinct pathotypes ofV. dahliae that vary in their aggressiveness to potato.     

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.     

Field performance of potato microtubers as propagation material

Summary In five field trials with the cultivars Gloria (early), Bintje (mid-early) Désirée (mid-late) and Morene (late), crops grown from conventional 28–35 or 35–45 mm seed were compared with crops grown from microtubers, of various sizes and from various sources, that had been producedin vitro. The following means to quicken initial crop development from the micropropagated material were tested: large microtubers, plastic soil cover, and transplanting of plantlets grown from microtubers pre-planted in a glasshouse. Crops grown from microtubers weighing less than half a gram yielded much less than crops grown from conventional seed crops but their yields were increased by each treatment. With the later-maturing cultivars, which generally produce few tubers per plant, the yields within seed grades from plants grown from transplanted microtuber plants were comparable with those of conventional crops. Reasons are given, why direct planting of microtubers, with or without plastic foil, is not a practical option.