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.     

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.     

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.     

Fungi Causing Dry Tuber Rots of Seed Potatoes in Storage in Scotland

The occurrence, pathogenicity (1997 isolates only) and fungicide sensitivity of pathogens causing dry tuber rots in Scottish seed potatoes was investigated over three storage seasons between 1997 and 2000 in relation to region of production, cultivar, class and generation of seed potatoes, type of temperature control in store and earthiness of tubers. A total of 156 samples, each comprising up to ten rotted tubers, was received over the three seasons. The relative importance of each pathogen in causing rots was summarised by calculating a mean rot index that combined the prevalence of a pathogen in the samples with the incidence of tubers affected by the pathogen within those samples. Phoma foveata (gangrene) had the highest rot index, which was five times greater than for P. exigua and ten times greater than for P. eupyrena. This relationship was mirrored by the relative pathogenicities of P. foveata and P. eupyrena, as measured by size of rots developing at inoculated wounds in test tubers. Fusarium avenaceum appeared to be the greatest cause of Fusarium dry rots, having a rot index at least twice as great as that for F. solani var. coeruleum. Infection by F. sulphureum was relatively uncommon. In the pathogenicity test, F. avenaceum, F. solani var. coeruleum and F. sulphureum produced rots of similar depths and widths and larger than those of F. culmorum. The mean rot index for Cylindrocarpon spp. was slightly more than that for F. avenaceum, whereas C. destructans produced smaller rots in the tuber pathogenicity test. Region of production affected the prevalence of P. foveata and F. avenaceum, but only the occurrence of P. foveata was affected by class and generation of seed potatoes. Isolate sensitivity to thiabendazole and imazalil was examined in vitro over 2 years. Six out of seven isolates of F. sulphureum were resistant to thiabendazole, with the remaining isolate being partially resistant. The growth of two out of 34 isolates of F. avenaceum was inhibited by more than 50% only at 100 mg thiabendazole l⁻¹. All isolates, except those of F. avenaceum,, were inhibited in their growth by more than 50% at either 1 or 10 mg imazalil l⁻¹. However, the growth of 40% of isolates of F. avenaceum was inhibited only at 100 mg l⁻¹. The various changes in pathogen prevalence and isolate sensitivity to fungicides recorded in this study highlight the need for regular monitoring programmes to be conducted in order that disease-control strategies can remain effective.     

Effect of haulm treatment and harvest time on incidence of tuber rots of potato (Solanum tuberosum L.) after standard wounding and on frequency of stem lesions caused byPhoma foveata Foister

Summary Various haulm treatments and harvest times differentially influenced the incidence of storage rots in potato tubers (Solanum tuberosum L.) in field experiments in Sweden performed over 4 years. The incidence of gangrene caused byPhoma foveata Foister (Phoma exigua var.foveata (Foister) Boerema) increased in cv. Bellona after spraying haulms with diquat as did the amount of infection caused byFusarium avenaceum (Corda ex. Fr.) Sacc. in all three cultivars (Bintje, Bellona, and Magnum Bonum). The lowestP. foveata infection levels occurred after pulling haulms. Late harvesting increased the frequency ofP. foveata in cvs Bintje and Bellona both at 2 and 4 wks interval between haulm treatment and harvest. Tuber and visible stem infection caused byP. foveata were correlated for cvs Bintje and Bellona, but not for cv. Magnum Bonum. The possible relation between tuber and stem infection is discussed.     

Characterization of forty-four potato varieties for resistance to common scab,black scurf and silver scurf

Common scab, black scurf and silver scurf belong to serious diseases of cultivated potato that especially impair tuber quality. Tuber infection in 44 potato (Solanum tuberosum ssp. tuberosum L.) varieties was evaluated using two measures – severity and incidence of infected tubers. Both measures were highly correlated. An important result derived from this study supports fungicide application against potato late blight secondarily acts on reducing tuber infection by all evaluated pathogens. Therefore, the conventional potato growing seems to be more effective compared to ecological growing. The pedigree analysis of 44 varieties indicated the presence of Solanum demissum genotypes in the variety genome may negatively affect variety response to Helminthosporium solani presence.     

Variation in pathogenicity among isolates ofPhoma exigua var.foveata on potato cultivars differing in resistance

Summary The size of gangrene lesions on potato tubers was significantly affected by isolate ofPhoma exigua var.foveata and cultivar. Inoculation methods based on the introduction of a wheat-germ/sand culture into the peripheral tissues of the tuber revealed greater differences among isolates and cultivars than one involving a deeper insertion of a mycelium/potato dextrose agar plug. The method of testing influenced the order of ranking of certain isolates and cultivars. The relative pathogenicity of isolates was similar over two years, but no cultivar-isolate interactions were found.     

Possible geographical origin ofPhoma exigua var.foveata,the principal causal organism of potato gangrene

Summary The highlands of South America are the probable geographic origin for the potato gangrene fungusPhoma exigua var.foveata (Foister) Boerema which was found parasitizing plants ofChenopodium quinoa in the Altiplano area of Peru. In vitro studies showed no dissimilarities between it and the antibiotic ‘E’ producing virulent European isolates ofP. exigua var.foveata. It is as pathogenic to potatoes as the virulent European isolates but onC. quinoa andC. album it is more pathogenic than the European isolates. So far the fungus has not been found on potato tubers in South America and the reasons why gangrene problems have not arisen there are discussed.     

Testing the resistance to silver scurf in commercial potato cultivars under controlled environmental conditions

Summary Optimum storage conditions to identify resistance to silver scurf among commercially grown potato cultivars were determined in a series of experiments. Inoculation of field-grown tubers with a conidial suspension ofH. solani (concentration 10⁴ conidia per ml of water) and incubation at 15°C with 95% RH for 1 month followed by 2 months at 85% RH produced the greatest differences in disease severity between potato cultivars. Results with glasshouse-grown tubers were comparable with those from field-grown tubers, indicating that the test could be used to identify resistance in wild species of potato and to understand the inheritance of this resistance.     

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.     

Influence of essential oils onPhytophthora infestans

Summary The effect of up to 19 essential oils (EOs) isolated from different types of plant species against potato late blight,Phytophthora infestans has been studied in vitro and under greenhouse conditions. Two isolates ofP. infestans were in a first screening exposed to an inhibition test using the EOs in a standard plate assay measuring the diameter of the development fungal zone. Representative oils selected from this first screening were then applied to susceptible potato plants and both the fungicidal and phytotoxic effects were examined. While most of the EOs tested in the fungus plate assay showed no desirable inhibitory action against the pathogen, four of the EOs produced an inhibition zone of 80%. EOs from thyme had e.g. an 89% inhibitory effect on the fungus isolates, but did not display protection in the potato study, and in addition caused phytotoxic symptoms. On the other hand, treatment with hyssop in the plate assay demonstrated only up to 45% of inhibition, but hyssop promoted good fungus protection for the plants and stimulated the plant growth without any signs of phytotoxity.     

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.     

Comparison of the interactions ofErwinia carotovora ssp.atroseptica withPhytophthora infestans, Phoma foveata andFusarium coeruleum in rotting potato tubers

Summary An interaction betweenP. infestans andE. carotovora subsp.atroseptica (Eca) in rotting tubers was confirmed and the biochemical basis for its occurrence investigated. The pH of tissue infected withP. foveata became alkaline whereas tissue infected withP. infestans orF. coeruleum did not rise above neutrality, which could promote pathogenicity of Eca by maintaining pH closer to the optimum for polygalacturonase activity. Polygalacturonase, pectate lyase and galactanase were detected in cultures of all three fungi grown on media containing cell wall material or pectin from tubers. AsP. infestans produced more polygalacturonase than the other two fungal tuber rot pathogens the possibility was investigated that oligogalacturonide products of enzymatic degradation of pectin byP. infestans stimulates pathogenicity of Eca. However, while tubers soft-rotted after infiltration with supernatant from fungal cultures grown on tuber cell wall material, controls showed that rotting resulted from infiltration rather than the products contained in the infiltrated water.     

Cultivar Effects on the Interaction between Free-Living Plant-Parasitic Nematodes and the Fungal Pathogen <Emphasis Type="Italic">Rhizoctonia solani</Emphasis> in Potato

Crop damage is associated with infection by plant pathogens but can also arise through abiotic factors. However, the plant pathogens are involved in biotic interactions with other plant pathogens, and these interactions may differ depending of the cultivar of the crop. Here, the interaction between the fungus Rhizoctonia solani (AG3) and free-living plant-parasitic nematodes was investigated in a pot experiment with different potato cultivars. No synergistic interaction between R. solani and plant-parasitic nematodes was found, instead there was an effect of treatment with lower tuber yield when nematodes occurred alone. There were differences among the cultivars regarding incidence of black scurf, dry weight of stems and tubers, and there was interactive effects between treatment and cultivar regarding dry weight of stolons and roots. Therefore, results concerning incidence and damage of R. solani and/or plant-parasitic nematodes found for one cultivar may not be applicable to other cultivars.     

Causes of differences in growth pattern,yield and quality of potatoes (Solanum tuberosum L.) in short rotations on sandy soil as affected by crop rotation,cultivar and application of granular nematicides

Summary A crop rotation experiment was carried out on a light sandy soil in 1979–1986 to study the effects of the frequency of potato cropping on yield, quality and on the occurrence of soil-borne pathogens other than potato cyst nematodes. Tuber yield decreased markedly with increasing cropping frequency, but also depended on what crops were grown in rotation with potato. Growth during the early part of the season, as well as the length of the growing period, were reduced in short rotations. The fungiVerticillium dahliae andRhizoctonia solani, and root-knot nematodes (Meloidogyne spp.) were the most important yield reducing pathogens. The effects of rotation depended on the cultivar used. The percentage of mis-shapen tubers increased with increasing cropping frequency and after application of granular nematicides, but the incidence of common scab (Streptomyces scabies) was not affected.     

Field study of the relative susceptibility of eleven potato (Solanum tuberosum L.) varieties and the efficacy of two fungicides against Rhizoctonia solani attack

Rhizoctonia black scurf is an economically important disease of potatoes in Tunisia and around the world. It reduces the quality and yield of potatoes and has become an important impediment for export of potatoes, especially to Europe. The information on comparative susceptibility of potato cultivars will help the growers make informed decisions regarding the management of this disease. The eleven potato cultivars used in this study showed a range of susceptibility reactions to Rhizoctonia solani but none of these was completely resistant to the disease. The Spunta variety showed the least percentage of infection of progeny tubers by R. solani sclerotia at harvest, while the varieties Nicola, Santana, Labadia, Liseta and Tango showed a high percentage of infection of progeny tubers. Seed treatment using fungicides, presently registered in Tunisia, are not effective in controlling the disease to growers’ satisfaction. Field experiments conducted to evaluate the efficacy of two newly introduced fungicides pencycuron (Monceren^®) and azoxystrobin (Amistar^®) showed that they were effective for the control of the disease. Seed potatoes treatment and in-furrow application of azoxystrobin consistently provided superior protection in two years of experimentation.     

Assessing the resistance to gangrene of progenies of potato (Solanum tuberosum L.) from parents differing in susceptibility

Summary The resistance to gangrene (caused byPhoma exigua var.foveata) of progenies from two hybridisation schedules involving cultivars resistant, susceptible, or of intermediate reaction to gangrene was assessed. Two methods of sampling tubers from field-grown single-plant plots of 18 clones representing each progeny were used: one tuber was taken from each plant, and a bulk sample of 18 tubers was selected at random from the whole progeny. Gangrene resistance was assessed after incubating tubers which had been rolled in cornmeal-sand inoculum ofP. exigua var.foveata, and was expressed as a percentage of tubers infected. Results from the two methods of sampling correlated closely, and agreed well with tests in a subsequent year on 10-tuber samples of each individual clone. It is concluded that an 18-clone sample adequately represented each progeny, and that the mid-parent gangrene score satisfactorily predicted the resistance of the resulting progeny. There are indications that susceptibility tended to be dominant. The implication of these findings to breeding for resistance to gangrene is discussed.     

Effect of soil water tension and harvest time on levels of infestation byPhoma foveata Foister in soil and on tubers and stems of potato (Solanum tuberosum L.) in northern Sweden

Summary The incidence ofP. foveata in soil and tuber samples from field experiments often increased significantly as the harvest season progressed and temperatures fell. Mean levels of tuber and soil infestation were significantly higher in irrigated rows compared with those artificially sheltered from precipitation, both at normal and late harvest dates. Soil infestation was most frequently detected within 10 cm of the parent tuber. No pycnidia were visible on the haulm, and very few latently infected stem segments and leaves were detected using various methods. In three out of four years, the incidence ofP. foveata in tubers after wounding was reduced by washing them gently in tap water immediately after harvest.     

Naturally-occurring aromatic compounds inhibit potato tuber sprouting

Several naturally-occurring aromatic aldehydes and an aromatic alcohol inhibited potato tuber sprouting when applied as volatiles or directly as emulsions. Exposure to volatiles was for 1 or 10 days prior to placement into fresh air. Salicylaldehyde inhibited sprouting of tubers exposed for 1 or more days; benzaldehyde, cinnamaldehyde, cuminaldehyde and thymol suppressed sprouting of tubers exposed for 10 days. Direct application of cinnamaldehyde and benzaldehyde as 1% and 10% emulsions completely inhibited sprouting 14 days after treatment without apparent tuber damage. These compounds applied without dilution inhibited sprouting but damaged tubers.     

Induction of hydrolytic enzymes and antifungal in vitro activity inS. tuberosum subsp.andigena after infection withFusarium solani f. sp.eumartii

Summary Fusarium solani (Mart.) Sacc. f. sp.eumartii (Carp.) Snyder & Hansen is the causative agent of two important types of disease in potato (Solanum tuberosum) plants: wilt in stems and leaves and dry rot in tubers. The aim of this work was to study the molecular response of the potato to fungal attack. Micropropagated plantlets were tested, using a biological assay to detect antifungal proteins. An inhibitory activity of spore germination was detected in protein fractions from infected plantlets 7 days after inoculation with the pathogen. In addition, β-1,3-glucanase activity (glucanase III) was measured in protein fractions with antifungal activity. Induction of hydrolytic enzymes and antifungal activity in infected plantlets ofSolanum tuberosum subp.andigena suggests that it could be a suitable source of resistance genes.