The influence of potato harvesting and grading machinery on contact spread of Phoma exigua var. foveata on tubers

The incidence of wounds infected by Phoma exigua var. foveata was increased if freshly damaged tubers (recipients) were shaken in a bag with diseased tubers (donors) to simulate the tuber-to-tuber contact that occurs during potato handling. An increase in the number of gangrene rots on damage points also occurred if the recipient tubers were wounded after contact with the diseased tubers, rather than before, and when the donor tubers were heavily infested with P. exigua var. foveata but were free of gangrene lesions. Increasing the proportion of donor to recipient tubers increased the percentage of infected wounds on recipients. Increased incidences of infection in recipient tubers also occurred after they had been passed over an elevator digger when it was lifting stocks of tubers heavily infested with P. exigua var. foveata. When spores of an E +ve isolate of P. exigua var. foveata were sprayed onto the webs of manned potato harvesters, tubers harvested immediately after developed gangrene rots from many of which the E +ve isolate was cultured. An E +ve isolate was also used to demonstrate the transfer of P. exigua var. foveata inoculum from tubers onto soil on riddles of a potato grader and from these soil-coated surfaces onto other tubers during grading.     

The effect on gangrene incidence of dusting seed potatoes with benomyl at time of planting

In 1 out of 2 years' field trials benomyl applied as a dust treatment at time of planting seed potatoes resulted in an increase in potato gangrene in the progeny of both gangrene-free tubers planted in land contaminated with Phoma exigua var. foveata and gangrene-diseased tubers planted in clean land. Gangrene-infected seed tubers treated with benomyl also produced more stems infected with var. foveata than untreated tubers. Two hypotheses are presented to account for this increase in gangrene which does not occur in seed potatoes treated shortly after lifting. In all cultivars tested an organo-mercury dip-treatment increased total numbers of tubers in the seed and chat-size grades without increasing total weight whereas benomyl dust increased the numbers in these grades in Majestic only. The treatment of gangrene-diseased seed with benomyl dust affected neither total weight nor total number of tubers.     

The role of seed-tuber and stem inoculum in the development of gangrene in potatoes

To determine the relative importance of seed tubers and stems as sources of inoculum for potato gangrene in progeny tubers, different levels of inoculum of Phoma exigua var. foveata were established in field experiments by planting rotting or contaminated seed tubers and by inoculating stems shortly before haulm desiccation. The pathogen was only occasionally detected by isolation from inside green stems in June and July on plants growing from contaminated but unrotted seed. The incidence of pycnidia on desiccated stems increased with increasing concentration of inoculum contaminating seed tubers and with increasing time interval between desiccation and harvest. Stem infection was probably derived from inoculum on seed tubers spreading via the soil to the stem bases. Stem inoculation of plants growing from uninoculated or uncontaminated seed greatly increased both the gangrene potential of progeny tubers (defined as % wounds with gangrene after uniformly damaging tubers and storing them at 5°C for 12 wk), and tuber contamination (defined as % wounds with gangrene after spreading tuber-borne soil onto test tuber slices and storing them at 5°C for 8 wk). However, when stems of plants growing from rotting or contaminated seed were cut at ground level and removed before desiccation, gangrene potential of progeny tubers was only slightly less than that of tubers from untreated plots. There was no evidence that soil inoculum or aerial spread played a significant role in disease development. Gangrene potential and contamination of progeny tubers were related to Contamination levels on seed tubers. Some transmission also occurred from rotted seed tubers to progeny. Inoculum levels around progeny tubers increased rapidly after desiccation even in plots where stems had previously been cut at soil level and removed to eliminate pycnidial development above ground as a source of inoculum. Both gangrene potential and contamination of progeny tubers were greater on cv. Ulster Sceptre than on cv. Pentland Crown. The results showed that the inoculum on seed tubers, whether from rots or surface contamination, contributed more to the contamination of progeny tubers at harvest than did the inoculum from pycnidia on stems following desiccation of the haulm.     

The significance of tuber damage and inoculum concentration of Phoma exigua var. foveata in the development of gangrene in stored potato tubers

The susceptibility to gangrene infection of wounds of various shapes and depths on potato tubers was studied by inflicting wounds using differently-shaped brass teeth and rods of different diameters. Inoculating wounds with spore suspensions or damaging tubers which had been previously contaminated with Phoma exigua var. foveata or which had been recently lifted from plots of field experiments showed that wounds in which tissue was crushed were most susceptible to infection. Over a wide range of inoculum concentrations and in experiments using several different cultivars the incidence of infection of any wound type was compared to that of the standard severe cut and crush wound. Using a probit transformation a linear relationship was established, the slope of the line indicating the relative susceptibility of the wound. In 1977 and 1978, crops of cv. Pentland Dell were surveyed for damage incidence, inoculum and inoculum potential on arrival at a commercial bulk store. Nets of tubers buried among the tuber bulk were recovered after storage and gangrene incidence compared with damage and inoculum assessments. Inoculum potential and incidence of severe damage both influenced disease development but damage incidence was of greater importance, showing that priority should be given to decreasing damage and to curing to promote rapid wound healing in endeavours to control the disease.     

The spread of Phoma exigua within the potato crop

The incidence of gangrene that developed in tuber samples harvested in a systematic way from potato crops planted with surface sterilized gangrene-free seed surrounding a small plot planted with gangrene-affected seed showed that in two out of the three years marked outward spread of Phoma exigua var.foveata occurred in an uneven manner to about 4 ft (1–2 m) from the central plot. The occurrence of var. foveata was restricted to the immediate area in and around the gangrene infection locus whilst P. exigua var. exigua was more widely and evenly distributed throughout the experimental area. The incidence of infection in and around the central plot varied from year to year and it is suggested that the main method of spread of var. foveata inoculum is by rain splash. Infection by both fungi appeared to be influenced in a similar manner in all three seasons.     

The infection of potatoes by Phoma exigua

The pathogenicities of Phoma exigua var. foveata and P. exigua var. exigua were compared by determining the relationship between incidence of infection of whole tubers or tuber pieces and various inoculum levels in soil. In natural soil var. foveata was more pathogenic than var. exigua, but in sterilized soil there were no clear differences between the two. Both varieties were isolated from some lesions derived from inoculum to which only one variety had been added indicating either natural contamination of soil or latent infection in tubers; the proportion of such lesions increased at the lower inoculum concentrations. In studies where potato plants were grown in infested soils, the incidence of stem disease and tuber disease increased at increased inoculum concentrations of var. foveata; var. exigua was not added to the inoculum but was isolated from some lesions, the proportions of such lesions again increasing at lower inoculum concentrations. The infectivity of the infested soil, assessed by a tuber-baiting method, declined soon after planting but increased prior to harvest. Laboratory studies indicated that in natural soil the infectivity was highest after 2–3 months and lowest after 10 months incubation, but in sterilized soil infectivity was highest after 5 months and lowest after 7 months incubation.     

Development and Validation of Conventional and Quantitative Polymerase Chain Reaction Assays for the Detection of Storage Rot Potato Pathogens, Phytophthora erythroseptica, Pythium ultimum and Phoma foveata

The diseases pink rot, watery wound rot and gangrene are important storage rot diseases of potato associated predominantly with Phytophthora erythroseptica (P.), Pythium ultimum (Py.) and Phoma exigua (Phoma) var. foveata respectively. Reliable molecular‐based diagnostic tests are required that will not only allow unequivocal identification of symptoms but will further advance epidemiological studies of these potato diseases to increase our understanding and contribute to more effective management and control strategies to the potato industry. Primers and probes were designed in specific regions of the internal transcribed spacer (ITS) regions to develop conventional and real‐time quantitative polymerase chain reaction (PCR) assays able to detect all possible fungal and oomycete pathogens causing pink rot, watery wound rot and gangrene. The specificity of each diagnostic assay was rigorously tested with over 500 fungal/oomycete plant pathogen isolates from potato and reference culture collections, and both conventional and real‐time PCR methods produced similar results. In terms of sensitivity, the detection limits for real‐time PCR went below ag DNA levels compared with pg DNA levels with conventional PCR. The real‐time PCR assays developed to detect Phoma foveata and Py. ultimum on tubers were suitable for the comparative C_t method (ΔΔCt) of quantification using the cytochrome oxidase gene of potato as a normalizer assay; an advantage as the need for a standard curve is eliminated. Each assay detected Phoma species (var. foveata or exigua) from naturally infected tubers showing symptoms of gangrene, and P. erythroseptica or Py. ultimum were also detected following inoculation of Russet Burbank tubers. Each diagnostic assay developed could reliably detect and distinguish between the pink rot, watery wound rot and gangrene‐causing potato pathogens.     

Aerial dispersal of the potato gangrene pathogen, Phoma exigua var. foveata

A semi-selective medium for isolating the potato gangrene pathogen, Phoma exigua var. foveata, from air is described. Airborne propagules of P. exigua var. foveata were generated for at least 57 min after the start of simulated raindrop impaction on senescent potato stems bearing pycnidia from a natural infection. The number of propagules generated as aerosols was 0·17% of that available for aerosolisation. The aerodynamic particle size of at least 92% of the aerosols was in the range 2·1–4·7 μm in diameter. Using a Casella High Volume Bacterial Sampler airborne propagules of P. exigua var. foveata were detected in the open air during rainfall from September to November at three sites in Southern Scotland. They were recovered at least 800 m down wind from the nearest potato crop. P. exigua var. exigua and P. eupyrena were also recovered from the air during rainfall. Very low numbers of P. exigua var. foveata were occasionally detected in the absence of rain. The role of airborne inoculum of P. exigua var. foveata is discussed in relation to reinfection of virus-tested stocks of seed potatoes derived from stem cuttings.     

The effects of various chemical and physical haulm treatments on the incidence of potato gangrene

In studies of the influence of haulm defoliants on gangrene incidence in storage the use of diquat dibromide was consistently found to exacerbate levels of Phoma exigua vzr.foveata infection particularly on tubers harvested 3–4 wk after burning down. Intermediate levels occurred where the haulm was chemically defoliated with dinoseb or was physically cut and removed and least where the defoliants were sodium chlorate or sulphuric acid. Pycnidia of P. exigua var. foveata developed within 10 days on stems desiccated with diquat dibromide, sulphuric acid or dinoseb and most prolifically on those treated with diquat dibromide but tuber infection was not always related to their abundance. Speed of kill was not considered to be important in determining effect on gangrene incidence.     

The effect of harvest date and duration of wound healing conditions on the susceptibility of damaged potato tubers to infection by Phoma exigua (gangrene)

In experiments extending over 3 yr, King Edward tubers harvested on various dates from early September until early November were uniformly wounded and kept at 15 ^oC and c. 95% r.h. for periods of up to 21 days for wounds to cure. When tubers were subsequently inoculated with Phoma exigua var.foveata and incubated at 5 ^oC for 12 wk, curing for 3 and 7 days decreased gangrene incidence to respectively c. 13 and 4% of the non-cured controls. When tubers were inoculated before curing and immediately after wounding, curing for 7 days decreased gangrene incidence to only c. 68% of the non-cured controls. Curing was also progressively less effective the later the date of harvesting, suggesting that there would be advantages in harvesting potato crops early in the autumn.     

Factors affecting the occurrence of gangrene (Phoma exigua) in potatoes

The presence in soil from Scotland and England of Phoma exigua f. sp. exigua and P. exigua f.sp. foveata, which cause gangrene, is confirmed by isolation, and it is established that infection of tubers occurs before lifting, and after lifting from soil adhering to tubers. The distribution of the disease is related to soil moisture, gangrene being most prevalent in tubers from the north-eastern counties of Scotland where the moisture content of arable soils remains high throughout the growing season. The incidence of the disease may be affected by the haulm, either as a source of infection—though not an important source—or by its effect on the maturity of the tubers. The incidence of gangrene is less where haulm destruction is rapid. The incidence of gangrene in a crop is not related to its incidence in the seed tubers planted and, unless infection is severe, the effect on yield is not serious. Symptoms of skin necrosis were associated with tubers from acid soils, infected with P. exigua f.sp. foveata and stored at low temperature.     

土壤灭菌-生物有机肥联用对连作马铃薯及土壤真菌群落结构的影响

甘肃省中部沿黄灌区是全国重要的加工型马铃薯生产基地,然而因集约化生产带来的连作障碍问题已经严重影响到当地马铃薯种植业的健康发展。结合田间试验和相关的室内分析,从马铃薯块茎产量和品质、植株生理特征和土壤真菌群落结构等角度,初步评估土壤灭菌和生物有机肥联用(Ammonia Disinfection plus Bio-organic Fertilizer Regulation,ABR)对马铃薯连作障碍的防控效果。同对照相比,ABR处理的块茎产量和商品薯率分别显著增加约71.1%—152.1%和39.2%—53.3%,但块茎化学品质变化不大。ABR处理叶绿素含量和根系活力较CK均显著增加,而叶片和根系丙二醛含量显著下降。PCR-DGGE分析发现,ABR处理显著影响了马铃薯连作土壤的真菌群落结构,表现为真菌群落的多样性指数较CK相比显著下降。ABR处理还有效抑制了土传病害的滋生,植株发病率和收获后的病薯率较CK分别显著下降约67.2%—82.2%和69.1%—70.5%。采用Real-time PCR评估连作土壤中3种优势致病真菌的数量变化,显示ABR处理下立枯丝核菌、茄病镰刀菌和接骨木镰刀菌的数量在生育期内较CK均有不同程度的下降。综合来看,土壤灭菌和生物有机肥联用技术在防控甘肃省中部沿黄灌区马铃薯连作障碍上具有较大的应用潜力,而对土传病害的抑制和微生物群落结构的改善是其主要的作用机理。     

Pathogenesis of Potato Gangrene Caused by Phoma exigua var. foveata. I. Location of Phosphatase Activity in Diseased Tuber Tissue

The nitrocellulose blotting method was adapted for locating phosphatase activities in gangrene‐diseased tuber tissue. Adenine and glucose phosphates were used as enzyme substrates. A strong dephosphorylation of high‐energy phosphates took place in the UV‐fluorescent tissue adjacent to dry rots caused by Phoma exigua var. foveata, but the enzyme remained at a low activity in the other areas of the diseased tuber. Phosphatases were probably induced by P. exigua var. foveata, and intense dephosphorylation of high‐energy compounds leads to the death of potato tuber tissue.     

Persistence of Phoma exigua xar. foveata and Polyscytalum pustulans in dry soils from potato stores in relation to reinfection of stocks derived from stem cuttings

The incidence of Phoma exigua var. foveata and Polyscytalum pustulans in dry soil and dust from potato stores was assessed at 10 farms in Scotland producing virus-tested stocks of seed potatoes derived from stem cuttings (VTSC). Samples were collected on three occasions during 1976: during the storage period (February-March), soon after the potatoes were planted (May) and just before the new crop was lifted and stored (August-October). Both pathogens were detected at all three sampling times but P. exigua var. foveata was more frequently detected at the last one than P. pustulans. Soil and dust from various sites within the stores, such as floors, ledges, graders and boxes, were contaminated by these pathogens. Propagules of both pathogens remained airborne for at least 12–17 min after the floors were swept. P. exigua var. foveata remained viable for at least 16 months in dry soil from tuber surfaces stored at 4–6°C. When VTSC tubers were dusted before planting with a dry store soil contaminated by P. exigua var. foveata the gangrene potential of the progeny tubers was much greater than that from untreated tubers or from tubers treated with an uncontaminated soil. The role of inoculum surviving in dry soil is discussed in relation to reinfection of VTSC stocks.     

Effects of rotation length, fungicide treatment of seed tubers and nematicide on diseases and the quality of potato tubers

In 1982 – 88, potatoes were grown in 2-, 4- and 6-course rotations with spring barley on a field infested with Globodera rostochiensis. Severity of stem canker and black scurf increased with increasing frequency of previous potato crops, and seed tuber treatment with tolclofos-methyl became less effective in controlling diseases. This suggested that previous crops had increased the amounts of soil-borne inoculum of Rhizoctonia solani. Oxamyl soil treatment increased stem canker in one year and decreased black scurf in four years. Seed tuber treatment with imazalil or prochloraz decreased stem base infection by Polyscytalum pustulans and skin spot and silver scurf on tubers. Black dot was prevalent on tubers in all years and was not affected by seed tuber treatment or previous cropping. Oxamyl increased black dot and common scab in five years and decreased % tuber dry matter in six years. Cysts of G. rostochiensis were found attached to Désirée but not to Maris Piper tubers in August. At harvest tubers of both cultivars were affected by superficial pitting and its severity was related to soil populations of G. rostochiensis at planting. This damage was controlled by oxamyl. It is suggested that the pitting developed from holes made in the tuber skin at larval invasion. In 1989, Désirée seed tubers and healthy mini tubers were planted in all plots and severity of stem canker and black scurf increased with increasing proximity of previous potato crops and with the number of previous crops. Black dot on stems and tubers was not affected by previous cropping but was much less severe in a plot that had not grown potatoes during the seven years of the experiment. The severity of common scab generally decreased as the number of preceding potato crops increased.     

Effects of delaying fungicide treatment on the incidence of gangrene in stored potato tubers

Potato titbers infested with Phoma exigua var. foveata were uniformly wounded and sprayed or dipped in fungicide suspensions either immediately or after periods of up to 21 days' storage at 5, 10, 15 or 20 °C. Tubers were then stored at 5 °C and gangrene assessed after 12 wk. Incidence of gangrene on untreated tubers was progressively decreased by increasing the length of storage at 15 or 20° (curing) but was not affected by 3 days' storage at any temperature. Fungicide treatment immediately after wounding gave best control of the disease; treatment after 3 days' delay was less effective and after 14 or 21 days was usually ineffective. Gangrene was decreased by fungicides more on tubers stored for 3 or 7 days at 5 °C than at higher temperatures. Control of gangrene by curing or fungicides diminished when the amount of inoculum on tubers was increased. Increasing the amount of fungicide applied improved control and fungicides were more effective in decreasing gangrene on cut and crush wounds than on cut wounds. At the arbitrary concentrations used in these experiments imazalil gave better disease control than thiabendazole, prochloraz, carbendazim plus quinolin 8-ol or triadimefon.     

Immunoenzymatic techniques for the detection of Phoma exigua in infected potato tissues

Antisera to Phoma exigua var. foveata and var. exigua were prepared by injecting rabbits and mice with protein solutions from mycelium. Specific antibodies were isolated and immunoenzymatic techniques (double antibody sandwich ELISA and indirect ELISA) were used to test for the fungus in inoculated tubers and sprouts and in stems grown from these tubers. The fungus was detected in these different tissues, with var. foveata being more aggressive, demonstrating the applicability and sensitivity of the techniques. The antibodies produced to the two varieties of the fungus were not specific to their own varieties. They also reacted with Phoma tracheiphila but did not react with several other common potato pathogens. Preliminary results obtained with antibodies from mouse ascite liquid suggest the possibility of producing specific monoclonal antibodies.     

Infection of potato tubers with soft rot bacteria

Stolons attached to developing potato tubers were inoculated with the soft rot bacterium Erwinia carotovora var. atroseptica. Almost all the stolons rotted, but soft rots developed in less than 10% of new tubers; the bacterium was isolated later from these tubers. No rots developed in the other tubers but the bacterium was later isolated from about half of them. It could not be isolated from tubers attached to inoculated stolons where the rot on them did not extend to the tuber or from tubers attached to stolons that were not inoculated though many of these rotted. The bacterium was reisolated from almost all arrested lesions in tubers inoculated 8 month earlier with E. carotovora var. atroseptica. Blackleg did not develop from plants grown fom these tubers under various soil conditions. It did develop in a large proportion of plants from tubers inoculated shortly before planting and grown in cool, wet soil. Less than 1% blackleg developed in plants grown from tubers from plants with blackleg or from plants immediately adjacent. The presence of pectolytic bacteria and E. caratovora var. atroseptica in seed and new tubers was investigated during June, July and August. Although E. caratovora var. atroseptica was obtained from c. 40% tubers, only c. 0·3% of c. 8400 plants developed blackleg. The bacterium was isolated from only three of 160 new tubers sampled during the summer.     

Survival of the causal agents of 'early-dying disease' (Verticillium wilt) of potatoes

Although the isolation of Verticillium albo-atrum and V. dahliae from soil and dried moribund stems following infection of a potato crop proved extremely difficult, both fungi were equally capable of overwintering in these substrates and of inducing disease in a subsequently planted susceptible crop. In the absence of a susceptible crop some weed species became colonized. The two species, however, appeared to differ in their capacity for survival both beneath a monocotyledonous crop and within the potato tubers. Colonization of the roots of wheat, barley, oats, rye and maize was observed with V. dahliae but not with V. albo-atrum. The latter appeared to be capable of prolonged survival in the tubers, whereas V. dahliae did not remain viable in storage over winter. Consequently only tubers infected with V. albo-atrum produced infected plants. The presence of the fungi within the tubers affected neither dormancy nor the initial development of the sprouts. Some correlation was noted between tuber size, the percentage of tubers infected, the distribution of V. albo-atrum within the tubers and the development of disease in plants subsequently grown from these tubers.     

Control of potato gangrene by thiabendazole in relation to time of harvest after haulm destruction and site of production

The efficiency of thiabendazole in controlling gangrene on potato tubers harvested on different dates was assessed over two years in Scotland (East Craigs) and England (Rothamsted). At East Craigs the incidence of gangrene on wounded and cold-stored tubers was greater than on those tubers kept at ambient store temperatures and graded in January, but the assessments were correlated. The incidence of gangrene increased with delay in harvest after haulm destruction although the pattern differed with year and site. Dipping tubers in thiabendazole or sodium hypochlorite almost eliminated gangrene on all harvest dates at Rothamsted but at East Craigs the incidence of gangrene on dipped tubers was much greater and tended to increase the longer the interval between haulm destruction and harvest. Furthermore, dipping tubers in thiabendazole was much more effective than spraying even though the fungicide residues were twice as large with the latter treatment. All 43 isolates of Phoma foveata collected in Scotland were sensitive to thiabendazole in agar plate tests. The results are discussed in relation to disease development at each of the sites.