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.     

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.     

A method for assessing the field susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica)

In experiments to develop a method for assessing the field susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica) seed tubers were stab-inoculated near the stolon (attachment end), with a suspension of the bacterium, or with water, before planting. Disease symptoms were recorded in three years (1980–1982) and plant growth and yield in 1982. Estima and Maris Bard were the most susceptible cultivars with many plants failing to emerge and most of those that did showing disease symptoms. Pentland Crown was the most resistant: few plants failed to emerge and few showed blackleg. Nevertheless compared with water-inoculated plants bacterial inoculation of the seed tubers of this cultivar caused loss of yield and differences in tuber size distribution. Cara, Wilja and King Edward showed intermediate reactions.     

Stem canker (Rhizoctonia solani) on five early and seven maincrop potato cultivars I. Infection of shoots, stolons and tubers

In 1983 and 1984, potato seed tubers of five early and seven maincrop cultivars were inoculated with cultures of Rhizoctonia soluni during planting to simulate severe seed infection. Shoot and stolon infection was assessed in June-August and black scurf on tubers recorded after harvest in October. Almost all shoots of all cultivars had stem canker in both years and disease on shoots, stolons and tubers was more severe in 1984 than in 1983. In 1983 similar amounts of disease developed on all early cultivars and between 11% (Ulster Sceptre) and 32% (Maris Peer) shoots were pruned off. Maris Peer had a stem canker score lower than other cultivars in 1984 but more than half the shoots were pruned off. Shoot pruning on Estima, Ulster Prince and Ulster Sceptre was more common on plants from sprouted than non-sprouted seed. Between 30 and 50% of stolons were pruned off. After harvest in 1985, black scurf was least prevalent on Arran Comet and Maris Peer tubers and in 1984 on Arran Comet and Estima tubers from non-sprouted seed. Of the maincrop cultivars, King Edward plants from sprouted seed had many shoots pruned off in both years. Shoot pruning was also prevalent on Maris Piper and Pentland Squire plants from non-sprouted seed. Record had fewest pruned shoots and stolons and the lowest stem canker score. The disease was more severe on Pentland Crown and Maris Piper plants from non-sprouted than sprouted seed. Black scurf was most common on Cara and King Edward tubers in 1983 and on King Edward and Record tubers in 1984. In both years few shoots but many tubers were infected on plants from non-inoculated seed and the significance of this is discussed.     

A comparison of methods of seed tuber inoculation for assessing the susceptibility of potato cultivars to blackleg (Erwinia carotovora subsp. atroseptica) in the field

Experiments were done with the aim of developing a reliable method for assessing the susceptibility of potato cultivars to blackleg caused by Erwinia carotovora subsp. atroseptica, in the field. Over four years seed tubers were either stab-inoculated at the heel-end prior to planting, vacuum infiltrated with bacteria just after harvest or vacuum infiltrated prior to sprouting and after ‘cutting’ or ‘pricking’. Stab inoculation produced fewer diseased plants but generally a greater range of symptoms than vacuum infiltration. Vacuum infiltrated ‘cut’ seed gave most disease whereas infiltration at harvest and infiltration of ‘pricked’ seed gave similar disease incidence. The cultivar Maris Piper was more resistant than Desiree in 1981 and 1982, and Pentland Javelin than Ulster Sceptre in 1983 and 1984, largely irrespective of the inoculation treatment. Irrigation improved yields but did not affect disease symptoms. In further field experiments over three years, each with a minimum of 20 cultivars, seed tubers were sliced just before planting at a standard distance from the apex and inoculated by applying a pad impregnated with inoculum. Differences were shown between cultivars and it is suggested that the method could be adapted for testing for blackleg susceptibility under controlled environment conditions.     

Stem canker (Rhizoctonia solani) on five early and seven maincrop potato cultivars. II Effects on growth and yield

In 1983 and 1984, potato seed tubers of five early and seven maincrop cultivars were inoculated with cultures of Rhizoctonia solani during planting in field experiments to simulate severe seed infection. The size of foliage was assessed during June-August and tuber yields recorded during growth and at harvest in October. Stem canker delayed shoot emergence, decreased the number and length of stems and caused increased variation in stem length; these effects were greatest with Maris Peer and Arran Comet (early cultivars) and King Edward and Pentland Squire (maincrop cultivars). Total weight of foliage was decreased, especially with earlies, dry matter of stems increased and the proportion of foliage on lateral stems increased. With the early cultivars, tuber yield from sprouted Maris Peer seed 11 wk after planting in 1983 was decreased by 24%, and 13 wk after planting in 1984 yields were decreased by 42% (Maris Peer), 40% (Ulster Sceptre), 34% (Estima), 30% (Arran Comet) and 17% (Ulster Prince) with sprouted seed and by 20, 29, 53, 39 and 28% respectively with non-sprouted seed. Decrease in total yield at harvest in October averaged 13% with sprouted seed and 10% with non-sprouted seed. In all cultivars the weight of small tubers was decreased and with Estima the weight of large tubers was increased. Tuber bulking was also delayed with all maincrop cultivars and at harvest yields from sprouted King Edward seed were decreased by 13% in 1983 and by 16% (sprouted seed) and 23% (non-sprouted seed) in 1984; yields of Pentland Squire were decreased by 5, 16 and 21% respectively. Yield losses with other cultivars ranged from 5–13% with sprouted seed and 0–16% with non-sprouted seed. The yields of small tubers were decreased with all cultivars and yields of large tubers were increased with Pentland Squire, Pentland Crown and Cara.     

Transmission of Fusarium solani var. coeruleum and F. sulphureum from seed potatoes to progeny tubers in the field

In experiments in three years, seed tubers were inoculated before planting with either Fusarium solani var. coeruleum or F. sulphureum to initiate a rot, or were contaminated by dipping in soil slurries containing spore suspensions of one or other of the pathogens. Transmission to progeny tubers was tested by uniformly wounding and incubating tubers and by dilution plating of soil samples. In two years, transmission of F. sulphureum was greater from highly contaminated than from rotting seed and was greater on cv. Pentland Crown than on cvs Desiree and Maris Piper. F. solani var. coeruleum appeared to be transmitted most readily from rotting seed and Maris Piper was the cultivar most extensively contaminated. In experiments with different harvest dates, transmission of both fungi from highly contaminated seed could be detected by late June or mid-July. More progeny tuber wounds rotted in F. sulphureum than in F. solani var. coeruleum plots and in one year, F. sulphureum caused more rots on cv. Record than on cv. Maris Piper. These differences between the pathogens may be related to their differing abilities to sporulate underground on the surface of seed tubers and on stem bases.     

Effects of dual infections of Puccinia hordei and Erysiphe graminis on barley, cv. Zephyr

Seed tubers of the varieties King Edward, Majestic and Pentland Crown selected as ‘clean’ (lesion-free), moderately, or severely affected by gangrene lesions were planted in field experiments. Infection delayed plant emergence, increased the number of stems/plant, sometimes caused gaps in crops and was associated with increased blackleg. On average severely affected seed yielded 20% less than ‘clean’ seed. Seed infection also increased the proportion of tubers in smaller size grades so that crops from severely infected King Edward seed averaged 1·4 ton/acre (3·5 t/ha) less small ware and 2·5 ton/acre (6·3 t/ha) less large ware than ‘clean’ seed. With Majestic, small ware was increased (0·7 ton/acre (1·8 t/ha)) and large ware decreased (4·4 ton/acre(11·0 t/ha)); Pentland Crown was similarly affected (small ware increased 0·8 ton/acre (2·0 t/ha); large ware decreased 3·9 ton/acre (9·8 t/ha)). In eight of twelve experiments unselected diseased stocks yielded significantly less than ‘clean’ tubers. Other experiments compared seed stocks with different proportions of gangrene-infected seed tubers. Yields decreased as the proportion of diseased seed tubers increased, but differences were significant only when more than 60% were affected. Surprisingly, yields from ‘clean’ tubers also decreased as the proportion of diseased tubers increased in the stocks from which they were selected. Gangrene on progeny tubers after storage was not always related to the amount of gangrene visible on the seed. It was increased by riddling or wounding and decreased by dipping tubers in organo-mercury fungicide before or soon after wounding.     

Fungus diseases on potato seed tubers planted in England and Wales, 1963-76

During 1963-76 samples of potato tubers from commercial seed stocks of cvs King Edward (14 yr), Pentland Crown (9 yr), Majestic (7 yr), Pentland Dell (3 yr), Record and Arran Pilot (2 yr) were received from farms in England and Wales. Fifty tubers from each sample were examined macroscopically for fungus diseases and eyes were excised from a 20-tuber sub-sample, incubated and examined for pathogenic fungi; 50 tubers were stored on trays to sprout and examined for diseases and sprouting in May and in most years samples of 50 tubers were wounded by dropping onto expanded metal, stored at 5° C and examined for gangrene and dry rot after 12 wk. Amounts of disease varied between years and during 14 yr black scurf and powdery scab on King Edward tended to increase and skin spot and late blight decrease. On average 44% of King Edward tubers were affected with skin spot, 25% with black scurf and 16% with powdery scab. Gangrene affected 5% of tubers and 97% of the isolates from rots were identified as Phoma exigua var. foveata. Wounding tubers increased the incidence of gangrene three-fold. During 1963-69 late blight affected 2% of King Edward tubers but fewer in later years and in other cultivars. Majestic had most common scab (44% tubers) and Arran Pilot most dry rot (9% tubers) and this disease was increased by wounding tubers. Conidiophores of Helminthosporium solani (silver scurf) were more common on excised eyes of Pentland Crown, Record and Arran Pilot than of other cultivars, and isolations from verticillate conidiophores that developed on the side of incubated eye plugs of King Edward and Majestic stocks gave pure cultures of Verticillium tricorpus (78%), V. nigrescens (9%) and V. nubilum (3%). Proportions of tubers with different diseases were affected by their country of origin; Scottish seed had most skin spot and gangrene, Irish seed most powdery scab and English seed most common scab, late blight and H. solani. There was also evidence of differing disease incidence in seed from different geographical areas in Scotland and England. Up to half the King Edward and Pentland Crown stocks examined in 1975 and 1976 were derived from stem cuttings and average amounts of diseased tubers were similar to those in stocks not derived from stem cuttings. Annual and cultivar differences in disease incidence and effects of date of receipt of seed on farms are discussed.     

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.     

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.     

Sampling potatoes for the incidence of tuber diseases and levels of inoculum

In three years, between three and six seed stocks of cv. Pentland Crown were planted in field experiments. Seed tubers, stem bases and progeny tubers from the growing crop and tubers at harvest and after storage were sampled intensively and estimates of inoculum and disease made for five principal tuber diseases: the blemishes skin spot (Polyscytalum pustulans), silver scurf (Helminthosporium solani) and black scurf (Rhizoctonia solani) and bacterial soft rot (Erwinia carotovora). The data were principally used to calculate sample variances after transformation of % values to logits. A variance of 0.010 was selected as giving acceptable accuracy for most purposes and to achieve such values substantial numbers of samples would have been needed. Five to ten 50-tuber samples of seed tubers or 15–20 samples of a harvested or stored crop would have been required for assessing disease symptoms and similar numbers of samples for assessing gangrene potential by a standard damage test. The eye plug test for assessing inoculum of the three blemishing diseases used 15 or 20-tuber samples and five such samples of seed or about ten from the growing crop would usually have been adequate. Variances were nearly always greater for black scurf assessments than for other blemishing diseases. The data also illustrated changes in inoculum and disease levels during the season and in store. Their significance for understanding the epidemiology of the diseases and for storage disease forecasting is discussed.     

Blackleg potential of potato seed: determination of tuber contamination by Erwinia carotovora subsp. Atroseptica by immunofluorescence colony staining and stock and tuber sampling

Two methods to determine numbers of the blackleg pathogen, Erwinia carotovora subsp. atroseptica, in tuber peel extract were compared; (1) growth and cavity formation on crystal violet pectate (CVP) medium (Pérombelon, Lumb & Hyman, 1987) and (2) immunofluorescent colony (IFC) staining (Van Vuurde & Roozen, 1990) using an antiserum against the bacterium conjugated with fluorescein isothiocyanate. Detection, identification and quantification of the bacterium based on the differential effect of temperature on growth in the CVP method were severely restricted and in some cases could not be done at low peel extract dilutions containing > 106 saprophytic bacteria ml“1 and > 103 cells ml-1 of E. carotovora subsp. carotovora. In contrast, although recovery was c. 50% relative to growth of E.c. atroseptica alone on nutrient agar, numbers of the bacteria could be determined by the IFC method regardless of numbers of saprophytic bacteria and E.c. carotovora present. Moreover, the tedium of counting colonies on a UV microscope could be avoided by automation using an imaging system on photograph film negatives of the microscope fields. Readily accessible tubers from the top layer of one tonne boxes in commercial stores were c. 10 times less contaminated than those from the middle of the boxes. For the two methods of peel extract preparation examined, the estimated sample size needed with an allowable error of log1010 E.c. atroseptica cells ml“1 extract with 95% confidence, was c. five tubers per box and 14 boxes for extract prepared from individual tubers and c. three lots of 10 tubers per box and 10 boxes for extract from 10 pooled tubers. A blackleg potential index for seed stocks was proposed based on the summation of the weighted number of individually tested tubers in different classes of contamination level.     

The susceptibility of stems of different potato cultivars to blackleg caused by Erwinia carotovora subsp. atroseptica

The susceptibility of stems of six potato cultivars to Erwinia carotovora subsp. atroseptica was assessed in two years (1981 and 1982) either by direct inoculation in the field or by inoculation of detached stems in the laboratory. These six and a further 22 cultivars were also assessed in three years (1982-84) by inoculating stems of glasshouse-grown plants. Different methods of inoculation and types of inocula were tested. In the field, wooden toothpicks rubbed in bacterial slime were more successful in establishing infection than when dipped in a bacterial suspension, but injection of bacterial suspension with a hypodermic needle was reliable in establishing infection over a range of concentrations. Detached stems were more readily infected and gave more consistent results compared with inoculation in the field. The range of reaction of the six cultivars was similar in both detached stem and glasshouse tests. The early cultivars Pentland Javelin and Ulster Sceptre were most susceptible and of the maincrop cultivars, Maris Piper was intermediate and Desiree and King Edward least susceptible whereas Pentland Crown showed greater resistance in the glasshouse than in the field. Glasshouse tests using hypodermic inoculation indicated a range of susceptibilities; the early cultivars Manna, Maris Bard and Estima were most susceptible and the maincrop Pentland cultivars Crown, Dell, Hawk, Ivory and Squire least susceptible.     

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 effect of Erwinia carotovora subsp. atroseptica (blackleg) on potato plants. I. Growth and yield of different cultivars

The effect of planting seed tubers inoculated either near the stolon attachment (heel end), among the eyes at the apex (rose end) or mid-way along the tuber with Erwinia carotovora subsp. atroseptica, was assessed in terms of growth of the plant, disease symptoms and yield. Invariably rose-end inoculation had the greatest and heel-end the least effect in decreasing yield when compared with uninoculated plants. Cultivars Majestic and King Edward were the most susceptible, Pentland Crown showed some resistance to invasion of stems (blackleg) although plant vigour, expressed in terms of plant height and stem number was affected and Maris Piper was the most resistant.     

Development of black dot disease (Colletotrichum coccodes (Wallr.) Hughes) and its effects on the growth and yield of potato plants

Seed tubers of cvs Désirée and Pentland Crown with different severities of black dot were planted in 1988 and 1989 at Rothamsted in fields in 4– or 7-course rotations, respectively. Tubers treated with prochloraz (1988) or imazalil (1989) were planted in some plots, and in others Colletotrichum coccodes inoculum was added to the soil at planting. In further experiments at Mepal, Cambridgeshire in 1989 and 1990 and at Rothamsted in 1990 on sites where potatoes had not been grown for more than 15 years, large amounts of inoculum were added to the soil around disease-free seed tubers of two (1989) or three (1990) cultivars at planting. In all experiments plants were sampled during the season and the effects of treatments on disease development, growth and yield were recorded. Disease on roots, stem bases and tubers was found early in the season and was more severe on Désirée than on Pentland Crown plants from fields in 4– or 7-course rotations. Severity increased throughout the season and with increasing amounts of disease on the seed tubers, especially with Desiree. Disease was also found on plants from disease-free tubers and was more severe in 1988 than 1989. At harvest, black dot on tubers was significantly more severe from severely affected than from disease-free seed, and was most severe where inoculum, especially large amounts, had been added at planting. Fungicide treatment decreased disease early in the season but had no effect on tuber infection at harvest. In 1989 the weight loss of seed tubers during sprouting increased with increasing amounts of black dot, but the disease had little effect on plant size through the season. At harvest the yield of ware tubers (>50 mm) decreased with severe disease but total tuber yields were not significantly affected. However, at harvest in 1988 severely affected seed yielded significantly less than healthy seed. Plants grown from mini-tubers were free from disease on sites where potatoes had not been grown for at least 15 years. Inoculum applied at planting caused severe disease on all cultivars in both years, whereas disease was slight on uninoculated plants. Inoculated plants senesced early at Mepal in 1990, but there were no significant differences in total tuber yield in any experiment. However, yields of ware tubers (>50 mm) were sometimes decreased and the total tuber number per plant increased.     

Healthier seed potatoes. I. Effects of inoculating stem cutting stocks with Polyscytalum pustulans and Rhizoctonia solani on growth, yield and disease

Potato seed tubers of seven cultivars derived from stem cuttings in 1965 (healthier seed) were grown in 1969–72 at two sites, one clay with flints soil (Rothamsted) and the other sandy loam soil (Woburn). Inoculating sprouted tubers at planting with Polyscytalum pustulans did not affect the number of stems/plant or total yield but increased stem base and tuber infection. The yields of large tubers (57–83 cm) were increased and small tubers (>57 cm) decreased, indicating a decrease in tuber numbers. Rhizoctonia solani inoculated at planting decreased numbers of stems/plant and yield by up to 14% at Woburn but not at Rothamsted. At both sites, yields of large tubers were increased with cvs Majestic and Record and decreased with Pentland Crown. Stem canker and tuber infection were increased but infection was also prevalent on tubers from non-inoculated seed at Woburn. When both pathogens were inoculated together yields of large tubers were increased in cvs King Edward, Majestic and Record and decreased in Pentland Crown. Infection of stem bases and tubers was sometimes less than when either pathogen was inoculated singly.     

Stem canker (Rhizoctonia solani) of maincrop potatoes.

In two years, potato plants were sampled at 1- or 2- weekly intervals from plots planted with seed tubers bearing sclerotia of Rhizoctonia solani (black scurf) and with seed without sclerotia either infested or not with cultures of R. solani at planting. Sprouted King Edward seed was used in 1981 and sprouted and non-sprouted King Edward and Pentland Crown seed in 1982. In both years 60–80% of shoots from seed with sclerotia and 90% of shoots from seed inoculated at planting were affected with stem canker. Most disease developed before shoots emerged although it gradually increased later when new shoots arising both from seed tubers or as branches on shoots with damaged apices (pruned shoots) became infected before they emerged. Sprouting seed tubers bearing sclerotia decreased the disease on both cultivars but with soil-applied inoculum the disease was more severe on plants from sprouted than non-sprouted seed. Some stolons were infected by R. solani soon after they developed and incidence of infection later increased. Thirty to 50% of stolons were infected on plants from infected seed tubers and 60% on plants with soil-applied inoculum. With both cultivars and sources of inoculum about 70% of the infected stolons had their apices killed (pruned).     

The effect of soil- and tuber-borne inoculum on the incidence of potato gangrene

Under optimum growing conditions neither tuber- nor soil-borne Phoma exigua var. foveata inoculum appreciably affected stand or yield of the subsequent potato crop. Seed tubers with gangrene rots caused high levels of stem and tuber symptoms when planted in var. foveata contaminated or uncontaminated land; contaminated seed tubers with no rots also produced progeny with a high gangrene potential. Sufficient soil-borne inoculum was carried over in land that produced a gangrene affected crop in the previous year to override the effect of tuber disinfection. Effective gangrene control was achieved by a combination of tuber disinfection shortly after harvest over successive years with a 1 in 5 yr potato crop rotation. Gangrene rots usually developed through injuries to the tuber periderm, rots in other tubers being associated with pustules of powdery scab (Spon-gospora subterranea).