The influence of cultivar and isolate on the development of gangrene (Phoma exigua var. foveata) in potato tubers

In studies on the infection of potato cultivars with different field isolates of Phoma exigua var. foveata, cultivar × isolate interactions, although sufficiently large to influence the relative pathogenicities of isolates on different cultivars, were not substantial enough to affect cultivar ranking order. Cultivar rank was markedly influenced by pathogen isolate only when both field and culture collection isolates were compared. This suggests that the complications of cultivar × isolate interactions can be avoided in cultivar screening tests by the use of recent field isolates of high pathogenicity. It was found that cultivar was considerably more important than isolate in determining gangrene lesion size. The need to consider both lesion diameter and depth when estimating rot size following point inoculation of different cultivars was confirmed.     

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.     

Infection of potato stems by the gangrene pathogen, Phoma exigua var. foveata

Limited transport of pycnidiospores of Phoma exigua var. foveata within transpiring stems was demonstrated by isolating from cut mature stems placed for 24 h in a spore suspension. Pieces of shoot dipped in spore suspension and subsequently rooted contained the pathogen which was redistributed as the plants grew but without causing symptoms until after desiccation. The fungus was only rarely detected within stems growing from rotted seed tubers suggesting that spore transport in the vascular system was relatively unimportant in these experiments. Stems were readily infected from spore suspensions applied to unwounded epidermis especially when the inoculum was held in cotton wool and secured with wax film. The progressive colonisation of growing stems was demonstrated by isolations made at different positions on the stem and at different times after inoculation. In field experiments in two years stems were inoculated with four spore concentrations on four dates from late June to late August. After desiccation the extent of pycnidial lesions was greatest from the earliest inoculation date and the largest spore concentration and these differences were reflected subsequently in the incidence of gangrene on damaged progeny tubers. Younger stems may be more susceptible and infection in early summer probably results in extensive growth within the stem before desiccation and thus in massive pycnidial production.     

Seasonal occurrence of the potato gangrene pathogen, Phoma exigua var. foveata, in the open air

Using a Casella High Volume Bacterial Sampler air samples were collected down wind of potato fields in central Scotland when it was raining and when it was dry between December 1978 and October 1982. Airborne propagules of Phoma exigua var. foveata were recovered from the open air during rainfall in most months of the year but the frequency of recovery was greatest in September, October and November. The numbers of colonies of P. exigua var. foveata detected during rainfall were much greater in September and October than during the rest of the year. Low numbers of colonies of P. exigua var. foveata were recovered in the absence of rainfall and their occurrence appeared to be associated with strong winds and dry soil conditions. These latter airborne propagules were detected during all the months except November to February when few samples were made. Sampling simultaneously up wind and down wind of potato fields in September and October demonstrated that the potato crop was the main source of airborne propagules caught down wind but small numbers of P. exigua var. foveata were recovered up wind in four out of the seven experiments.     

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.     

Factors affecting the assessment of resistance of potatoes to gangrene (Phoma exigua var. foveata)

Three experiments designed to elucidate the effects and interactions of different inoculation methods, inoculation positions, strains of Phoma exigua var. foveata and clones of potato tubers inoculated were analysed using as data dimensions of the lesions and a more general surface score. The assessment of susceptibility was affected by all these factors. The ranking of varieties for resistance altered considerably depending upon whether the surface area or the depth of lesions was considered. Derivation of more complex scores combining these measurements reduced the relative genetic component and hence would be of no value to potato breeders.     

Variation in pathogenicity among field isolates of Phoma exigua var. foveata

Experiments carried out using a point inoculation method to infect potato tubers with Phoma exigua var. foveata demonstrated considerable variation in pathogenicity among field isolates. This variation was unlikely to be due to differential reductions in isolate pathogenicities during axenic culture. However, fresh field isolates generally produced larger lesions than stored isolates. An investigation of the distribution of pathogenicity variation in the fungus revealed that differences among isolates from different lesions taken from the same potato stock were greater than those between stocks, but pathogenicity variation within each lesion isolate was small relative to that between isolates. The importance of using isolates with a high level of pathogenicity, hence recent field isolates, in studies of this pathogen is stressed.     

The Influence of Glycoalkaloids and Impact Damage on Resistance to Fusarium solani var. coeruleum and Phoma exigua var. foveata in Potato Tubers

Potato glycoalkaloids were determined in tubers of a wide range of genotypes in order to study their influence on resistance to storage rots caused by Fusarium solani var. coeruleum and Phoma exigua var. foveata. No relationships were found. This indicates that breeders selection for low glycoalkaloid potato clones is not in conflict with the demands for high levels of resistance to these pathogens. It was also observed that genotypes susceptible to skin-breaking types of, damage were more easily infected than genotypes susceptible to other types of damage.     

Pathogenesis of Potato Gangrene Caused by Phoma exigua var. foveata: II. Activities of some Hydrolases and Dehydrogenases

The location of enzyme activity in gangrene‐diseased tubers was determined using the nitrocellulose blotting method. The activity of aminopeptidase and esterase was located in tissues adjacent to dry rot caused by Phoma exigua var. foveata and in other apparently healthy tissues. The activity of glucuronidase, succinic and glucose‐6‐phosphate dehydrogenases (G‐6‐PDH), however, was confined to tissues adjacent to the rotted tissue. The pathogen produces very active α‐ and β‐glycosidases, so their highest activity occurred in rotten tissue that was filled with fungal mycelium. Results suggest that all these enzymes are involved in alteration of cell metabolism and the destruction of diseased tuber tissue.     

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.     

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.     

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.     

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.     

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.     

Developing a biocontrol strategy to protect stored potato tubers from infestation with potato tuber moth species in the Andean region

Heavy infestations of stored potato (Solanum tuberosum L.) tubers by the two potato tuber moth species Symmetrischema tangolias (Gyen) and Tecia solanivora (Povolny) frequently occur in Andean potato‐growing regions of Ecuador. The aim of the study was to develop a biological control strategy for both species using powder formulations made of inert substances, Phthorimaea operculella (Zeller) granulovirus (PhopGV) and Bacillus thuringiensis Berliner subsp. kurstaki (Btk). The LC₅₀ of PhopGV on T. solanivora was 0.33 LE/L, and Btk caused 82.7% mortality at a concentration of 100 g/L in bioassays. The efficacy of talcum, kaolin, calcium carbonate and sand ranged between 76.2% and 98.7%. Calcium carbonate was highly effective to control both species; however, its efficacy was affected by the relative humidity and dropped to 55.4% at relative humidity of 100%. PhopGV at concentrations of five larvae equivalents (LE) per kg kaolin and Btk at a concentration of 60 g Btk/kg talcum caused 95.7% and 88.1% mortality of T. solanivora, respectively. In storage experiments, the efficacy of calcium carbonate alone and in combination with PhopGV (20 LE/kg) and Btk (15 g/kg) caused 95.0–99.8% mortality of T. solanivora in all treatments and reduced infestation on potato tubers by 83.6%–91.0%. In the case of S. tangolias, Btk significantly increased mortality to 96.5% compared with calcium carbonate alone and reduced tuber infestation by 83.4%. Storage of potato tubers in thin layers enhanced the efficacy of the calcium carbonate treatment compared with storage in bags. It was concluded that calcium carbonate alone seems to be appropriate for the control of T. solanivora, and an addition of 15 g Btk/kg would improve the control of S. tangolias. It is suggested to test these new formulations under on‐farm storage conditions.     

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.     

Contamination of seed potato tubers grown from stem cutting or microplant-derived tubers by Phoma foveata and Polyscytalum pustulans in Scotland: the influence of site and fungicide treatment at harvest

A survey was made over 5 years of the incidence of contamination by Phoma foveata and Polyscytalum pustulans on 3-year-old virus-tested stem cutting (VTSC) stocks assessed on three occasions during the storage season, and on their daughter tubers at five farms. The contamination occurring on plants grown in small plots from stem-cutting/microplant tubers was also assessed at the farms.
Fungicide treatment soon after harvest reduced the incidence of both pathogens, especially P. pustulans . Tuber contamination was similar for samples collected after fungicide treatment and before planting. The incidence of contamination on daughter tubers, relative to that on the untreated mother tubers, was less with P. foveata but was similar or greater with P. pustulans . Contamination by both pathogens varied between farms and was reflected in the degree of contamination occurring on daughter tubers from stem-cutting/microplant tubers.
The incidence of P. foveata on stems, soil and tubers of seed stocks classified at Foundation Stock grade from various regions of Scotland was assessed over 2 years. The incidence of tuber contamination was correlated ( P < 0.05) with the amount of tuber-borne soil contamination for all comparisons, but was correlated with stem inoculum in only 7 out of 13 comparisons. The largest amounts of stem inoculum of P. foveata were found in stocks in Aberdeenshire.
These results are discussed in relation to the spread of both pathogens to healthy VTSC stocks.     

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.     

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).