Tuber Surface Research Articles

Biological control of postharvest fungal rot of yam (Dioscorea spp.) withBacillus subtilis

The potential of isolates of Bacillus subtilis from yam farm soil to control rot of yam in storage barns was investigated. Yam tubers inoculated in vivo with B. subtilis showed no rot while those inoculated with Aspergillus niger, Botryodiploidia theobromae or Penicillium oxalicum showed considerable rot. The set of yams in which B. subtilis and the fungi were simultaneously inoculated produced rot whereas those in which B. subtilis was inoculated a day before the fungi was inoculated were totally reduced or free of rot. Many fewer fungi were isolated from the surface of tubers treated with B. subtilis than from the untreated (control) and there was high recovery of B. subtilis (99-100%) throughout the period of storage. Rot build up was faster in uninoculated control tubers or those inoculated with a spoilage fungus, while those treated with the antagonist were totally reduced or free of rot. The culture filtrate of B. subtilis prevented spore germination in some spoilage fungi. The importance of this study in relation to farmers in developing countries is discussed.

Influence of Nitrogen Nutrition on Tuber Quality of Potato with Special Reference to the Pathway of Nitrate Transport into Tubers

The influence of nitrogen (N) nutrition on tuber quality of potato (Solanum tuberosum L.), i.e., specific gravity, starch, nitrate, and protein content as well as ascorbic acid content was investigated in a pot experiment. As expected, specific gravity and starch content decreased with increasing fertilizer rates. Protein and especially nitrate content were positively related to nitrogen fertilization, while the effect on l-ascorbic acid content was relatively small. Only at the highest N level, which led to drastic yield depressions (3.8 g N pot−1), l-ascorbic acid content was significantly reduced. Since is considered phloem-immobile and xylem transport into the tubers is restricted (because of the tuber's low transpiration rate), the pathway of nitrate translocation into the tubers is unclear. For Ca2+, a direct uptake from the soil solution by the tuber surface is well documented. In order to check whether this pathway may also be of importance for , potato plants were cultivated in solution culture. The growing tubers were wrapped with paper tissue soaked with Ca(15NO3)2-labelled tuber medium, while contact of the tubers with the nonlabeled liquid growth medium was strictly avoided. 15N-content of tubers was measured on a mass spectrometer. The experiment demonstrated that with high concentration at the tuber surface, up to 40% of total tuber content can be taken up through the tuber skin.

Enumeration of plant pathogenic Streptomyces on postharvest potato tubers under storage conditions

To determine the potential for spreading pathogenic Streptomyces on seed tubers into field soil, we enumerated populations of pathogens on diseased and healthy tuber surfaces. Since nonpathogenic Streptomyces strains are indistinguishable from pathogenic ones by colony morphology, we used colony polymerase chain reaction to detect txtA, a gene of the thaxtomin biosynthetic pathway. Of the total Streptomyces spp. isolated from the tubers of potato ‘Yukon Gold’, 99% were pathogenic from scabby lesions, whereas only 12% were pathogenic from visually healthy surfaces of scabby or scab-free tubers. The density of pathogenic populations declined from 109 to 108 colony-forming units per gram of lesion tissue after storage of tubers for 18 weeks. Populations also declined from 105 to 103 colony-forming units per gram of tissue on healthy surfaces of scabby tubers and scab-free tubers after storage. Scabby tubers obtained from growers, prior to spring planting, also harbored large populations of pathogenic strains.

Occurrence and epidemiological aspects of potato silver scurf in California

A new potato tuber disease has been observed in the Tulelake region, California, USA, since 1995, with tuber symptoms suggestive of silver scurf disease (Helminthosporium solani). In this work we isolated, identified and demonstrated the nature of the causal agent of this potato disease in California. In addition, the distribution of H. solani in potato fields and the inoculum potential at harvest time were investigated. Disease progress and H. solani spore populations were also characterised under commercial storage conditions. The main fungal genera associated with potato tubers in storage were Helminthosporium solani, Colletotrichum sp., Fusarium sp., and Rhizoctonia sp. The results of Koch's postulates indicated that H. solani is responsible for the outbreak of silver scurf in the Tulelake region. In a disease survey in three commercial potato fields naturally infested, H. solani infections occurred in all fields. However, the extension of the infections differed significantly between the fields. During potato storage, silver scurf usually increased over time. The percentage of the tuber surface covered by silver scurf varied from 3.5% up to 35.5% during the storage period. The number of H. solani lesions per tuber also progressively increased from 6% up to 35%, six months after storage. H. solani spore populations also increased over time in all studied potato stores; nevertheless, they followed no consistent pattern, exhibiting multiple and variable peaks of increase and reduction during the period of storage.

Growth in vitro and infectivity of Colletotrichum coccodes on potato tubers at different temperatures

To investigate the ability of black dot symptoms to develop on infected potato tubers during storage, the growth of Colletotrichum coccodes was followed in vitro on malt agar at temperatures ranging from 5–27°C, and in vivo on artificially infected potato tubers kept at 5, 10 and 15°C. In vitro, 13 isolates from different geographical origins grew at all temperatures tested; growth started with a delay of 10 days at 5°C and of 4 days at 10°C, and was fastest at 27°C. All isolates had similar growth patterns and produced conidia and sclerotia at all temperatures. Minitubers were successfully infected at 5, 10 and 15°C by depositing either a mycelial plug or a drop of conidial suspension on the tuber surface. Sclerotia were observed after 7 days at the point of inoculation. Symptoms extended in all cases, although more slowly at 5 and 10 than at 15°C. Latent infections were detected in up to 21% of tubers without black dot symptoms at harvest. These results show that latent infections by C. coccodes are probably quite frequent, and that the pathogen is able to develop at low temperatures in controlled conditions. This suggests that black dot symptoms can increase during storage if stores are not adequately managed.

Heterologous pectate lyase isoenzymes are not different in their effects on soft rot resistance in transgenic potatoes

Pectate lyase (PL) isoenzymes secreted by Erwinia carotovora ( Ec) bacteria are of central interest to both basic and applied plant physiology because they depolymerise cell wall pectin into unsaturated oligogalacturonates that trigger various plant defence responses. Therefore, the effects of heterologously expressed PL1 and PL3 were examined with emphasis on plant resistance to soft rot disease caused by Ec bacteria. Agrobacterium-mediated, transformation resulted in transgenic potato lines of cv. Désirée, with activities of the recombinant PL1 ranging between 22 and 1402 mU ml −1. Compared with the non-transgenic plants, potato lines expressing PL1 provided enhanced resistance to soft rot-causing Erwinia bacteria. Thus, the degree of cell lysis caused by bacterial maceration as well as the extension of Ec-rotting was significantly diminished in PL-expressing tuber tissue. This was coincident with a strengthened formation of necrosis on the wound surface of tubers. Spreading of Ec bacteria appear to be prevented—most likely because of enhanced activities of polyphenol oxidase, phenylalanine ammonia-lyase and peroxidase in PL-transgenic tubers. However, key finding reveal that there are no significant differences in resistance to soft rot between potato lines expressing PL1 and PL3. Despite differences in their action patterns and reaction products, it is shown that both PL isoenzymes share similar molecular roles in the events leading to induction of plant defence responses.

Ultrastructure of the infection process of potato tuber by Helminthosporium solani, causal agent of potato silver scurf

Silver scurf is an important postharvest disease affecting potato tubers worldwide, caused by Helminthosporium solani. In the present study, key steps of infection of potato tubers (cv. 'Dark Red Norland') by H. solani were described using transmission (TEM) and scanning electron microscopy (SEM). The fungus entered potato tubers mainly via hyphae, although germ tubes were also able to directly penetrate the tubers. An extracellular sheath was observed around hyphae growing over the surface of tubers and the host cell wall appeared lyzed at the point of penetration. Observations suggested that both mechanical and enzymatic processes are involved in periderm penetration. Hyphae of H. solani, 9 h after tuber inoculation, were present intracellularly mostly in the periderm and in some cortical cells. Two days after inoculation, host cells were invaded and both infected and neighbouring host cells showed signs of necrosis (disrupted cytoplasm, absence of typical organelles or endomembrane systems, collapsed peridermal cells) that were not observed in healthy control tubers. Four days after inoculation, completing the infection cycle, conidiophores emerged from peridermal cells directly by erupting through the host cell walls.

Deep sunken lesions – an atypical symptom on potato tubers caused by Colletotrichum coccodes during storage

Colletotrichum coccodes causes a potato tuber-blemishing disease, commonly known as black dot (Lees & Hilton, 2003). Typical black dot symptoms are silvery-to-brown patches on the tuber surface, bearing black microsclerotia (Dillard, 1992). Severe infection can cause tuber shrivelling (Hunger & McIntyre, 1979). Furthermore, Mooi (1959) attributed deep lesions observed on infected tubers stored at −1°C to C. coccodes. Similar symptoms have also been observed occasionally on tubers kept in commercial cold stores under irregular temperature regimes (Gaucher, 1998). To our knowledge, these symptoms have never been reproduced in control inoculation experiments, so the implication of C. coccodes as the sole causal agent remains unclear. Deep sunken lesions, similar to those reported by Mooi (1959) and Gaucher (1998), were obtained on potato tubers from commercial stores that had been kept for several weeks at 5–15°C after an artificial inoculation with C. coccodes. The inoculation was performed by depositing a 10 µL drop of a conidial suspension (calibrated at 3 × 106 spores mL−1) of C. coccodes (isolate 91·22 g from the INRA culture collection) on each end of healthy mini-tubers of potato cv. Charlotte. Water droplets without inoculum were used on control tubers. Inoculated tubers were incubated in the dark at 5, 10 and 15°C and 100% relative humidity (rh). After approximately 10 weeks incubation, dark brown, irregular-shaped lesions with clear contours were observed. After 5 months incubation, symptoms had extended over the whole tuber surface. C. coccodes was the only pathogen isolated from these lesions. The proportion of tubers showing deep lesions was higher at 10°C (∼20%) than at 5 or 15°C (∼5%). These lesions were never observed on tubers inoculated in the same way but kept at 20–25°C and 100% rh. On these tubers only, typical black dot symptoms developed. No lesions or black dot symptoms were observed on control tubers. These observations therefore demonstrate that C. coccodes alone can cause deep tuber lesions on potato tubers stored for extended periods at 5–15°C.

Specific detection of Ralstonia solanacearum 16S rRNA sequences by AmpliDet RNA

The potential of AmpliDet RNA for specific detection of Ralstonia solanacearum in potato tuber samples and surface water was demonstrated. AmpliDet RNA is a procedure based on nucleic acid sequence based amplification (NASBA) of RNA sequences and homogeneous real time detection of NASBA amplicons with a molecular beacon. The procedure is carried out in sealed tubes, thus reducing the risks for carry-over contamination. AmpliDet RNA enabled reliable detection of specific 16S rRNA sequences of R. solanacearum in total RNA extracts from potato tuber samples in 90 min at a level of 10 cells per reaction, equivalent to ca. 104 cells ml-1 of sample. In surface water, AmpliDet RNA allowed detection of R. solanacearum at a level of 10 cfu ml-1, after concentrating bacteria from 200 ml of surface water into 1 ml of surface water by centrifugation. All strains of R. solanacearum and a strain of R. syzygii were positive in AmpliDet RNA, but not other (related) bacterial species. Ralstonia solanacearum (race 3, biovar 2) could be detected reliably in 18 naturally infected potato tuber samples containing varying concentrations of cells. Ninety-one negative tuber samples, from which no R. solanacearum was isolated, were tested in AmpliDet RNA, including 23 samples containing bacteria (cross-) reacting with antibodies against R. solanacearum in immunofluorescence (IF) cell-staining. Only one negative sample, containing high numbers of IF-positive cells, was positive in AmpliDet RNA

Occurrence of Potato Powdery Scab Caused by Spongospora subterranea in Korea

Potato tubers with scab lesions were found in several regions of Gangwon province in Korea during disease surveys in 2000 and 2001. The lesions appeared as shallow scabs with torn and raised skin around the periphery of the lesion. Sometimes the lesions expanded on tuber surface and formed hollowed-out areas. The causal organism was identified as Spongospora subterranea based on morphological characteristics using light and electron microscope. Ten cultivars of potato were evaluated to select resistant cultivars against powdery scab. Among the 10 cultivars, Jowon, Superior, and Atlantic were resistant, while Sephody, Gawon, and Irish Cobbler were susceptible to powdery scab. In the pathogenicity test, typical symptoms of powdery scab were formed on potato tubers, and galls developed on root hairs. This is the first report of powdery scab of potato caused by S. subterranea in Korea.

Phytophthora erythroseptica에 의한 감자 홍색부패병 발생

1999년 강원도 평창의 고랭지 감자 재배지에서 감자 홍색부패병이 발견된 이후 2001년과 2002년에는 전남 보성의 감자 재배지에서도 발생이 확인되었고 발병이 심한 지역에서는 약 5%정도의 이병율을 보였다. 병든 감자의 주요 병징은 괴경 외부가 연갈색으로 변색되어 부패증상이 나타나고 피목과 눈이 흑갈색으로 변화되어 건전 부위와 이병 부위가 쉽게 구분되었다. 또한 괴경 내부는 삶은 감자처럼 스폰지 형태로 변하고 냄새가 없었으며 괴경 절단면은 공기 중에 노출된지 30분 이후에 핑크색으로 변색되는 특징을 확인할 수 있었다. 균사는 배지에 따라 방사형, 장미꽃 또는 솜털 모양으로 특징적인 생육을 하였고, 균사생육온도는 5.0~3<TEX>$0^{\circ}C$</TEX>범위이고 최적온도는 <TEX>$25^{\circ}C$</TEX>였다. 유주자낭은 유두돌기 없이 선단 부위가 평편하고, 계란 또는 서양배 모양의 다양한 형태로 크기는 41.3~69.6<TEX>$\times$</TEX>26.8~47.4(평균 55.5<TEX>$\times$</TEX>37.1) <TEX>$\mu\textrm{m}$</TEX>이었다. 병원균은 동종생식에 의하여 유성생식기관을 형성하여 동일 배지에서 배양초기부터 엷은 황색의 난포자를 대량으로 생산하였고 장난기 형태는 구형으로 크기가 30<TEX>$\times$</TEX>46 <TEX>$\mu\textrm{m}$</TEX>이었고, 장정기는 저착성이고 형태가 신장된 타원형으로 크기가 13<TEX>$\times$</TEX>14 <TEX>$\mu\textrm{m}$</TEX>이었다. 이들 병원균의 배양적, 형태적 특성에 의거, Phytophthora erythroseptica에 의한 감자 홍색부패병으로 동정되어 국내에서 처음으로 보고한다. Pink rot of potato (Solanum tuberosum L.) occurred at Pyeongchang in Gangwon and at Bosung in Junnam province since 1999. The disease incidence in the surveyed areas was about 5% of harvested potatoes in 2002. Affected tubers showed a dull brown appearance and the lenticels and eyes on tubers turned dark brown. The cut surface of the diseased tuber colored faint pink and the entire surface of the diseased tuber becomes deep salmon pink within 30 min. The pathogen isolated from the diseased tubers was identified as Phytophthora erythroseptica based on morphological and cultural characteristics. Mycelial mat was fairly fluffy, rosette or stellate patterns and rounded or angular hyphal swellings were farmed in water, Temperature for mycelial growth was ranged from 5 to 3<TEX>$0^{\circ}C$</TEX> and optimal temperature was <TEX>$25^{\circ}C$</TEX>. Non-papillate sporangia were persistent on stalk and ellipsoid, ovoid, obpyriform or distorted in shape, often with a constriction distal in the middle. Size of sporangia was 41.3~69.6<TEX>$\times$</TEX>26.8~47.4 (av, 55.5<TEX>$\times$</TEX>37.1) <TEX>${\mu}{\textrm}{m}$</TEX>. Sexuality of Phytophthora erythroseptica was homothallic. Oogonia were 30~46 <TEX>${\mu}{\textrm}{m}$</TEX> in diameter and oospores were 28~35 <TEX>${\mu}{\textrm}{m}$</TEX> in diameter, Elongated or cyclindrical antheridia were all amphigynous. This is the first report on potato pink rot caused by Phytophthora erythroseptica in Korea.

Black dot (Colletotrichum coccodes): an increasingly important disease of potato

In recent years black dot (Colletotrichum coccodes) has become an economically important disease problem in potato (Solanum tuberosum). It is characterized by silvery lesions on the tuber surface which result in a deterioration in skin quality. In addition to causing tuber blemish symptoms, C. coccodes also causes symptoms on stems and foliage, resulting in crop losses in some countries, and is implicated as a factor in the potato early dying disease complex. In the past, the incidence and severity of black dot might have been underestimated, as tuber symptoms were often mistaken for silver scurf (Helminthosporium solani). Inoculum of C. coccodes can be both seed tuber‐ and soilborne, and disease control is difficult as there are few chemical control methods and little resistance in commercial cultivars. Cultural control options offer the only potential means to control this disease at present. Current developments in rapid and specific PCR‐based detection methods are being used to address epidemiological questions.

Fungi associated with peels of post harvest yams (dioscorea spp.) in storage

Fungi on surface of three cultivars of white yam (Dioscorea rotundata Poir) and a cultivar of water yam (D. alata L.) were investigated. The objective of the study is to show vertical distribution (Head, middle and tail regions) of mycoflora of yam surface. The mycoflora consisted principally of Aspergillus niger Van Tiegh, Botryodiplodia theobromae Pat, Trichoderma sp., Rhizopus sp., Fusarium solani (Mart) Sacc, Choanephora cucurbitarum (Berk and Fav.) Thaxt, Mucor sp. and Penicillium oxalicum Currie and Thom in that order of their frequency of occurrence. The head region showed significant difference in occurrence of fungi (p < 05) more than the middle and tail regions. The frequency of occurrence of A. niger and B. theobromae was considerable in all the cultivars except in water yam where the occurrence of A. niger was relatively sparse. The implication of tuber surface mycoflora on the post harvest storage of yams is discussed. Key words: yam, Peel, tuber surface, mycoflora (Global Journal of Pure and Applied Sciences: 2002 9(1): 19-24)

First Report of Nattrassia mangiferae as a Postharvest Fungal Pathogen of White Yam (Dioscorea rotundata) in Nigeria.

A survey of the yam growing areas of Nigeria was conducted in January 1999 to identify the major fungal pathogens responsible for considerable tuber rot losses in storage. A total of 150 tubers of white yam showing rot symptoms were collected from 30 barns at 18 locations spanning three agroecologies (guinea savanna, forest/savanna transition, and forest) within the yam belt of Nigeria. Tuber pieces were cut at the interface between healthy and infected tissue, surface-sterilized in 10% bleach solution (0.05% sodium hypochlorite), rinsed in sterile distilled water, and plated on potato dextrose agar (PDA; Difco Laboratories, Detroit, MI) amended with lactic acid (1ml/liter) to reduce bacterial contamination. Cultures were incubated at 28°C for 4 days. Isolations from the guinea savanna agroecology showed that Nattrassia mangiferae Sutton & Dyko (synonym Hendersonula toruloidea) (3) had 42% frequency of occurence, Botryodiplodia theobromae Pat. (10%), Penicillium oxalicum Curie & Thom. (12%), and Aspergillus niger Tiegh. had 50% frequency of occurrence of the total fungi isolated from this region. In the forest/savanna transition agroecology, N. mangiferae had 28% frequency of occurrence, while B. theobromae, P. oxalicum, and A. niger had 10, 14, and 50% frequency of occurrence, respectively. In the forest agroecology, N. mangiferae and B. theobromae had 6% frequency of occurrence, while P. oxalicum and A. niger had 17 and 72% frequency of occurrence, respectively. N. mangiferae was more frequent in occurrence than the two known major fungal pathogens (B. theobromae and P. oxalicum) of yam tuber in storage. Hence, its pathogenicity was tested on healthy tubers. Fifteen healthy tubers were wounded and inoculated individually with 7-day-old mycelial plugs of each of 10 isolates of N. mangiferae from each agroecology using the cork borer technique (2), placed in polythene bags, and stored in a barn for 14 days at 27± 2°C. Control tubers were inoculated with sterile agar plugs. Tubers were sliced at the points of inoculation, and lesion development was measured. N. mangiferae was found to be pathogenic in yam tubers causing soft, wet, brown rots similar to those observed in natural infections, and it appeared as dark necrotic lesions on tuber surface. In culture however, colonies are fluffy with olivaceous gray coloration that gradually darkens as the fungus matures. The fungus grows rapidly on PDA and is able to attain a radial diameter of 90 mm by the fourth day after inoculation. Cultures reisolated from the inoculated tubers matched those used for inoculation. Isolates obtained from the forest/savanna transition agroecology caused significantly (P < 0.05) larger rot lesions (24.67mm) than those obtained from the guinea savanna (18.33mm), and the forest (21.67mm). In another experiment, tubers were inoculated with N. mangiferae without wounding the tubers (mycelial disks were firmly placed on the tuber surface to make adequate contact), and the control tubers were inoculated with sterile agar plugs. Tubers inoculated without wounding remained asymptomatic when assessed for rot development. N. mangiferae has been reported as a root and stem rot pathogen of cassava in West Africa (1). To our knowledge, this is the first report of N. mangiferae causing tuber rot of yams in storage.

Influence of Timing of Harvest in Relation to Haulm Killing and Planting Date on Potato Tuber Rot Caused by Phytophthora infestans.

Killing potato haulms 2 to 3 weeks prior to harvesting tubers is recommended for the management of potato late blight to eliminate the foliage as a source of tuber blight inoculum. Some potato growers in the Columbia Basin of Washington and Oregon, however, harvest tubers within days of killing potato haulms or harvest tubers without killing potato haulms. The susceptibility of potato tubers harvested from green haulms to late blight is unknown. From 1996 to 1998, Russet Burbank and Ranger Russet seed tuber pieces were planted on three different dates and then harvested simultaneously to obtain daughter tubers at different levels of maturity as determined by physiological degree day (P-day) accumulation. Tubers from two trials in 1996 were harvested within 1 to 4 days of haulm killing, tubers from a single trial in 1997 were harvested 16 days after haulm killing, and tubers from two trials in 1998 were harvested 1 and 21 days after haulm killing. Tubers from each plot were divided into two samples; one sample was inoculated the day after harvest and the other sample was inoculated 6 months after harvest on the buds and periderm tissue with a US-8 isolate of Phytophthora infestans. Severity of tuber blight on tuber surfaces and internal tuber tissue was highly correlated. When tubers were harvested within 1 to 4 days after haulm kill, the youngest tubers had the highest percentage of skinning and the lowest percentage of tuber blight. These tests, however, were not designed to test the hypothesis that increased tuber skinning resulted in increased tuber rot. Severity of tuber blight was greater for Ranger Russet tubers than for Russet Burbank tubers. The correlation between the severity of tuber blight at harvest and after 6 months in storage was significant. In some cases, differences in susceptibility to tuber blight due to planting date were observed after 6 months in storage. When tubers were harvested within 1 to 4 days after vine killing, tubers from plants that had been growing longer were more susceptible to late blight tuber rot than tubers from plants that had been growing for a shorter period of time.

Gibberellin-induced flowering in small tubers of Zantedeschia ‘Black Magic’

Small tubers (1.5 cm diameter) of Zantedeschia ‘Black Magic’ were used to study the induction of flowering by gibberellic acid (GA 3) and gibberellins 4+7 (GA 4+7). When planted these tubers remained vegetative unless treated with gibberellin. To control gibberellin dose, tubers were dipped into treatment solution (0–1000 mg l −1) for defined periods (0–16 h) and then rinsed with water to remove gibberellin from the tuber surface. The tubers were then planted and grown in a greenhouse. Flower emergence was recorded and plants were then dissected to determine whether initiation had occurred on non-flowering plants and whether axillary buds were vegetative or floral. No differences were found between responses to GA 3 or GA 4+7 treatments. The proportion of shoots that initiated inflorescences increased with an increase in both gibberellin concentration and/or treatment duration. Increased gibberellin dose also increased in the proportion of initiated flowers that subsequently emerged and the proportion of axillary buds that initiated flowers. Plants treated with high gibberellin doses also flowered earlier and with a lower leaf number. Our results indicate that high gibberellin doses induce more rapid floral initiation than lower doses. In addition to their role in floral initiation, gibberellins play a role in floral development. Uptake of gibberellin is shown to be primarily through the buds on the upper surface of the tuber.

Induction of defence responses against Erwinia soft rot by an endogenous pectate lyase in potatoes

Pectate lyase (PL) enzymes are major virulence factors of Erwinia carotovora (Ec) bacteria. They degrade plant cell wall pectin into unsaturated oligogalacturonates (OG) known to elicit plant defence responses. Therefore, a gene encoding the isoenzyme PL3 of Ec ssp. atroseptica was transformed by means of Agrobacterium into potatoes of cv. Désirée. Four PL-transgenic potato plant lines selected on the basis of greenhouse experiments were grown over a period of 4 years (1997–2000) in the field. It is shown that the heterologous PL enzyme mediated an enhanced resistance to Erwinia soft rot in field grown tubers. Thus compared to the non-transgenic counterpart extension of rotting was significantly reduced (P<0.001) on the wound surface of PL-expressing tubers. Moreover, the threshold density ofEc -bacteria causing a progressive soft rot was up to 19-fold higher on tuber tissue containing the PL enzyme. An induction of plant defence responses in PL-transgenic potatoes may be indicated by an enhanced resistance of tuber tissue cell walls to Erwinia -derived enzymes, an increased PPO- and PAL-activity in tuber tissue as well as by a strengthened formation of necrosis on the wound surface of tubers after infection with Ec -bacteria.

The detection of tuber necrotic isolates of Potato virus Y, and the accurate discrimination of PVY O, PVY N and PVY C strains using RT-PCR

Potato tuber necrotic ringspot disease (PTNRD) is a damaging disease of potatoes, causing unsightly necrotic rings on the surface of tubers. The causal agent is thought to be tuber necrotic isolates of Potato virus Y, known as PVY NTN. The disease spoils tubers for processing and table use, and the lack of a diagnostic method makes control especially difficult. The development of an RT-PCR assay for the reliable detection of PVY NTN and discrimination of all the main strains of PVY (PVY O, PVY N and PVY C) is described. An assay was developed, exploiting a recombination site in the coat protein of PVY NTN, allowing more reliable diagnosis of these isolates. Although the conserved nucleotide differences observed between the strains was very small, competitive PCR and mutagenically separated PCR were both employed in the development of a robust assay. The assay was found to be more reliable than the most commonly used RT-PCR method, and should prove to be an important tool in the confirmation of symptoms and for the detection of PVY NTN in symptomless tissue, in disease surveys and seed health schemes.

Mycoflora of tuber surface of white yam (Dioscorea rotundata Poir) and postharvest control of pathogens with Bacillus subtilis.

Bacillus subtilis (Enrenberg) Cohn was investigated for its antagonistic properties against surface mycoflora of yam (Dioscorea rotundata Poir) tubers in storage. Yam tubers inoculated with a spore suspension of B. subtilis in potato dextrose broth using a knapsack sprayer showed a drastic reduction in the range and number of mycoflora, including pathogens of the tuber surface in contrast to the control tubers, during the five-month storage period in a traditional yam barn. However, B. subtilis maintained a high frequency of occurrence during the same period. Botryodiploidia theobromae Pat, Fusarium moniliforme Wollen and Reink., Penicillium sclerotigenum Yamamoto, and Rhizoctonia sp. were displaced completely on the treated tubers. The antagonism of B. subtilis was so effective that the normal tuber surface mycoflora was greatly reduced throughout the storage period of five months by a simple initial application of the antagonist.

Variability within Clones of Potato cv. Russet Burbank to Infection and Severity of Common Scab Disease of Potato

Significant variability in the level of resistance to common scab disease (to both disease incidence – tuber surface area affected, and severity – lesion depth) within potato (Solanum tuberosum) cv. Russet Burbank clonal lines and other selected processing cultivars was shown. Across two trials, cvs. Russet Nugget and Shepody and Russet Burbank clone British Columbia were consistently grouped within the cluster of least disease incidence whereas cv. Russet Norkotah and Vancouver unit four were grouped within the cluster showing greatest disease incidence. Russet Nugget and British Columbia were also consistently grouped within the cluster showing least disease severity, whereas Shepody and Russet Burbank clone Starks were grouped within the cluster with greatest severity scores. For some clones, consistency of rating across trials was not always apparent. Of note for the Tasmanian industry, there were differences within the five commercial Vancouver clonal units used interchangeably, with unit 4 showing significantly greater disease severity and incidence than some of the other units. However the extent of increased disease never exceeded 1.75 times the best of the other Vancouver units and the relevance of these results to the relative field performance of unit 4 remains to be tested.