Scab Severity Research Articles

Gene Expression During Acibenzolar-S-Methyl-Induced Priming for Potentiated Responses toVenturia nashicolain Japanese Pear

Pretreatment of leaves of the Japanese pear plant with the synthetic SAR activator acibenzolar-S-methyl (ASM) significantly reduced the severity of scab caused by the fungal pathogen Venturia nashicola. In this study, we have cloned three defence-related genes that are involved in the signalling network from Japanese pear, including a non-expresser of pathogenesis-related protein 1 (jpNPR1), a respiratory burst oxidase homologues (jpRBOH) and a mitogen-activated protein kinase (jpMAPK). We studied their expression in the same conditions that allow protection afforded by ASM, using quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR). Transcript level of jpNPR1 was not significantly elevated in Japanese pear leaves upon treatment with ASM and during infection with V. nashicola, and was highly and transiently primed in ASM-treated leaves after scab inoculation. Transcripts encoding jpRBOH and jpMAPK were directly responsive to ASM and were also early potentiated after pathogen inoculation. Involvement of NPR1 and reactive oxygen species in ASM-induced priming or potentiation is discussed.

Auxin-Induced Resistance to Common Scab Disease of Potato Linked to Inhibition of Thaxtomin A Toxicity.

Production of the phytotoxin thaxtomin A by pathogenic Streptomyces spp. is essential for induction of common scab disease in potato. Prior studies have shown that foliar application of sublethal concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and other auxin or auxin-like compounds significantly reduced severity and occurrence of common scab in subsequently produced tubers. However, the means of disease suppression by these compounds was not known. We confirm the disease suppressive activity of 2,4-D. Detailed tuber physiological examination showed that lenticel numbers, lenticel external dimensions, and periderm thickness and structure, physiological features believed to be critical to Streptomyces scabiei infection, were not substantially changed by 2,4-D treatments, negating a possible mechanism for disease suppression through alteration of these structures. In contrast, our studies show accumulation of 2,4-D in tubers of treated plants occurs and is associated with an enhanced tolerance to thaxtomin A. Applying 2,4-D to cultures of S. scabiei did not significantly alter in vitro growth of the pathogen. Thaxtomin A production by the pathogen was inhibited by 2,4-D, but only at the highest rate tested (1.0 mM), which is at least 200-fold more than is found in 2,4-D treated tubers. These data suggest 2,4-D has no direct effect on the pathogen or its virulence. Confirmatory evidence from studies with Arabidopsis thaliana seedlings demonstrated that the auxins 2,4-D and IAA ameliorate thaxtomin A toxicity. The evidence presented whereby auxin treatment inhibits toxicity of thaxtomin A secreted by the pathogen suggests a novel indirect means of disease suppression.

Paclobutrazol Soil Drenches Provide Partial Reductions in Symptoms of Apple Scab of Ornamental Trees and Guignardia Leaf Blotch of Horse Chestnut

Abstract A three year field trial was conducted using established apple (Malus cv. Crown Gold) and horse chestnut (Aesculus hippocastanum L.) to assess the efficacy of paclobutrazol (PBZ) root drenches against the foliar pathogens apple scab (Venturia inaequalis (Cooke) G. Wint.,) and Guignardia leaf blotch (Guignardia aesculi (Peck) VB Stewart). In the case of horse chestnut, pathogen severity of Guignardia leaf blotch was less (23–79%) in each of the three-year experimental periods in PBZ-treated trees compared to non-treated controls. Pathogen severity of apple scab was not affected during the first year after PBZ application; however, less (25–73%) disease severity was recorded in years 2 and 3 compared to non-PBZ treated controls. An increased PBZ concentration was associated with lower pathogen severity of both fungal pathogens. Irrespective of species, less pathogen severity in PBZ-treated trees was accompanied with greater leaf chlorophyll fluorescence (16–49%) values as measures of leaf photosynthetic efficiency. Marked differences in growth regulation between apple cv. Crown Gold (tolerant) and horse chestnut (sensitive) were recorded. PBZ applications resulted in less mean stem extension in both tree species but only reduced mean leaf size of horse chestnut. Based on the results of this investigation it is suggested that PBZ root drenches potentially offer a means of reducing the severity of apple scab and Guignardia leaf blotch for professionals involved with the nursery industry. However, where a zero pathogen control policy is required supplementary fungicide sprays would be needed. Similarly, the potential loss of aesthetics due to excessive growth regulation needs to be weighed against the benefits of pathogen protection and failure of PBZ to achieve total pathogen control may result in strong selection pressure for PBZ resistance in surviving populations.

A during-infection spray strategy using sulphur compounds, copper, silicon and a new formulation of potassium bicarbonate for primary scab control in organic apple production

In a field experiment conducted over two growing seasons, the effectiveness and phytotoxicity of inorganic fungicides such as sulphur, lime sulphur, copper, silicon and Armicarb (a new formulation of potassium bicarbonate) was compared with water for the control of primary apple scab infections in Belgium on high, medium and low scab-susceptible cultivars (cvs ‘Pinova’, ‘Pirouette’ and ‘Reinette des Capucins’, respectively). In order to drastically reduce the amount of fungicide applied in the orchard, two approaches were used: (1) a strategy involving spraying during the infection process, before fungal penetration and (2) a tunnel sprayer machine for treatment applications. Under field conditions highly favourable for disease, low rates of elemental sulphur (31.8 and 38.6 kg ha−1 year−1 in 2005 and 2006, respectively) combined with low rates of copper (2.1 kg ha−1 year−1 in both years) provided the best scab control and reduced scab severity on the fruits of cv. ‘Pinova’ by 97 and 98% compared with water control in 2005 and 2006, respectively. Lime sulphur was much more effective than wettable sulphur and appeared to be efficient at temperatures below 10°C, but its effectiveness against apple scab decreased if the treatments were applied 12–24 h later than in the ‘during-infection’ spray strategy. Armicarb used alone significantly reduced apple scab severity on the leaves and fruits of the three cultivars compared with the water control. Its effectiveness was as good as wettable sulphur applied using the same timing and dosage. Silicon reduced apple scab on fruits very slightly, but not on leaves. The amounts of wettable sulphur, lime sulphur, copper, silicon and potassium bicarbonate used in this experiment to control apple scab were not phytotoxic, did not increase fruit russet, did increase the yield of each cultivar and did not affect summer density of the beneficial Typhlodromus pyri. The potential and limitations of ‘during-infection’ spraying as a protection strategy against apple scab in organic farming are discussed.

The Identification and Detection of Spongospora subterranea and Factors Affecting Infection and Disease

Powdery scab caused by Spongospora subterranea is a serious disease, affecting both the quality and quantity of ware and seed potatoes, particularly in northern Britain where soil conditions are conducive to disease development. To enable accurate quantification of S. subterranea DNA for epidemiological studies, a real-time PCR assay specific to this pathogen was developed. This assay could reliably detect and quantify DNA from sporeballs, zoospores and plasmodia/zoosporangia of S. subterranea. The assay was used to measure the viability of sporeballs in soil, was combined with a tomato bait plant technique in order to study zoospore release and was used to study infection levels in potato under various environmental conditions. Both the incidence and severity of powdery scab were found to be influenced by temperature, soil type and soil moisture regime when tested under controlled conditions. Soil inoculum level did not have a significant effect on infection and disease development and even low levels of inoculum were regularly found to result in severe disease symptoms. Evidence from A GB-wide trial and a five year survey in England and Wales confirmed that there is no simple or consistent relationship between powdery scab inoculum on seed and disease developing on the progeny crop. Disease risk appears to be more related to the relative conduciveness of environmental conditions rather than inoculum level. Integration of control measures is likely to be the most effective way for growers to reduce the impact of the disease.

Aerobic Compost Tea, Compost and a Combination of Both Reduce the Severity of Common Scab (Streptomyces scabiei) on Potato Tubers

Aerobic Compost Tea, Compost and a Combination of Both Reduce the Severity of Common Scab (Streptomyces scabiei) on Potato Tubers

Reduction of potato tuber disease with organic soil amendments in two Prince Edward Islands fields

Single applications of liquid swine manure (LSM; ca. 49 hL/ha), ammonium lignosulfonate (ALS; a product derived from the pulp and paper processing industry, 50% solids at ca. 10 hL/ha), and Nature Safe (NS) 10:2:8 (N:P:K; an organic fertilizer made primarily of poultry feathers, ca. 8.6 t/ha) were made at two commercial potato (Solanum tuberosum) fields (sites P-I and P-II) in Prince Edward Island in the spring of 1999. Potato tubers were planted 3 weeks later and again in 2000 and 2001 without further addition of amendments. The effects on potato scab, black scurf, silver scurf, fusarium dry rot, verticillium wilt, and tuber yield were determined. Potato scab severity was significantly reduced (77%–84%) by NS compared with the controls at both sites in the first year. Scab severity remained significantly lower in the second year, but there was no difference from the controls in the third year. ALS significantly reduced scab severity (54%) compared with the controls at both sites in the first year, and scab severity remained lower than the control at only one site (P-I) and only for the second year. The mechanism by which ALS reduced scab severity is not known. LSM significantly reduced scab severity (38%) at one site only (P-II) and only in the first year. The control at site P-II was likely due to the favourable pH (5.4) for biotoxic activity of volatile fatty acids (VFAs) in the LSM, whereas the soil pH of 6.4 at site P-I was unfavourable for VFA toxicity to Streptomyces scabiei. The severity of black scurf was significantly reduced compared with the control at both sites by LSM (51% at P-I and 48% at P-II) and NS (40% at P-I and 61% at P-II) but in the first year only. There was no significant effect on the severity of silver scurf or fusarium dry rot or tuber yield by any amendment in any year. Verticillium wilt was significantly reduced compared with the control by ALS (88%) and NS (66%) at only one site (P-I) and only in the second year. Marketable yield was increased at both sites for NS and ALS, and at one site for LSM.

Doses de aplicação de nitrogênio e potássio em relação à podridão parda e sarna em ameixeira 'Reubennel' na região de Araucária, Paraná

O estado nutricional do hospedeiro pode influenciar na severidade de varias doencas de plantas, embora exista pouca informacao relacionando nutricao mineral com podridao parda e sarna em ameixeira. O objetivo do presente trabalho foi avaliar durante tres safras o efeito da adubacao nitrogenada e potassica, aplicada ao solo, na incidencia da podridao parda e na incidencia e severidade da sarna para ameixeira 'Reubennel', em pomar comercial no Municipio de Araucaria PR. Os tratamentos consistiam de cinco doses de nitrogenio (40, 80, 120, 160 e 200 kg ha-1 ano-1) aplicado na forma de ureia, e duas de K (55 e 110 kg ha-1 ano-1), aplicado na forma de cloreto de potassio. A incidencia de podridao parda foi avaliada na colheita determinando-se a incidencia em todos os frutos produzidos. A incidencia de sarna foi avaliada em amostra de 100 frutos, 20 desses considerados para avaliacao da severidade. A maior incidencia de podridao parda foi obtida na maior dose de K de 110 kg ha-1 ano-1 e nas maiores doses de N de 160 e 200 kg ha-1 ano-1 comparado com as outras doses utilizadas. O potassio na dose de 110 kg ha-1 ano-1 reduziu a severidade de sarna nos fruto em 21,2% para a safra de 2003/04. Ao contrario, a maior dose de N aumentou a severidade de sarna para a safra de 2004/05. A incidencia de sarna nao apresentou diferenca em funcao da adubacao nitrogenada utilizada. Nao houve interacao entre as doses de nitrogenio e o potassio na incidencia ou severidade de sarna e podridao parda.

Edaphic Soil Levels of Mineral Nutrients, pH, Organic Matter, and Cationic Exchange Capacity in the Geocaulosphere Associated with Potato Common Scab

ABSTRACT In order to determine possible relationships between geocaulosphere soil properties and severity of common scab of potato caused by Streptomyces scabies, soils were collected from representative commercial potato fields in Canada: in Simcoe and Dufferin Counties, Ontario and across Prince Edward Island (PEI) in August 2004. Soils immediately adjacent to tubers were sampled and analyzed for select edaphic factors and for pathogen presence using polymerase chain reaction (PCR) tests with primers that amplify a region of the TxtA gene involved in regulating the biosynthesis of the thaxtomin toxin family. Individual tubers were assessed visually for scab severity. The relationships between soil chemical factors and disease severity were investigated for each region to detect the strongest relationships. Principal component analysis revealed a distinctive clustering of samples with respect to disease severity in PEI but not in Ontario soils. Total and percent saturation of K (%K) were the only factors found associated with high disease severity in soils from both provinces. In PEI soils, pH, Mg, Ca, Cu, and %K, %Mg, %Ca, and %Na were associated with high disease severity, whereas cation exchange capacity (CEC) and Al were correlated with low disease severity soils. In Ontario, high Mn content was strongly correlated with low disease severity soils, whereas %K and organic matter content were correlated with disease severity. Partitioning samples into presence or absence of the TxtA PCR product with corresponding high or low severity showed further significant relationships in the data. There was an excellent correlation between Streptomyces spp. presence as detected by PCR and disease severity in PEI soils; however, the relationship was not as clear in Ontario soils, where many PCR-positive soils had low disease incidence. Principal component and partial least square analysis indicated that disease severity was predicted by soil factors such as organic matter, CEC, pH, Al, %Ca, %Mg, and %K for PEI but not for Ontario soils. The data reveal that the relationship between scab severity and soil chemical components is complex and potentially soil specific.

High proportions of nonpathogenic Streptomyces are associated with common scab-resistant potato lines and less severe disease

Streptomyces isolates were obtained from potato tubers with common scab lesions from 2 fields over a 3 year period in Minnesota and a 5 year period in Maine. Isolates were obtained from different potato cultivars or breeding lines and types of scab lesions. A majority of isolates could be classified as putative pathogens based on the presence of genes for biosynthesis of the pathogenicity determinant, thaxtomin, but large numbers of streptomycetes lacking genes for thaxtomin biosynthesis (presumably nonpathogenic) were also recovered. Most Streptomyces isolates recovered from raised and pitted lesions were pathogens, whereas mostly nonpathogenic isolates were recovered from unblemished potato skin or nonscab lesions. Fewer pathogenic than nonpathogenic isolates were recovered from the most resistant potato lines. The proportion and diversity of nonpathogenic isolates recovered was higher in Maine than in Minnesota. The association between greater numbers of nonpathogenic Streptomyces and less severe common scab suggests that the interaction between plant genotype and Streptomyces microbial community is important in determining the severity of common scab on potato, and emphasizes the role of complex interactions between plants and microbial populations on and near plant roots in plant disease outcomes.

Spongospora subterranea soil contamination and its relationship to severity of powdery scab on potatoes

We investigated soil contamination by Spongospora subterranea f. sp. subterranea (Sss) and disease severity of powdery scab in 29 potato fields in Hokkaido, Japan, using a hydroponic culture method with tomato seedlings as bait plants. The quantity of Sss infection on the roots of bait plants was evaluated using the polymerase chain reaction (PCR) and expressed in terms of the infection potential in the soil. The infection potential was positively correlated with the disease severity of harvested tubers, whereas the spore ball density determined using PCR had an indistinct relationship with disease severity. The infection potential can be useful in evaluating soil contamination and in applying countermeasures against powdery scab.

Characterization of a leucine-rich repeat receptor-like protein kinase (LRPK) gene from Japanese pear and its possible involvement in scab resistance

A gene encoding a leucine-rich repeat receptor-like protein kinase (LRPK) was identified from Japanese pear (Pyrus pyrifolia var. culta) using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) PCR. The Japanese pear LRPKp protein was composed of a leucine-rich extracellular ligand-binding domain, a transmembrane-spanning segment, and a cytoplasmic kinase domain; it had the highest sequence homology with apple LRPKm and Arabidopsis receptor-like protein kinase (RLK). Southern blot analysis revealed that the LRPKp belongs to a multigenic family, and a polymorphism in restriction fragment length was observed among Asian and European pear cultivars. The LRPKp transcripts were present at a high level in leaves and at a low level in flowers, while no expression was detected in fruits. The expression pattern of LRPKp was also studied by real-time RT-PCR in the scab susceptible cv. Kousui and resistant cvs. Kinchaku and Flemish Beauty after challenge inoculation with Venturia nashicola, the causal agent of Japanese pear scab. Transcripts of LRPKp were transiently enhanced in both the susceptible Kousui and to a great extent in the non-host-resistant Flemish Beauty. However, in resistant Kinchaku, expression of this gene was only weakly enhanced. Moreover, treatment with a resistance inducer, acibenzolar-S-methyl (ASM), which significantly reduces scab severity on potted trees of Kousui, was also able to induce and to potentiate the expression of this gene after inoculation. This suggests that the LRPKp gene is possibly involved in ASM-induced resistance.

Effect of fish emulsion used as a preplanting soil amendment on verticillium wilt, scab, and tuber yield of potato

Fish emulsion is used mainly as a fertilizer for crop production but, in a previous study, we have demonstrated its efficacy in suppressing damping-off [Rhizoctonia and Pythium spp.] in radish and cucumber in a peat-based substrate or soil. In the present study, fish emulsion was tested as a preplanting soil amendment to control soilborne diseases of eggplant and potato such as verticillium wilt [Verticillium dahliae and Verticillium albo-atrum] and scab [Streptomyces spp.]. Tests were conducted on 11 soils with different characteristics (pH, 5.2–7.2; organic-matter content, 1.0%–3.7%), originating from commercial potato fields in Ontario, New Brunswick, and Prince Edward Island with a history of verticillium wilt and scab. The amendment of soil with fish emulsion at rates of 0.5% and 1% (m/m) protected eggplant from verticillium wilt, and the 1% fish-emulsion treatment increased fresh and dry plant biomasses in the greenhouse. Microplot experiments with potato showed that the 1% fish-emulsion treatment significantly (P < 0.05) reduced scab severity (by 0.6–2.1 units on a scale of 0–6) in seven soils with low to moderate scab-disease pressure and significantly increased total tuber yield (by 41%–170%) in nine soils, compared with the control treatments. Fish emulsion also reduced potato petiole infection by V. dahliae in one soil. In potato field trials at two sites in 2004, treatment with 1% fish emulsion (20 000 L/ha) significantly (P < 0.05) reduced scab severity (by 0.8–1 units) and significantly increased scab-free tubers (by 132%–366%) and marketable-tuber (surface scab < 5%) yield (by two fold), compared with the control treatments. Total tuber yield was reduced at one site and not affected at the other. No effect of fish emulsion on scab and verticillium wilt or tuber yield was observed in a 2005 field trial. The reduction of scab or verticillium wilt by fish emulsion was not soil-specific and varied from year to year. The results indicated that fish emulsion was not effective in soils with high disease pressure.

Reaction of potato varieties toPotato mop-top virus infection in the andes

The unexpected detection ofPotato mop-top virus (PMTV) in potatoes growing at several locations in the U.S.A. and Canada in 2002 has led to the realization that this soil-borne virus may be widespread throughout the potato-producing regions of both countries. A lack of information concerning the response of U.S. cultivars to PMTV infection caused us to examine the effects of PMTV on field-grown plants at three locations in the Peruvian Andes where the virus and its fungal vector,Spongospora subterranea, are endemic. All 21 U.S. cultivars tested proved susceptible to PMTV infection, especially at La Victoria where the overall incidence of both PMTV and powdery scab was high. Symptom expression in the foliage proved to be an unreliable indicator of virus infection, and thus, more reliable serological (NCM-ELISA) and nucleic acid-based techniques (NASH and RT-PCR) were used to monitor tuber tissue for the presence of PMTV. As much as 25% of field-grown tubers of cvs Kennebec, Monona, and Norland were infected with PMTV, but no correlation between virus infection and either the incidence or severity of powdery scab for any variety was apparent. Although PMTV infection did not result in the appearance of spraing or other internal necrotic reactions resembling those reported from many North American and European locations, a disproportionate proportion of infected tubers did exhibit a reticulate surface cracking. In hydroponic culture, the proportion of cracked tubers increased dramatically when the nutrient solution was seeded with virus-infestedS. subterranea. Comparatively few tubers of cvs Monona and Russet Burbank exhibited this surface cracking, suggesting that these two varieties may be somewhat tolerant to PMTV infection.

Effect of leaf litter management on scab development in an organic apple orchard

Ascospores of Venturia inaequalis produced on scabbed leaves in the leaf litter are the main source of primary inoculum, causing infections in apple orchards. The purpose of this 2-year experiment, carried out in a commercial organic orchard, was to assess the effect of combining leaf sweeping from the alleys with leaf ploughing in within the row on scab inoculum and development. In 2003, scab severity was monitored at different distances from the unremoved leaf areas to estimate ascospore spreading. Scab incidence and severity were low in 2003 and high in 2004. At fruit harvest, both years, the leaf litter removal method reduced fruit scab incidence by 82.5% and 54.6% respectively, and fruit scab severity by 74.0% and 67.7%, respectively. Measures of scab lesion gradient indicated that ascospore spreading was not important beyond a 20 m distance from the source. The number of trapped ascospores observed in 2004 in the leaf removal treatment was reduced by 95%. Results from this 2-year experiment showed that leaf litter management by leaf removal allowed a reduction in apple scab inoculum and development and demonstrated the benefit of a complete removal of the leaf litter, when combining leaf sweeping from the alleys with leaf ploughing in within the row.

Resistance of potato genotypes to common and netted scab‐causing species of Streptomyces

Common and netted scabs are two disfiguring bacterial diseases of potato tubers, caused by various groups of Streptomyces species. Common scab, caused primarily by Streptomyces scabies and Streptomyces europaeiscabiei, is characterized by more or less deep pustules on the tuber surface, while symptoms of netted scab, caused mainly by Streptomyces reticuliscabiei, are superficial, corky alterations of the tuber periderm. Some isolates of S. europaeiscabiei are able to induce both common and netted scab symptoms, and therefore constitute a third pathogenicity group. Like most bacterial diseases, potato scabs would be best controlled by using resistant cultivars. Repeated experiments with soil artificially infested with isolates of three species representative of the three pathogenicity groups showed the level and stability of cultivar resistance, as well as the existence of a range of aggressiveness among different isolates. The distribution of scab severity indexes recorded on a collection of 16 potato cultivars and 27 breeding clones grown in soil infested with common scab‐inducing isolates was continuous, suggesting isolate nonspecific quantitative resistance. Least susceptible cultivars were Nicola, BF15, Sirtéma, and Charlotte, while Urgenta, Désirée, Ondine and Bintje were very susceptible. The same genotypes proved either highly susceptible (e.g. cvs Bintje, Désirée or Carmine) or highly resistant (e.g. cvs Charlotte, Sirtéma, Monalisa, BF15 or Belle de Fontenay) to isolates forming netted scab symptoms, suggesting isolate‐specific qualitative resistance. The ability was confirmed of some isolates of S. europaeiscabiei to induce one or the other type of symptoms depending on cultivar and soil temperature.

Effect of soil inoculum level and environmental factors on potato powdery scab caused by Spongospora subterranea

The effects of soil inoculum level and three environmental factors (soil type, soil moisture regime and temperature) on the incidence and severity of powdery scab caused by Spongospora subterranea were investigated in potato plants grown under controlled environmental conditions. Symptoms of powdery scab on tubers were assessed visually, after which DNA was extracted from tuber peelings and quantified in a real‐time polymerase chain reaction assay using primers and a TaqMan® probe specific to S. subterranea to establish tuber infection levels. Soil inoculum concentration of S. subterranea did not significantly affect the incidence and severity of either tuber infection or powdery scab symptoms at maturity. No significant differences in disease incidence and severity were found between sandy, loamy and clay soils, although the two lighter soils yielded more powdery scab than clay soil. Constant dampness of the soil resulted in significantly more disease than a fluctuating moisture regime. Infection and disease levels were high at all three temperatures tested (9, 12 and 17°C), but symptoms were most severe at 12°C. The percentage of plants with infected tubers did not increase after tuber initiation, although the amount of S. subterranea DNA detected in tubers and the severity of powdery scab symptoms increased in mature plants. Latent tuber infections were found to be common, especially under conditions suboptimal for disease development. This new information may be important for the prevention of powdery scab in potato‐growing areas around the world.

The use of molecular diagnostics to investigate the epidemiology of potato diseases

In recent years, quantitative molecular diagnostic assays based on real-time PCR have been developed for many pests and pathogens of potato. In addition, simple sequence repeat markers have been developed and used to track isolates of Phytophthora infestans. These diagnostic assays are now being used as tools to study unresolved questions in the epidemiology of potato diseases including late blight, powdery scab and black dot. Examples of various investigations designed to examine the relative contribution of seed and soil-borne inoculum in causing black dot and powdery scab on progeny tubers, the effect of environmental factors on the incidence and severity of powdery scab and the survival of asexual and sexual inoculum of P. infestans in soil are described. Consideration is given to the development of appropriate diagnostic assays, their use in conjunction with relevant and robust sampling techniques, and the interpretation of results to inform disease risk assessment and control strategies through industry collaboration.

Influence of crop rotation and conservation tillage practices on the severity of soil-borne potato diseases in temperate humid agriculture

A field trial was established in 1994 in Prince Edward Island, Canada, to determine the effects of 2- and 3-year crop rotations, with either conventional or minimum tillage treatments, on the severity of potato diseases caused by soil-borne pathogens. The 2-yr rotation consisted of spring barley and potato (cv. Russet Burbank) and the 3-yr rotation was barley (undersown with red clover), red clover and potato. In the potato years only, the main plots were split to examine conventional and minimum tillage (sub-plot) treatments. Potato diseases were assessed during the period 2000–2002, 6–8 yr after the initial establishment of the field trial. In each year of the study, potato plants grown in 3-yr rotations developed significantly (P = 0.05) lower levels of stem and stolon canker and black scurf (Rhizoctonia solani) compared with those from 2-yr rotations. Overall, rotation duration (not tillage type) was the main factor contributing to reduction of disease caused by R. solani. The severity of silver scurf (Helminthosporium solani) was significantly (P = 0.05) lower in tubers from plots managed in 3-yr rotations under minimum tillage practices in 2000, significantly higher in plots managed with minimum tillage in 2001, and unaffected by treatment in 2002. The severity of dry rot (Fusarium spp.) was significantly (P = 0.05) lower in tubers from plots managed with 3-yr rotations under minimum tillage practices in 2000, but was unaffected by either tillage regime in 2001 and 2002. The severity of common scab (Strep tomyces scabiei) was low and not influenced by tillage or rotation in any year of study. In summary, a 3-yr crop rotation combined with minimum tillage reduced potato diseases caused by R. solani and did not affect disease caused by other soil-borne pathogens. From a crop health perspective, there appear to be no constraints to the use of minimum tillage practices in association with 3-yr rotations in potato production in regions of temperate humid agriculture. Key words: Crop rotation, minimum tillage, potato, Solanum tuberosum, soil-borne pathogens, Rhizoctonia solani

Moisture Sources in Relation to Conidial Dissemination and Infection by Cladosporium carpophilum Within Peach Canopies

ABSTRACT Cladosporium carpophilum, the causal agent of peach scab, overwinters in lesions on 1-year-old twigs, from which conidia infect the developing fruit during spring and early summer. Twig lesions constitute the sole source of initial inoculum; therefore, the mode of dissemination of conidia from such lesions to the fruit is of considerable interest. In a 4-year study, we determined the relative importance of air- versus water-borne conidia and their interaction with different fruit wetness sources (splash, twig runoff, and dew) in a peach orchard with areas that had been treated or not treated with fungicide the previous year. The rareness of scab twig lesions in the previously sprayed trees implied that fruit infection in these trees would occur primarily by airborne conidia from unsprayed trees nearby (located within the same tree row or the adjacent row). In the unsprayed areas, additional infections could occur by short-distance waterborne dissemination of conidia from locally abundant twig lesions via splashing or runoff. Beginning at calyx fall, individual fruit were protected from splash by rain shields, protected from runoff by cotton wicks placed proximal to the peduncle, or left untreated. Rain shields were adjustable, allowing rain or dew to be excluded selectively. Various combinations of the shield and wick treatments were implemented in the previously sprayed and unsprayed areas, and statistical comparison of fruit scab severity between individual treatments by linear contrasts allowed us to untangle the relative contributions of the various sources of inoculum and fruit wetness. Results showed that aerial dissemination of conidia contributed little to fruit scab development, even in the presence of fruit surface wetness caused by splashing, runoff, or dew. In contrast, waterborne conidia contributed considerably and significantly (P < 0.0001) to disease development. This was due primarily to the importance of splash in disseminating conidia from twig lesions to the fruit, given that exclusion of splashing via rain shields decreased disease severity by >90%. Runoff water from the twig to the fruit via the peduncle also contributed to scab development, as evidenced by the fact that exclusion of runoff by cotton wicks reduced disease severity by 31.6 to 44.9%; however, this effect was not always statistically significant. The exclusion of dew did not reduce scab severity (P > 0.4), suggesting that it played a limited role in infection in the presence of other fruit wetness sources.