Scab Severity Research Articles

The Influence of Inducing Agents Applied by Soil Drenches on Disease Severity of Apple and Pear Scab

Apple and pear scab are foliar diseases of ornamental and fruiting apple and pear trees. Unmanaged, yield and aesthetic losses can be severe. Overreliance on synthetic fungicides means novel means of disease management are required. Field trials were conducted using apple (Malus cv. Crown Gold) and pear (Pyrus communis ‘Williams Bon Chrétien’) to assess the efficacy of a range of commercially available inducing resistance (IR) agents (harpin protein, potassium phosphite, salicylic acid derivative, and chitosan) as root drenches against both scab diseases. A synthetic fungicide (penconazole) spray program used within the UK for apple and pear scab control was included for comparison. Each IR agent was applied four times, (i) before the visible appearance of scab (April through June, i.e., preventatively) or (ii) after symptoms of scab were visibly observed (June through August, i.e., curatively). Limited efficacy as scab protectants was demonstrated when IR agents were applied curatively. Likewise, limited efficacy was recorded when IR agents were applied once or twice as a preventative measure. However, when IR agents were applied as root drenches greater or equal to three times, efficacy as scab protectants was confirmed (increased leaf chlorophyll content, increased fruit yield, reduced leaf and fruit scab severity). A synthetic fungicide penconazole spray program provided the greatest protection against apple and pear scab in all trials when sprayed preventatively rather than curatively. Results suggest application of at least three root drenches from April through June with an appropriate IR agent provides a useful addition to existing methods of apple and pear scab management under field conditions.

Standard area diagrams to assess scab severity of entire‐margined leaves of sour passion fruit

AbstractScab caused by the Cladosporium cladosporioides complex is one of the most important diseases affecting passion fruit yield and fruit quality. Standard area diagrams are useful tools to improve the accuracy and reliability of the estimates of disease severity in many pathosystems. The interpretation of the accuracy of the resulting disease data may be the basis for providing better disease management. This study aimed to develop and validate standard area diagrams (SADs) to quantify scab severity of entire‐margined leaves of sour passion fruit plants. The SADs are composed of ten images of scab severity values (0.4%, 3.5%, 5.4%, 8.0%, 11.1%, 16.2%, 26.1%, 38.2%, 47.9% and 58.9%). Twenty raters (ten experienced and ten inexperienced ones) evaluated the SADs. The raters estimated the same set of fifty images twice. In the examination, the SADs were used, while in the second examination, they were not used. Accuracy was significantly improved by using the SADs. The coefficients of bias (Cb) were 0.96 and 0.98 for the experienced raters and 0.75 and 0.98 for the inexperienced raters without and with SADs, respectively. The correlation coefficient (r) values were 0.95 and 0.97 for the experienced raters and 0.89 and 0.97 for the inexperienced raters without and with the use of the SADs, respectively. Lin's concordance correlation coefficient (ρc) values were 0.91 and 0.95 for the experienced raters and 0.66 and 0.95 for the inexperienced raters without and with the use of the SADs, respectively. Moreover, the coefficient of determination (R2) significantly improved when the SADs were used. The SADs proposed in this study, if properly used, may improve both the accuracy and reliability during the evaluation of scab severity of entire‐margined leaves of sour passion fruit plants.

Designing a crop rotation strategy to manage Streptomyces scabies causing potato scab in north India

AbstractCommon scab of potato caused by various species in the genus Streptomyces has assumed serious proportions in certain potato‐growing regions of North India. Although it does not have significant effect on tuber yield, it downgrades quality and reduces marketability. Being soil‐borne, this disease is often difficult to manage. Therefore, the present studies were conducted to evaluate different potato‐based crop rotations prevalent in north India as a part of a strategy to manage Streptomyces scabies population in scab sick fields. Our results showed that S. scabies population in soil can be effectively reduced either by keeping the field fallow after potato or by including mungbean or sunhemp as green manure, in a single year cropping sequence over a period of four years. Three crop rotations, viz. fallow–rice–potato, mungbean–rice–potato and sunhemp–rice–potato, showed maximum decrease in the population of S. scabies in soil resulting in reduced scab severity. The mungbean‐ and sunhemp‐based crop rotations also enhanced rhizospheric soil microflora especially fluorescent pseudomonads and Trichoderma spp.

Low-dose foliar treatments of the auxin analog 2,4-D reduce potato common scab and powdery scab for multiple potato cultivars and enhance root development

Common scab and powdery scab are major soilborne diseases of potato (Solanum tuberosum). The agricultural cost of both diseases is due to discounting or rejection of infected seed tubers or the formation of lesions on the surface of the tubers that make them unmarketable or, for powdery scab, yield loss. There are no widely accessible and efficacious disease management options for either common scab or powdery scab. In prior work, low-dose foliar treatment with the auxin analog herbicide 2,4-D has been shown to reduce common scab severity of select S. tuberosum cultivars in field and greenhouse settings. In the present study, we address the broad applicability of 2,4-D in disease management by testing the efficacy of 2,4-D treatment across diverse potato cultivars and in suppressing multiple scab diseases of potato. Specifically, we tested whether low-dose 2,4-D treatment is broadly efficacious on multiple white potato cultivars used in Eastern United States potato production. Additionally, we sought to determine whether 2,4-D is efficacious for mitigating powdery scab for Russet Burbank potatoes in a commercial field setting. In two years of field trials, low-dose foliar 2,4-D treatment significantly reduced common scab of all tested potato cultivars with no impact on total tuber yield. In two years of field trials in Tasmania, Australia, low-dose foliar 2,4-D treatment also reduced powdery scab of Russet Burbank potato with no impact on total tuber yield while stimulating potato plant root growth. The ability of 2,4-D to suppress two major tuber diseases and stimulate potato root growth warrants further investigation and optimization for integration into commercial growing systems.

A Comparison of Ground-Based Air-Blast Sprayer and Aircraft Application of Fungicides to Manage Scab in Tall Pecan Trees.

Pecan scab (caused by Venturia effusa) is a destructive disease of pecan in the southeastern United States. Susceptible cultivars must be sprayed with fungicide every 10 to 21 days to ensure yield and kernel quality. Fungicide is most often applied using large orchard air-blast sprayers. Pecan trees grow tall, and air-blast sprays result in a gradient in spray deposition and consequently of scab. Aerial fungicide application is also practiced. Disease distribution and spray deposition of the two methods have not been compared but will provide information aiding decisions on spray application methods. We compared air-blast, aerial, and air-blast + aerial applications for efficacy controlling scab at five heights in the canopy of 25-m cultivar Schley pecan trees. There was a negative relationship between scab severity and height in control trees, a positive linear relationship with height in air-blast treated trees, and a generally negative linear relationship between scab severity and height in aerially treated trees. Air-blast + aerial treatments resulted in low severity of scab at all heights. Spray deposition on water-sensitive cards indicated a declining gradient with height using an air-blast sprayer, whereas aerial applications resulted in a low deposition at all sample heights. Air-blast sprays tended to result in less good control at heights >12.5 m, and aerially treated trees at ≤7.5 m. The results provide insight into the efficacy and advantages of these methods for applying fungicide to control scab in tall pecan trees; further research is needed to better understand the impact of frequency and timing of these two methods.

Management and Soil Conditions Influence Common Scab Severity on Potato Tubers Via Indirect Effects on Soil Microbial Communities.

Common scab, caused by Streptomyces scabies and related species, is a potato tuber blemish disease that causes reductions in marketable yield worldwide. Evidence of suppression of common scab by indigenous soil microbial populations has been found in several studies. However, we lack a comprehensive understanding of how common scab severity relates functionally to potato varieties, farming systems, soil physical and chemical properties, and soil microbial communities. These factors may affect disease directly or indirectly by affecting one of the other variables. We performed a survey of 30 sampling locations across 12 fields in Wisconsin and used structural equation modeling to disentangle the direct effects of potato market classes, farm management (conventional versus organic), and soil physiochemical properties on common scab severity from their indirect effects mediated through soil bacterial and fungal communities. We found that, although potato market classes affected disease severity directly, the effects of farm management and soil physiochemistry were best explained as indirect, mediated by their impacts on soil bacterial communities. This suggests that evaluating the consequences of specific management practices for soil microbial communities may be useful for understanding disease pressure across fields.

Standard area diagram set for scab quantification in trilobed leaves of the sour passion fruit

ABSTRACT: The study aimed to develop and validate a standard area diagram set (SADs) to improve accuracy, precision, and reproducibility in the assessment of scab severity in trilobed leaves of the sour passion fruit. The SADs comprised six severity levels (1%; 3%; 6%; 11%; 21%; and 33%). The SADs increased the accuracy of the estimates by reducing the constant and systematic errors, and the tendency to overestimate the actual severity, limited the number of evaluators who presented this trend, in addition to increasing the bias correction factor. The inexperienced evaluators were the ones that benefited the most from the SADs use. Once the SADs was introduced, 80% and 100% evaluators gave estimates free from constant and systematic errors, respectively. The improvement in precision was verified by the increase in the coefficient of determination and Lin’s concordance correlation coefficient, by reducing absolute errors and increasing the reproducibility of the estimates.

Monitoring of Wheat Scab Using the Specific Spectral Index from ASD Hyperspectral Dataset

It is highly important to accurately monitor wheat scab and provide technical guidance for the crop pests and diseases. In this study, relevant analysis was performed among spectral reflectance, first-derivate data, and the disease severity data through ASD hyperspectral data. Two sensitive spectral wavelength ranges of 450–488 nm and 500–540 nm were selected. Then, a new wheat scab index (WSI) consisting of the two bands was proposed. The inversion models of the scab severities were comparatively built by unitary linear regression and multiple stepwise regression techniques. The results showed that the WSI had a significant linear relationship with severity of disease compared with other commonly used spectral indices. The fitting R2, testing R2, and RMSE were 0.73, 0.70, and 13.41, respectively. The multiple stepwise regression model established using the WSI, SDg/SDb, NBNDVI, and SDg as independent variables was better than the single-variable model. Our results suggest that WSI can be used to provide scientific guidance for monitoring and precise management of wheat scab disease.

Effects of Quantitative Ordinal Scale Design on the Accuracy of Estimates of Mean Disease Severity

Estimates of plant disease severity are crucial to various practical and research-related needs in agriculture. Ordinal scales are used for categorizing severity into ordered classes. Certain characteristics of quantitative ordinal scale design may affect the accuracy of the specimen estimates and, consequently, affect the accuracy of the resulting mean disease severity for the sample. The aim of this study was to compare mean estimates based on various quantitative ordinal scale designs to the nearest percent estimates, and to investigate the effect of the number of classes in an ordinal scale on the accuracy of that mean. A simulation method was employed. The criterion for comparison was the mean squared error of the mean disease severity for each of the different scale designs used. The results indicate that scales with seven or more classes are preferable when actual mean disease severities of 50% or less are involved. Moreover, use of an amended 10% quantitative ordinal scale with additional classes at low severities resulted in a more accurate mean severity compared to most other scale designs at most mean disease severities. To further verify the simulation results, estimates of mean severity of pear scab on samples of leaves from orchards in Taiwan demonstrated similar results. These observations contribute to the development of plant disease assessment scales to improve the accuracy of estimates of mean disease severities.

Control of Insects and Diseases with Intelligent Variable-rate Sprayers in Ornamental Nurseries

Abstract Intelligent spray technology can reduce pesticide use and safeguard the environment; however, its ability to effectively control insects and disease must be validated before its adoption by growers. Comparative tests for two different laser-guided variable-rate intelligent sprayers and the same sprayers with conventional constant-rate mode were conducted to control pests at two ornamental nurseries in two growing seasons in Ohio. Crabapple [Malus ‘Sutyzam’ (Sugar Tyme®), M. sargentii], apple (Malus pumila), maple [Acer ×freemanii ‘Jeffersred' (Autumn Blaze®), A. rubrum ‘Franksred' (Red Sunset®) and A. rubrum], birch (Betula nigra and Betula populifolia ‘Whitespire'), London planetree (Platanus ×acerifolia ‘Bloodgood') and dogwood (Cornus florida) were used as the test plants. Intelligent spray technology reduced pesticide use by 56.1% and 51.8% on average at the two nurseries, respectively. Compared to conventional air-assisted sprayers, severity of scab on apple trees and powdery mildew in dogwood was reduced on intelligent spray-treated plants at one nursery, and there were equal or fewer leafhoppers in maple trees and aphids in birch trees when sprayed using intelligent spray technology at both nurseries. These results suggest that intelligent, variable-rate sprayers achieve equivalent or greater insect and disease control in ornamental tree nurseries compared to conventional, constant-rate sprayers. Index words: aphid, apple scab, environmental protection, leafhopper, precision spray, powdery mildew, sustainable. Species used in this study: apple (Malus pumila Mill), birches (Betula nigra L, Betula populifolia Marsh. ‘Whitespire'), crabapples [Malus ‘Sutyzam' (Sugar Tyme®), M. sargentii Rehder], dogwood (Cornus florida L.), maples [Acer ×freemanii E. Murray ‘Jeffersred' (Autumn Blaze®), A. rubrum L. ‘Franksred'(Red Sunset®) and A. rubrum L.], London planetree [Platanus ×acerifolia (Ait.) Willd. ‘Bloodgood'].

Standard Area Diagram Set for Scab Evaluation in Fruits of sour Passion Fruit

Scab (Cladosporium spp.) significantly comprises the commercial acceptance of sour passion fruit (Passiflora edulis) because of the deformed and atrophied fruit appearance resulting from the development of the lesions. Therefore, the objective of this study was to elaborate and validate a standard area diagram set (SADs) for the severity evaluation of scab in fruits of sour passion fruit. The SADs comprised eight severity levels (0.6; 1; 2; 4; 8; 16; 37; and 46%) and was validated by 20 raters (G1 and G3, inexperienced; G2 and G4, experienced). Initially, all raters performed a non-aided SADs evaluation of the scab severity. Afterward, G1 and G2 completed the second evaluation without the proposed SADs, whereas G3 and G4 performed a SADs-aided assessment of the disease severity. The accuracy and precision of the evaluations were determined by simple linear regression and by the Lin’s concordance correlation coefficient. Constant and systematic errors decreased with the use of the SADs, demonstrating an approximation between the estimated and the actual values. Precision increased with an increase in the coefficient of determination. Also, the absolute error reduced by 66% (G3) and 47% (G4). Therefore, 94.4% (G3) and 98.8% (G4) of the estimates had up to ±10% of errors, which corresponds to a 20.4% (G3) and 5.6% (G4) increment in the estimates with errors within this variation range. As a result, accuracy and precision were higher in the SADs-aided groups. Inexperienced raters were the most benefited by the use of the SADs. The accuracy and precision of the non-aided groups had a slight or no increase when compared with the SADs-aided groups.

Efficacy of some bio-control agent formulations on scab of cowpea and groundnut late leaf spot in the Nigerian savanna

Field experiment on fungicidal efficacy of Trichoderma harzianum, Pseudomonas fluorescens and Bacillus cereus formulations against cowpea scab and groundnut late leaf spot was conducted. The trials were conducted in 2016 and 2017 rainy seasons on two farms of the Institute for Agricultural Research located at Samaru, Zaria (110 12′ N, 070 37′ E) in the Northern Guinea savanna and Minijbir, Kano (08031' E., l20 03' N) in the Sudan savanna. The formulations were applied as seed treatment, foliar spray and also incorporated into the soil by broadcasting the powdered formulations on the ridges. The experiment was laid in Randomized Complete Block Design replicated three times with five treatments which consisted of the three formulations, Funguforce (Mancozeb + Carbendazim) and control. Data were taken on plant emergence, pod yield, disease incidence and severity. Irrespective of the treatments, incidence of scab at both Samaru and Minjibir were at par. However, Funguforce application gave lowest mean scab severities of 10% and 8.5% at Minjibir and Samaru respectively in 2017. In 2016 at Samaru, P. fluorescens had the lowest incidence (50.57 %) while B. cereus had the lowest severity (14.40 %) of groundnut late leaf spot. At Minjibir however, T. harzianum recorded the lowest incidence (48.75 %) and severity (18.22 %). The incidence and severity of groundnut late leaf spot at Samaru in 2017 did not significantly vary while at Minjibir lowest incidence (9.77 %) and severity (9.73 %) were recorded on Funguforce. The microbial formulations recorded an average pod yield increase of 10 – 58 % which was comparable with that of Funguforce. The use of formulations of T. harzianum, P. fluorescens and B. cereus in the management of fungal diseases of cowpea and groundnut in the Nigerian savanna is therefore recommended.Keywords: Genetic-crosses, RILs, Polymorphisms, Genetic-Markers, EPN

The effect of phosphonate rate on control of scab on pecan

Scab, caused by the plant pathogenic fungus Venturia effusa, is the major disease of pecan in the southeastern USA. In widely grown susceptible cultivars, frequent application of fungicides is the only option to control the disease. Unfortunately resistance to some fungicides has developed so management of the chemistries available is critical for the industry. Understanding how rate affects disease control is important to ensure that control is maximized. We compared the effect of different rates of phosphonate products in reducing scab in a total of six field experiments from 2015 to 2018 at two locations in Georgia. Cultivar Desirable was used in four experiments, and cv. Wichita was included in 2 years at one location. Depending on experiment, four rates of ProPhyt® (2.3, 3.5, 5.3 and 7.0 L/Ha), three rates of Rampart® (2.3, 4.7 and 7.0 L/Ha), or two rates of K-phite® 7LP (2.3 and 7.0 L/Ha) were tested. Also depending on experiment, 5 to 10 sprays of phosphonate product were applied. Incidence of scab was invariably high on fruit from the non-treated, control trees and most often high on treated trees. However, treatment with the higher rates of phosphonate product most often significantly or numerically reduced severity of scab compared to the lower rates of the products applied on foliage and fruit. The label-stated weight of the active ingredient (phosphonate) per unit volume is similar in these products and they reduced the severity of scab similarly compared to the control and to each other at comparable rates. We found that applying the higher rates we tested (3.5–7.0 L/Ha) most often controlled scab better when compared to the lowest rate (2.3 L/Ha). In 2015 fruit weight was increased where phosphonate application resulted in reduced disease, but in 2016 the effect was unclear due to a late and mild epidemic of scab likely having limited impact on the fruit size. Regardless of epidemic intensity, higher rates of phosphonate provided more efficacious control. Phosphonate products are a vital chemistry to manage scab for the pecan industry in the southeastern USA and our results demonstrate that higher rates contribute to more efficacious control of this yield-limiting disease.

Development and validation of a standard area diagram to assess common scab in potato tubers

Common scab (Streptomyces scabies) is a severe disease of the potato (Solanum tuberosum L.), which can seriously reduce the commercial value of tubers. More accurate and precise estimates of incidence and severity are required for its effective control. Standard area diagrams (SADs) were developed to aid the visual assessment of common scab severity in potato tubers. A set of 115 tuber images, with and without symptoms, was used to develop the scale, which contained 12 images of tubers with different symptoms. The SADs were then tested by 12 evaluators with no experience in appraising plant diseases, who assessed a set of 50 images 4 times, first without the aid of the SADs, then twice using the proposed scale, and then again using the scale of James (Canadian Plant Disease, 51(2), 39–65, 1971). The use of SADs increased the accuracy (coefficient of bias (Cb) = 0.99, 0.95 and 0.95 with SADs, without SADs and with the James SADs, respectively) and agreement (Lin’s coefficient (ρc) = 0.97, 0.90 and 0.88 with SADs, without SADs and with James SADs, respectively) of the severity estimates. Severity estimates were also more reproducible with the use of the scale (determination coefficient (R2) = 0.93 using SADs and R2 = 0.84 without the use of SADs). The results obtained with the scale were also repeatable (intra-rater determination coefficient (R2) = 0.94). The proposed SADs have been shown to be useful in evaluating common scab in potatoes, increasing the accuracy, precision and reliability of disease severity estimates.

The occurrence of potato common scab correlates with the community composition and function of the geocaulosphere soil microbiome

BackgroundSoil microorganisms can mediate the occurrence of plant diseases. Potato common scab (CS) is a refractory disease caused by pathogenic Streptomyces that occurs worldwide, but little is known about the interactions between CS and the soil microbiome. In this study, four soil-root system compartments (geocaulosphere soil (GS), rhizosphere soil (RS), root-zone soil (ZS), and furrow soil (FS)) were analyzed for potato plants with naturally high (H) and low (L) scab severity levels. We aimed to determine the composition and putative function of the soil microbiome associated with potato CS.ResultsThe copy numbers of the scab phytotoxin biosynthetic gene txtAB and the bacterial 16S rRNA gene as well as the diversity and composition of each of the four soil-root system compartments were examined; GS was the only compartment that exhibited significant differences between the H and L groups. Compared to the H group, the L group exhibited a lower txtAB gene copy number, lower bacterial 16S copy number, higher diversity, higher co-occurrence network complexity, and higher community function similarity within the GS microbiome. The community composition and function of the GS samples were further revealed by shotgun metagenomic sequencing. Variovorax, Stenotrophomonas, and Agrobacterium were the most abundant genera that were significantly and positively correlated with the scab severity level, estimated absolute abundance (EAA) of pathogenic Streptomyces, and txtAB gene copy number. In contrast, Geobacillus, Curtobacterium, and unclassified Geodermatophilaceae were significantly negatively correlated with these three parameters. Compared to the function profiles in the L group, several genes involved in “ABC transporters,” the “bacterial secretion system,” “quorum sensing (QS),” “nitrogen metabolism,” and some metabolism by cytochrome P450 were enriched in the H group. In contrast, some antibiotic biosynthesis pathways were enriched in the L group. Based on the differences in community composition and function, a simple model was proposed to explain the putative relationships between the soil microbiome and CS occurrence.ConclusionsThe GS microbiome was closely associated with CS severity in the soil-root system, and the occurrence of CS was accompanied by changes in community composition and function. The differential functions provide new clues to elucidate the mechanism underlying the interaction between CS occurrence and the soil microbiome, and varying community compositions provide novel insights into CS occurrence.

A comparison of organic fungicides: alternatives for reducing scab on pecan

In the southeastern USA, the most widespread and damaging disease of pecan is scab, caused by Venturia effusa. Although scab can be controlled using conventional chemical methods, organic pecans that attract a premium price mandate the use of organic fungicides. Also, organic production is an environmentally sustainable method. However, where susceptible pecan cultivars are grown, there are limited options for organic management of scab. We conducted experiments to compare organic fungicides to control scab on the susceptible cv. Desirable in 2011, 2012, 2014, 2015, and 2016. The alternatives compared included Bordeaux mixture, compost tea, sodium bicarbonate, Bacillus subtilis, sulfur, cuprous oxide, and extract of the Giant Knotweed (Reynoutria sachalinensis). Rainfall and scab severity differed between seasons. There was consistently low severity on foliage, with little or no difference between treatments. Similarly at the time of the first fruit assessment, the severity was low and the differences in severity small and inconsistent between seasons and treatments. However, by the time of the second fruit assessment, severity of scab had increased and consistent differences among treatments existed (except in the drought year of 2011, when scab severities were very low and similar to the control). In all other years, the control treatment had significantly more severe scab compared to some (2012 and 2014) or all other treatments (2015 and 2016). Extract of the Giant Knotweed as a fungicide was included in 2012, 2014, 2015, and 2016, and fruit on those trees had less severe scab in all years compared to that on fruit of the control trees. In three seasons (2012, 2015, and 2016), applications of Bordeaux mixture resulted in a reduction in scab severity. Compost tea, Sodium bicarbonate, B. subtilis, sulfur, and cuprous oxide significantly reduced scab compared to the control in one or two seasons, but were not consistent among seasons, and were never more efficacious compared to the extract of the Giant Knotweed. Extract of the Giant Knotweed and Bordeaux mixture appear to offer the greatest potential as organic approaches for managing scab in pecan. However, wherever possible, planting of scab resistant cultivars should be considered as a first line of defense.

Evaluation of Silicon Fertilizers and a Resistance Inducing Agent for Control of Apple and Pear Scab under Field Conditions

Two field trials were conducted using established apple (Malus cv. Golden Delicious) and pear (Pyrus communis ‘Williams’ Bon Chrétien’) to assess the efficacy of a range of silicon (Si) fertilizers, a resistance-inducing (IR) agent based on salicylic acid and a combination of Si + IR when applied at four growth stages of tree development (i.e., bud break, green cluster, 90% petal fall, early fruitlet) against the foliar pathogens apple (Venturia inaequalis) and pear (V. pirina) scab. In addition, a comparative evaluation of a conventional synthetic fungicide (penconazole) used within the UK for apple and pear scab control was performed. Greatest protection against apple and pear scab was provided by a combination of a Si fertilizer + IR agent and the synthetic fungicide penconazole in both the 2014 and 2015 field trials where statistically comparable degrees of control were achieved. Efficacy as scab-protectant compounds (i.e., reduced scab severity of leaves and fruit) enhanced leaf chlorophyll content and fruit yield, and was confirmed when each Si fertilizer was applied at four growth stages during two growing seasons; however, there was little difference in the degree of control conferred by each Si fertilizer. The IR agent used alone proved the least effective scab protectant compound.

Efficacy of Bordeaux mixture in reducing pecan scab in the southeastern USA

Pecan scab (Venturia effusa) is the most important yield-limiting disease of pecan (Carya illinoinensis) in the southeastern USA. On susceptible cultivars, which are widely grown, conventional fungicides are applied intensively to minimize losses to the disease. Producers of organic pecan must either select resistant cultivars at planting or find alternative methods to manage scab if the cultivars are susceptible. There has been little research on the efficacy of organic products against scab, although historically, Bordeaux mixture was applied (until the late 1950s). We assessed the efficacy of Bordeaux mixture on the cultivar Stuart at two locations in Georgia from 2011 to 2016. Rainfall varied among seasons and locations and impacted severity on the non-treated controls. Thus, on mature fruit at the end of the season, the most severe scab occurred at the Colquitt location (46.63 to 71.84% fruit area diseased) compared to the Byron location (2.98 to 25.27% fruit area diseased). There was a significant reduction in the severity of scab on fruit due to the Bordeaux mixture treatment at the Byron location in 2011 (F = 17.0, P < 0.0001) and 2014 (F = 4.5, P = 0.04) and at the Colquitt location in 2014 (F = 30.3, P < 0.0001) only. In other seasons, there was no significant effect at either location. In most seasons and assessments, the severity of scab was associated with reduced fruit weight. The results are discussed in relation to tree height and fungicide coverage, the cultivar, and weather conditions. Based on the applied spray schedule, the results indicate Bordeaux mixture has limited effect in reducing pecan scab on cv Stuart, and was somewhat unreliable between seasons. We believe further testing of Bordeaux mixture is warranted on other cultivars, and efficacy should be compared to that of other organic fungicides. If considering planting a new pecan orchard to be managed organically, growers in the southeastern USA should consider selection of scab-resistant cultivars as an option.

Effect of Diverse Compost Products on Soilborne Diseases of Potato

ABSTRACTSoilborne diseases result in major economic losses for potato producers. Compost application can reduce soilborne diseases, however the effects of compost products on potato disease severity and incidence are still unclear. Diverse compost products were compared for their effects on soilborne diseases of potato in New Brunswick, Canada using field and growth room experiments. In the field, five products were applied at 45 Mg ha−1 dry weight to field plots in October of 2014 and 2015. In the growth room experiment, seven products were mixed to a 5% w/w ratio with naturally infested soil. Tubers were assessed for disease severity and incidence and compared with a no compost addition control. Severity of symptoms of silver scurf, black scurf (BS), common scab (CS), and powdery scab varied among treatments, experiments, and years. In the field experiment, BS severity was significantly greater in the control than in the poultry manure compost treatment (3.26% versus 0.90%) in 2016. Common scab severity and incidence in the field were positively related to soil pH and negatively related to soil particulate organic matter C and compost C concentrations. In the growth room experiment, CS severity was significantly greater in the control (8.98%) than in the municipal source separated organic compost and sea-waste compost treatments (1.72 and 2.47%, respectively). In this study, compost products had a significant, but inconsistent, suppressive effect on soilborne potato diseases. The quantity of compost C, rather than compost quality, was likely the most important factor in disease suppression in this study.

Biological control of potato common scab by Bacillus amyloliquefaciens Ba01.

Potato common scab, which is caused by soil-borne Streptomyces species, is a severe plant disease that results in a significant reduction in the economic value of potatoes worldwide. Due to the lack of efficacious pesticides, crop rotations, and resistant potato cultivars against the disease, we investigated whether biological control can serve as an alternative approach. In this study, multiple Bacillus species were isolated from healthy potato tubers, and Bacillus amyloliquefaciens Ba01 was chosen for further analyses based on its potency against the potato common scab pathogen Streptomyces scabies. Ba01 inhibited the growth and sporulation of S. scabies and secreted secondary metabolites such as surfactin, iturin A, and fengycin with potential activity against S. scabies as determined by imaging mass spectrometry. In pot assays, the disease severity of potato common scab decreased from 55.6 ± 11.1% (inoculated with S. scabies only) to 4.2 ± 1.4% (inoculated with S. scabies and Ba01). In the field trial, the disease severity of potato common scab was reduced from 14.4 ± 2.9% (naturally occurring) to 5.6 ± 1.1% after Ba01 treatment, representing evidence that Bacillus species control potato common scab in nature.