Contribution of Mid-Season Cover Sprays to Management of Peach Brown Rot at Harvest.

Four protectant fungicides applied as midseason cover sprays were quantitatively assessed for their ability to reduce brown rot caused by Monilinia fructicola during the preharvest fruit ripening periods in the 2012 through 2015 growing seasons. No fungicides were applied during bloom or during the preharvest period. Treatment programs consisted of captan, sulfur, ziram, and thiram applications beginning at early shuck-split stage and ending with the final cover spray at 23 to 26 days before harvest. The incidence of brown rot at harvest was determined by examining 41 to 91 fruit for symptoms of rot on each of four replicate trees for each treatment. The incidence of sporulating blossom blight cankers was assessed during the preharvest period at 8, 15, and 22 days after the final cover spray. An in vivo bioassay was also conducted at 7, 14, and 21 days after the final cover spray to ascertain the level of fungicide residue during the preharvest period. The bioassay uses conidia germination as a quantitative indicator of effective residue. Results of the harvest assessment showed that captan cover sprays significantly reduced brown rot incidence in all years of the study. Furthermore, results of the bioassay demonstrated that fungicide residue was the mechanism by which this control occurred. None of the other fungicide cover spray programs contributed significantly to brown rot control at harvest in any year, and bioassay results showed insufficient residue to inhibit conidial germination. Antisporulant activity against blossom blight cankers was not observed for any fungicide program, indicating that reducing inoculum production from this source was not a mechanism for brown rot control. The captan and sulfur programs provided very good control of peach scab incidence and severity, caused by Fusicladium carpophilum, while the ziram and thiram programs failed to control this disease. These findings demonstrated that captan cover sprays can contribute significantly to control of brown rot at harvest, thereby augmenting the efficacy and consistency of management by preharvest fungicide programs. Furthermore, any reduction of the M. fructicola population by the captan cover sprays should reduce selection pressure against the site-specific fungicides commonly used during the preharvest period, thereby prolonging their useful life for brown rot control.

An In Vivo Bioassay for Estimating Fungicide Residues on Peach Fruit.

Recent fungicide efficacy studies indicated that brown rot fruit rot at harvest, caused by Monilinia fructicola, was being controlled by residual activity from protectant fungicides applied during the time between bloom and the preharvest fruit ripening period. To determine the extent of this residue, a simple in vivo bioassay was developed by assaying M. fructicola spore germination directly on sampled fruit. A 1.5-cm section of clear flexible tubing was placed upright on harvested fruit to create a small incubation well. After the tubing-fruit interface was sealed using silicon grease, a suspension of M. fructicola conidia was pipetted into the well. The spores were suspended in a buffer-substrate medium consisting of 0.025 M potassium phosphate, 0.1% sucrose, and 0.1% yeast extract. A rubber stopper with an aeration hole was inserted into the well's top and the fruit was placed in an incubator at 25°C. Results of a time-course study indicated that the optimal conidial incubation time was 6 h. Bioassay sensitivity was evaluated by examining test results from varying concentrations of captan fungicide. Results indicated that captan residue levels as low as one-thousandth the standard field rate could be detected using spore germination as the predictor. Fitting of the logistic decline model to the data created a standard curve to allow quantitative estimation of fungicide residue based on observed level of spore germination. A modified version of the bioassay, which can be used to detect carbohydrate or nutrient sources on the fruit surface, was also demonstrated.

Influence of Trifloxystrobin on Primary Inoculum and Progression of Scab Epidemics on Stone Fruit.

The ability of the QoI fungicide trifloxystrobin to reduce production of conidia by Fusicladium carpophilum on twig lesions was quantitatively assessed over a 3-year period from 2005 through 2007. Four annual treatment programs, consisting of two consecutive trifloxystrobin applications at pink + bloom, bloom + petal fall, and petal fall + shuck-split, plus a single application timing at petal fall, were examined in a 'Redgold' nectarine orchard harboring high levels of overwintering scab lesions. Sporulation potential, the ability of twig lesions to produce conidia under optimum environmental conditions, was subsequently assessed five to six times during each spring and early summer. In each year of the study, all four treatments significantly reduced the area under the sporulation curve or peak sporulation. The petal fall + shuck-split program provided the greatest antisporulant activity, reducing conidia production at peak sporulation by 82 to 92%. Furthermore, examination of results over the 3-year period showed that the programs significantly slowed the annual rate of increase in peak sporulation; however, none of the programs completely halted or caused a decline in the annual rate. Although no fungicide was applied after the treatment programs, results from fruit disease assessments showed that these programs, applied as much as 3 months earlier, significantly reduced disease incidence and, in particular, disease severity. A comparison of four QoI fungicides in 2008 indicated that trifloxystrobin and azoxystrobin provided the most control of fruit scab, while pyraclostrobin + boscalid and fluoxastrobin yielded minimal or no benefit. Results of this study demonstrate that certain QoI fungicides, in particular trifloxystrobin and azoxystrobin, can probably improve the efficacy of current protectant programs used for peach scab control by providing season-long control of F. carpophilum sporulation on twig lesions. Such program enhancement may be critical when orchards have high inoculum levels and/or environmental conditions are very favorable to disease development.

Modeling sporulation of Fusicladium carpophilum on nectarine twig lesions: relative humidity and temperature effects.

The production of conidia by Fusicladium carpophilum on twig lesions was quantitatively modeled as a function of temperature and duration of high relative humidity. During peak sporulation periods in 2007, 2008, and 2009, 1-year-old twigs bearing abundant overwintering lesions were removed from a heavily infected 'Redgold' nectarine orchard, placed in trays at high relative humidity (>95%), and incubated at eight constant temperatures for seven durations, resulting in a factorial design of 56 treatment combinations. Conidia numbers were estimated with a hemacytometer. Results from a six-stage modeling process indicated that, at any given temperature, spore production during high relative humidity periods increased in a monomolecular- to Gompertz-like pattern. The Richards model, with shape parameters of 0.79 to 0.90, was found to provide the best overall fit. When the asymptote and rate parameters were derived as functions of temperature using Gaussian and quadratic models, respectively, the duration of high relative humidity and temperature described 90 to 94% of the variation in conidia production. Predictions of the final models were highly correlated with observed levels of sporulation (r > 0.94; P < 0.0001), indicating an excellent fit to the data. The optimum temperature for sporulation, based on fitting a Gaussian model to the maximum sporulation levels at each temperature, was 17.9 to 20.2°C, with an overall average of 18.8°C. The derived models give a quantitative description of sporulation by F. carpophilum and may have potential use in simulators and forecasting systems.

Effect of QoI Fungicides on Colonization and Sporulation of Monilinia fructicola on Peach Fruit and Blossom Blight Cankers.

Three studies were conducted to examine the curative activity of azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid against Monilinia fructicola, causal agent of brown rot on peach. In the first study, 'Autumnglo' peach trees were treated with each of the three fungicides both before and after fruit inoculation. In the second study, the effects of fungicide active ingredient, rate/volume, and inoculation timing were examined on inoculated 'Suncrest' peach fruit. Results of these studies showed that sporulating area, on average across all treatments, was reduced by 15.9, 42.4, and 0.4% for azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid, respectively. In any single treatment, trifloxystrobin provided the greatest benefit with two consecutive sprays, reducing sporulating area by 53 to 60%. In contrast to sporulation activity, the three fungicides exhibited less of an inhibitory effect on fruit colonization. When applied at maximum labeled rates in the various treatments, azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid reduced colony growth, on average, by 12.3, 7.5, and 7.4%, respectively. Because the pathogen was inoculated into the mesocarp, this low level of activity against colonization may be due to a lack of deeper systemic movement of the fungicides into fruit tissue. In the final study, the three fungicides were examined for their antisporulant activity on blossom blight twig cankers. Unlike results observed on fruit, significant reductions in spore production on cankers were observed for all three fungicides. Azoxystrobin, trifloxystrobin, and pyraclostrobin + boscalid provided 56, 71, and 53% reductions, respectively, in the number of conidia produced per unit canker length. Overall results of these studies indicated that quinone outside inhibitor fungicides, in addition to their known protectant activity, also possess varying levels of curative activity against M. fructicola. In particular, trifloxystrobin demonstrated good antisporulant activity on both fruit infections and cankers.

First Report of the Peach Brown Rot Fungus Monilinia fructicola Resistant to Demethylation Inhibitor Fungicides in New Jersey.

Reduced sensitivity and resistance of Monilinia fructicola to demethylation inhibitors (DMIs; fungicide group 3) have been previously found in stone fruit orchards in Georgia, South Carolina, Ohio, and New York (2). Resistance development is a major concern because of the importance of DMIs for brown rot management. Eleven single-spore isolates, originally collected during 2006 from separate commercial peach (Prunus persica) orchards in southern New Jersey, were removed from cold storage (5°C) in early 2008 and examined in vitro for resistance to the DMI propiconazole (Orbit 3.6EC; Syngenta Crop Protection, Inc., Greensboro, NC). After 19 months at 5°C, isolate 7 was inhibited 53.4% in growth on potato dextrose agar (PDA) amended at the discretionary dose of 0.3 µg/ml propiconazole; inhibition of the remaining isolates ranged from 81.4 to 100%. Inhibition values were based on two replications of eight colonies per isolate performed after incubation at 25°C for 4 days. Because of the previously reported relationship between duration of cold storage and propiconazole sensitivity, isolate 7 was tentatively deemed resistant (1). To confirm the in vitro results, isolates were grown at 25°C for 7 days on cellophane over PDA. Genomic DNA was isolated from mycelium with the DNeasy Plant Mini Kit (Qiagen, Inc., Valencia, CA). PCR with primers INS65-F and INS65-R was conducted on a GeneAmp thermal cycler (Applied Biosystems, Inc., Foster City, CA) as described previously to amplify a 65-bp region named 'Mona' associated with DMI resistance (2). PCR products were separated via electrophoresis on 0.8% agarose gel. The primers amplified a 376-bp fragment from isolate 7 and a 311-bp fragment from all other isolates, thus indicating the presence of Mona in isolate 7. Restriction fragment length polymorphism analysis using the BsrBI enzyme, specific to a single restriction site within Mona, was conducted on the amplified fragments from all isolates. Electrophoresis results showed digestion of the 376-bp fragment from isolate 7 into 140-bp and 236-bp fragments, thereby confirming the presence of Mona; none of the 311-bp fragments from the remaining isolates were cut by BsrBI. Although economic loss from brown rot has not been reported in New Jersey, these results show that propiconazole-resistant strains have been detected since 2006 and it is most likely that resistant strains of the pathogen are still present in commercial peach orchards. To combat this risk, current brown rot control recommendations are incorporating quinone outside inhibitors (QoIs; fungicide group 11) and carboxamides (fungicide group 7) into control programs as a resistance management strategy. More extensive sampling is planned to ascertain the prevalence and location of resistant strains. References: (1) K. D. Cox et al. Phytopathology 97:448, 2007. (2) C.-X. Luo et al. Plant Dis. 92:1099, 2008.

Phytotoxicity of Copper-Based Bactericides to Peach and Nectarine.

Organometallic copper, consisting of a mixture of copper abietate, copper linoleate, and copper oleate (CuALO), is important for postbloom management of bacterial spot in New Jersey peach and nectarine orchards. Rotation of CuALO with oxytetracycline reduces cost and helps prevent (or delay) resistant organisms. However, because copper is also phytotoxic, higher rates and inorganic coppers have not been utilized. A study was conducted on 'Encore' peach and 'Redgold' nectarine to determine the quantitative relationship between copper concentration (metallic equivalent) and phytotoxicity. Different rates of CuALO and copper hydroxide (CuOH) were applied seven times postbloom. Foliar injury and defoliation increased with number of applications and copper concentration. Maximum 'Encore' defoliation was 10 to 17%, while 'Redgold' sustained 30 to 35% leaf loss. CuOH caused similar or less leaf injury and defoliation than CuALO. No injury was observed on fruit, even at three times the labeled metallic copper rate of CuALO. Fruit size and soluble solids were not influenced by copper. Estimates of foliar density and tree volume indicated that sufficient photosynthetic capacity existed to allow acceptable leaf damage and loss from copper applications. These results showed that inorganic coppers may be viable alternatives to organometallic copper for postbloom bacterial spot control; nectarine may be more sensitive to copper injury than peach; and higher rates of metallic copper, above the current commercial rate, can be applied to peach. In the latter case, greater amounts of copper may provide longer residual activity and improved bacterial spot control on highly susceptible cultivars.

Quantitative Models for Describing Temperature and Moisture Effects on Sporulation of Phomopsis amygdali on Peach.

ABSTRACT Twigs with constriction cankers were pruned from a Prunus persica 'Jerseyglo' orchard and placed in incubators under high humidity (>95%) at constant temperatures of -6, 1, 10, 17, 24, 31, 38, and 45 degrees C. Cankers were removed for observation after 2, 6, 24, 48, 72, and 96 h incubation. Sporulation was expressed as the percentage of pycnidia producing cirri and the number of conidia per pycnidium or canker. The experiment was first performed during 1998 to 1999 and then repeated in 1999 to 2000 and 2000 to 2001. Sporulation was modeled by fitting the Richards function to each dependent variable with duration of high relative humidity as the independent variable. The asymptote and rate parameters of the Richards model were expressed as Gaussian and quadratic functions of temperature, respectively. Models created from the first two experimental repetitions were validated by statistical comparison to those created independently from the third repetition. When models were fit to the pooled data, temperature and moisture described 69 to 80% of the variation in sporulation. The models specified a temperature ranging from 0 to 37 degrees C (optimum temperatures for cirri formation and conidia production range from 19 to 20 and 22 to 23 degrees C, respectively), and the majority of sporulation occurred between 16 and 48 h from initiation of the high humidity period. These results show that the environmental criteria for sporulation coincide with those that prolong the susceptibility of infection courts during fall and spring.

Seasonal availability of inoculum for constriction canker of peach in new jersey.

ABSTRACT The seasonal sporulation ability of Phomopsis amygdali was evaluated over a 2-year period by sampling twig cankers from commercial 'Encore' and 'Jerseyglo' peach orchards. Canker size, pycnidia per canker, percent pycnidia forming cirri, and spore production were evaluated once each month from January 1997 through December 1998. Average canker size and number of pycnidia per canker, which followed a sinusoidal pattern of change, were at lowest values in spring and at maximum values in fall. In 1997, the ability of pycnidia to sporulate (produce cirri) increased significantly from a minimum during winter to a maximum during summer. In contrast, pycnidia sporulation in 1998 increased slowly throughout the year with a maximum in the fall. Inoculum potential, measured as number of spores per canker, peaked between June and August and was lowest in winter. Although pycnidia numbers in 1997 were about 50% of that observed in 1998, approximately twice as many conidia per canker were produced in 1997; consequently, the number of conidia produced per canker was equivalent in both years. These results indicated that P. amygdali acts as an r strategist by maintaining abundant pycnidia and sporulation capability throughout much of the year. Therefore, infection is most likely not limited by the availability of inoculum.

Evaluation of a Disease Warning System for Downy Mildew of Grapes.

An electronic warning system for grape downy mildew- based on models for the infection of leaves of Vitis lambrusca, production of sporangia by Plasmopara viticola in lesions, and sporangial survival-was tested over 7 years in Ohio. Grapevines were sprayed with metalaxyl plus mancozeb (Ridomil MZ58) when the warning system indicated that environmental conditions were favorable for sporulation and subsequent infection. Over the 7 years, plots were sprayed from one to four times according to the warning system, and from four to 10 times according to the standard calendar-based schedule (depending on the date of the initiation of the experiment). The warning system resulted in yearly reductions of one to six sprays (with median of three sprays). Disease incidence (i.e., proportion of leaves with symptoms) in unsprayed plots at the end of the season ranged from 0 to 86%, with a median of 68%. Incidence generally was very similar for the warning-system and standard-schedule treatments (median of 7% of the leaves with symptoms), and both of these incidence values were significantly lower (P < 0.05) than that found for the unsprayed control, based on a generalized-linear-model analysis. Simplifications of the disease warning system, where sprays were applied based only on the infection or sporulation components of the system, were also effective in controlling the disease, although more fungicide applications sometimes were applied. Effective control of downy mildew, therefore, can be achieved with the use of the warning system with fewer sprays than a with a standard schedule.