Refinement of Peach Cover Spray Programs for Management of Brown Rot at Harvest.

Peach cover spray applications of the protectant fungicide captan were previously shown to significantly reduce brown rot caused by Monilinia fructicola during the preharvest fruit ripening periods in the 2012 through 2015 growing seasons. The protectants sulfur, ziram, and thiram failed to yield this benefit. Percentage disease control with captan ranged from 50 to 69%. Results of a bioassay indicated that the mechanism for this control was the creation of an effective, persistent fungicide residue on the fruit surface. Given these findings, the current 2017 to 2018 study was initiated to further refine the cover spray program. Cover spray applications of captan were made at lower rates and fewer timings with the goal of minimizing fungicide usage while maintaining control. High concentrations of the protectants sulfur and ziram were also examined in cover spray programs to determine whether greater concentrations could improve control. Results of the captan treatments from both years showed that the concentration could be reduced 17%, from 3.36 to 2.80 kg/ha active ingredient, without a significant increase in rot at harvest. Disease control at this medium rate was 69% in 2017 and 51% in 2018. The late season timing treatment, which consisted of the final two cover sprays at fifth and sixth cover, significantly reduced brown rot at harvest and provided control equivalent to the full cover spray program consisting of seven applications. Thus, a buildup of residue from many cover sprays is not needed to achieve control. As hypothesized, the midseason treatments, which consisted of two sprays at third and fourth cover, did not provide control of brown rot at harvest. The bioassay confirmed that insufficient residue remained on fruit for adequate control. However, the early season treatment, which consisted of sprays at shuck split, first cover, and second cover, provided 40% control, even though the bioassay showed that an effective residue was not present during the preharvest period. Brown rot management for this treatment was probably caused by inhibition of quiescent or latent infections on young green fruit. If confirmed, this novel finding indicates that high levels of latent infections are possible in eastern U.S. peach growing regions. Finally, higher rates of sulfur and ziram cover sprays were still ineffective for providing brown rot control at harvest. Comparison of half maximal effective concentration values calculated from the dose-response models confirmed that the sulfur and ziram intrinsic efficacies were too low for adequate control, even at the highest registered rates. These findings demonstrated that late season captan cover sprays can contribute significantly to control of brown rot at harvest, thereby augmenting the efficacy of preharvest fungicide programs. The year-to-year consistency of control should also be improved because heavy rainfall during the preharvest period did not reduce control by the captan residue. 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. The development of resistance to captan, a multisite protectant fungicide, is not likely, so this resistance management strategy should be sustainable.

Evaluation of hydrophobic and hydrophilic kaolin particle films for peach crop, arthropod and disease management.

Hydrophobic and/or hydrophilic kaolin particle film treatments to peach (Prunus persica (L) Batsch) trees were evaluated for crop and pest management capabilities in six studies from 1997 to 2000. Unsprayed control and standard treatments, the latter consisting of a commercial pesticide program, were included for comparison. Treatments in initial studies were applied via handgun, which resulted in a uniform and heavy deposit of kaolin after the first application. In contrast, treatments in subsequent studies used airblast equipment, which provided a uniform but less dense coverage, even after multiple applications. Results showed that both formulations of kaolin provided control of oriental fruit moth (Grapholita molesta (Busck)), plum curculio (Conotrachelus nenuphar (Herbst)) and Japanese beetle (Popillia japonica Newman) that was comparable with or better than the standard pesticide program. Effective management of late season catfacing insects (tarnished plant bugs Lygus lineolaris (Palisot de Beauvois) and stinkbugs Acrosternum hilare (Say), Euschistus servus (Say), and E tristigmus (Say)) and leafrollers (undetermined species) was also observed, although kaolin applications significantly increased phytophagous mite (Panonychus ulmi (Koch)) levels. In contrast to arthropod management, kaolin failed to control either peach scab (Cladosporium carpophilum (Von Thumen)) or rusty spot (Podosphaera leucotricha (Ell and Ev) ES Salmon) in any of the 4 years of the study. However, hydrophobic kaolin provided effective brown rot (Monilinia fructicola (G Winter) Honey) control when applied via handgun, and partial control when applied via airblast; hydrophilic kaolin failed to provide any control. These results suggest that hydrophobicity and deposit density may be important factors for effective disease management. The application of kaolin significantly delayed fruit maturation, increased fruit size and increased soluble solids relative to the standard. This effect, attributed to a reduction in plant stress, also resulted in increased fruit number and yield on young trees, indicating that an accentuated beneficial response from kaolin applications may be possible.

Peach Rusty Spot Epidemics: Management with Fungicide, Effect on Fruit Growth, and the Incidence-Lesion Density Relationship.

Different numbers of consecutive fungicide applications, beginning at petal fall and continuing into the summer, were examined for their effect on rusty spot epidemics. Disease progressions for each fungicide level were quantified by fitting either the logistic or monomolecular model. When the weighted absolute infection rate (ρ) and maximum disease level (Kmax) parameters were expressed as functions of the number of applications, the logistic decline model provided the best fit for five of six data sets. This model described a gradual decrease in ρ and Kmax in response to the initial fungicide application, a rapid decline in parameter values with the addition of one or two applications, and a diminished parameter response as fungicide applications continued toward the end of the epidemic. Based on examination of model behavior across all 3 years of the study, adequate management was achieved with a total of three to five fungicide applications. Additional analyses of area under the disease progress curve and final disease intensity at harvest supported these results and indicated that further reduction in fungicide usage may be possible. Unlike earlier findings, rusty spot did not significantly decrease fruit volume or weight at midseason or at harvest; as lesion density increased, fruit volume remained constant. The relationship between disease incidence and lesion density within any given year was best explained by the zero-intercept version of the exponential model. However, comparison of model parameters across years revealed significant seasonal variation. Nevertheless, the incidence-lesion density relationships were fairly uniform across years at incidence values below 0.5, where lesion density increased gradually and in a near-linear fashion.

Peach Rusty Spot Epidemics: Temporal Analysis and Relationship to Fruit Growth.

Incidence and severity of peach rusty spot were monitored throughout the growing seasons of 1999 to 2001. Graphical and statistical analysis revealed that disease increased from the shuckoff stage of fruit development until 60 days after full bloom; epidemics typically lasted from 17 to 30 days. An analysis of fruit growth indicated that the early-season epidemic coincided with the first stage of stone fruit development, physiologically characterized as the period of cell division. During this period, as fruit growth slowed and approached initiation of pit-hardening, the rate of disease increase slowed. Since fruit infection was greatest during the period of fruit growth, disease progression was modeled as a function of plant growth instead of time. Temporal analysis revealed that the logistic function was appropriate for describing both growth processes, and a synchronous logistic/logistic composite disease progression/fruit growth model was fit to all data sets. No change in disease levels occurred during midseason, which coincided with the second stage of fruit development, a period of slow growth. Subsequently, disease incidence and severity significantly declined on average by 26% and 1.3 lesions per fruit, respectively, during the 20 to 30 days prior to harvest. This decline phase coincided with the third stage of fruit growth, the period of cell enlargement and coloration. These disease reductions may be related to physical changes in fruit size and pigmentation, as opposed to resistance development, causing younger, less established lesions to become undetectable.

Effect of Fungicides, Application Timing, and Canker Removal on Incidence and Severity of Constriction Canker of Peach.

The fungal plant pathogen Phomopsis amygdali, causal agent of constriction canker, initiates infections of peach twigs through petiole scars in fall and bud scale scars and flowers in spring. Fall fungicide treatments, consisting of seven to eight sprays during leaf abscission, reduced canker incidence by 45 to 63%. In contrast, spring applications consisting of four to five sprays from bud-break through bloom, provided only 10 to 28% control. When applied during both fall and spring, chlorothalonil (46 to 71% control) and captan (46 to 69% control) provided the lowest canker incidence and severity, followed by azoxystrobin (41% control) and myclobutanil (28 to 44% control). Removing cankers by pruning significantly reduced disease incidence by 42% 1 year but had no effect in another year. Possible causes for the inability of any treatment to achieve >90% disease control include: (i) none of the fungicides tested were highly effective against the pathogen, (ii) additional infections may have occurred outside the time period during which fungicides were applied, or (iii) use of single-tree plots surrounded by heavily infected nonsprayed trees. Also, the relationship between disease incidence and severity was quantified and observed to be the same for the two cultivars examined. When modeled using the power and quadratic functions, incidence explained 89% of the variation in severity.

Estimating Yield and Economic Loss from Constriction Canker of Peach.

Constriction cankers, caused by Phomopsis amygdali, girdle and kill fruiting twigs which results in a direct crop loss. To quantitatively determine this loss from 1996 to 1998, the number of fruit lost per infected shoot was estimated as a function of disease incidence in 21 severely infected orchards in New Jersey. For each cultivar in 1997 and 1998, the distribution of fruit sizes at harvest and prices at shipping were used to calculate total crop value for typical expected yields. Economic loss was then calculated from yield loss and crop value estimates. The overall percent yield loss mean across all sites and cultivars, unadjusted for fruit remaining on infected shoots, was 22.2, 30.7, and 23.7% for 1996, 1997, and 1998, respectively. The frequency of these losses were not normally distributed, and the nonparametric Friedman test indicated that yield loss was significantly different among years. Assuming the remaining fruit on infected shoots were harvested, yield losses for 1997 and 1998 were 28.5 and 21.0%, which translated into average economic losses of $4,009 and 2,803/ha, respectively, for an expected yield level of 14,010 kg/ha. These loss values justify control measures for management of constriction canker in severely infected orchards.