A biological control agent, Bacillus subtilis, and a biorational fungicide, potassium bicarbonate, were examined in integrated programs with myclobutanil for management of peach rusty spot during the 2004 and 2005 growing seasons. The research was conducted on ‘Bounty’ peach blocks located at three commercial orchards in southern NJ. Based on past epidemiological research, four fungicide applications beginning at petal fall and ending at endocarp sclerification (pit hardening) were necessary for effective control. Thus, integrated programs consisted of the standard fungicide (S) and each biofungicide (B) applied in alternation (SBSB). Integrated programs were tested with both materials at low and high rates, resulting in four treatment combinations. A fifth standard treatment consisted of only myclobutanil. Treatments were arranged in a randomized complete block design with grower sites acting as the replicates. Analysis of areas under the disease progress curves indicated that all four integrated programs were as effective as the standard. Final fruit disease incidence on integrated and standard treatments ranged from 1-20% and 3-21%, respectively. In comparison, non-treated ‘Autumnglo’ trees located at the research center had 30-51% infected fruit. These results demonstrated at a commercial level that the integration of reduced-risk fungicides in disease management programs can reduce usage of conventional fungicides by 50% without any loss of control.
Rusty spot of peach is a serious problem on fruit of susceptible cultivars and can be found throughout the United States (4). This disease is reportedly caused by the apple powdery mildew fungus Podosphaera leucotricha (7, 9). However, controlled etiological experiments have not been conclusive, and it is possible that additional causal agents exist (6). Infection results in the formation of small orange-to-brown lesions that usually increase in size to cover large areas of the fruit surface; multiple lesions often coalesce. As lesions age, the trichomes detach and the epidermal cells become lignified, causing the affected surface to appear smooth and russetted, rendering the fruit unmarketable. In New Jersey, incidence of fruit infection on unprotected susceptible cultivars can approach 100%. Commercial crop loss has been estimated as high as $204/A (8).
Given the need to grow high quality cultivars which are often susceptible, management of rusty spot has become heavily dependent on application of fungicides. Of the current fungicides available, myclobutanil (Nova 40W) is the most effective and has become the accepted standard for peach rusty spot control (5). Current control recommendations stipulate that fungicide protection should begin at the shuck split stage, the time at which the calyx separates from the young fruit, and continue on an eight- to ten-day schedule (4). This disease control program can result in as many as six to eight consecutive applications, depending on time of fruit maturation.
In a three-year temporal analysis study conducted from 1999-2001, results indicated that fruit develop ontogenetic resistance at initiation of endocarp sclerification (pit-hardening), approximately two months prior to harvest (2). These results indicated that adequate disease control should be attainable with much fewer fungicide applications. In additional research on optimization of fungicide timing, we determined that on average only four applications are necessary for adequate control, resulting in a 33% to 50% reduction in fungicide usage (3).
In general, biofungicides have not provided the level of control typically attained with use of conventional fungicides. For example, in a field screen of nine biofungicides in 2000-2001, application of the biocontrol Bacillus subtilis (Serenade) or the biorational compound potassium bicarbonate (Armicarb, Kaligreen) resulted in significantly less rusty spot than on the non-treated control, but only provided partial control relative to the standard (1). However, when these biofungicides were used in an integrated program with conventional fungicide, a 50% reduction in conventional fungicide usage was obtained without any loss in disease control or increase in crop loss (unpublished data).
(1) Furman, L. A., Lalancette, N., and White, J. F. 2001. Peach rusty spot biological / biorational disease management. Phytopathology 91:S30.
(2) Furman, L. A., Lalancette, N., and White, J. F. 2003. Peach rusty spot epidemics: temporal analysis and relationship to fruit growth. Plant Dis. 87:366-374.
(3) Furman, L. A., Lalancette, N., and White, J. F. 2003. Peach rusty spot epidemics: management with fungicide, effect on fruit growth, and the incidence–lesion density relationship. Plant Dis. 87:1477-86.
(4) Grove, G. G. 1995. Rusty spot. Page 15 in: Compendium of Stone Fruit Diseases. J. M. Ogawa, E. I. Zehr, G. W. Bird, D. F. Ritchie, K. Uriu, and J. K. Uyemoto, eds. APS Press, The American Phytopathological Society, St. Paul, MN.
(5) Lalancette, N., and Robison, D. 1999. Comparison of fungicides for control of rusty spot and brown rot on peach. Fung. & Nemat. Tests 54:60.
(6) Lalancette, N., Furman, L. A., and White, J. F. 2001. Evidence on the etiology of peach rusty spot. Pages 162-170 in: Proceedings of the Cumberland-Shenandoah Fruit Workers Conference, 77th Annual Meeting, Winchester, VA.
(7) Manji, B. T. 1972. Apple mildew on peach. (Abstr.) Phytopathology 62:776.
(8) Polk, D., Schmitt, D., Rizio, E., and Peterson, K. 1998. The economic impact of peach pests in New Jersey 1996-1997. The New Jersey State Horticultural Society. Horticultural News 78(1):3-1
(9) Ries, S. M., and Royce, D. J. 1978. Peach rusty spot epidemiology: Incidence as affected by distance from a powdery mildew infected apple orchard. Phytopathology 68:896-899.
The overall project goal was to demonstrate to growers that highly efficient and effective disease management of peach rusty spot is possible through a combination of optimized application timing and judicious use of biofungicides. Specifically, we wanted to:
(1.) Validate the effectiveness of an optimized four-application integrated rusty spot control program at the commercial level
(2.) Demonstrate that integrated biorational / conventional fungicide programs, utilizing Bacillus subtilis and potassium bicarbonate, can provide efficacy equivalent to a conventional program
The demonstration experiment was conducted over two growing seasons. In 2004 the work was funded by the U.S. EPA Region 2 Food Quality Protection Act / Strategic Agricultural Initiative Program. In 2005, the USDA/NE-SARE Partnership Grants Program provided the necessary funding. This final report contains combined results from both years.
Design. The experiment was conducted in three commercial peach orchard blocks, each consisting of the susceptible cultivar ‘Bounty’, based on data indicating a history of high rusty spot incidence. Treatments at each site were randomly assigned to 0.4 – 0.6 ha portions of the selected orchard. Fungicide applications followed the optimized four-application program (petal fall, shuck split, first cover, and second cover). Integrated treatments consisted of biofungicide and conventional fungicide applied in alternation. Conventional fungicide treatments were included for comparison. Grower sites represented statistical blocks.
Treatments. Biofungicide treatments were Bacillus subtilis (Serenade 10WP) and potassium bicarbonate (Kaligreen 82SP). The conventional fungicide was myclobutanil (Nova 40W). Five different treatments were applied: myclobutanil at standard rate, M; alternation of B. subtilis and myclobutanil at standard rates, B/M; alternation of B. subtilis and myclobutanil at high rates, B/M-H; alternation of potassium bicarbonate and myclobutanil at standard rates, P/M; alternation of potassium bicarbonate and myclobutanil at high rates, P/M-H. The standard rates were myclobutanil at 70.0 g a.i./ha; B. subtilis at 0.56 kg a.i./ha; and potassium bicarbonate at 2.3 kg a.i./ha. High rates were applied at 2X the standard rates. All treatments were applied by grower cooperators using their own airblast sprayers.
Assessment. Nine to ten disease assessments were conducted during the progressive phase of the epidemics. At each assessment, a total of 40 fruit were arbitrarily picked from each tree and evaluated for number of rusty spot lesions. From these data, disease incidence was expressed as proportion infected fruit and lesion density, a measure of disease severity, as number of lesions per fruit. As an indication of disease pressure in each year, assessments were also made on non-treated ‘Autumnglo’ trees located at the research center.
Analysis. Two dependent variables, area under the disease incidence curve (AUDIC) and area under the disease severity curve (AUDSC), were calculated for each treatment. These data were subjected to an analysis of variance using the general linear models procedure of SAS (Statistical Analysis System v8.0). Area means were compared using the Waller-Duncan K-ratio t-test (P≤0.05; K=100).
Disease progression. During 2004, a low but sufficient amount of disease occurred. On the non-treated ‘Autumnglo’, fruit infection reached a maximum of 30% on day 153 with 0.36 lesions/fruit. In 2005, disease pressure was greater. Maximum incidence and severity of 51% fruit infection and 0.88 lesions/fruit were observed on day 171. On ‘Bounty’ peach at the grower sites, disease incidence and severity were much lower on fruit receiving the fungicide treatments. In 2004 and 2005, incidence across all treatments ranged from 1 to 3% and 10 to 21%, respectively, at the end of the epidemics (Fig. 1).
Analysis of variance. All four ANOVA models describing AUDIC and AUDSC data for 2004 and 2005 had no significant treatment main effects (0.28 < P < 0.59). F-values ranged from 0.75 to 1.52 (df = 4). Treatment comparisons. In both years of the study, areas under the disease incidence and severity curves for all integrated treatments were not significantly different from those of the conventional program (Table 1). However, in both years, the AUDIC and AUDSC values for the standard integrated treatments were consistently lower than those values observed for the conventional treatments. Among the integrated programs, the standard rate treatments were as effective as the high rate treatments, even under the higher disease pressure experienced in 2005 (Fig. 1B). However, in both years, the AUDIC and AUDSC values for the higher rate treatments were consistently lower than those values observed for the standard rate treatments (Table 1).
Earlier studies on the epidemiology and control of peach rusty spot showed that only four fungicide applications were necessary for adequate disease control (2, 3). The results of recent studies conducted in single-tree plots from 2002 to 2004 further demonstrated that biorational fungicides can be effectively integrated into this optimized four-application rusty spot disease control program. In these integrated programs, two applications or 50% of the standard fungicide, Nova 40W, were replaced by either biofungicide without any significant loss in disease control.
Results of the current study showed that the biological control agent B. subtilis and biorational fungicide potassium bicarbonate can be effectively utilized in these integrated programs at the commercial level. These integrated programs, each consisting of two biofungicide applications and two myclobutanil applications, provided disease control equivalent to the conventional program consisting of four applications of myclobutanil. Although higher rates of the biofungicide and myclobutanil were observed to reduce the overall intensity of the epidemics relative to the standard, the differences were not observed to be significant.
In addition to providing effective disease control, the integrated programs yield other benefits. The successful alternation programs also act as a resistance management strategy for the standard fungicide. Furthermore, incorporation of less toxic biofungicides reduces overall toxicity of the program. The two biofungicides are rated as slightly toxic (“Caution”) relative to the moderately toxic Nova (“Warning”). Serenade and Kaligreen are also listed by the Organic Materials Review Institute (OMRI) as acceptable for use by organic growers.
Education & Outreach Activities and Participation Summary
Although the research portion of the project is completed, much extension activity will be forthcoming in the year ahead. These activities can be divided into professional and grower stakeholder outreach.
Professional. Initial outreach has been to plant pathology colleagues in other regions of the United States and world so that they can transfer this information to their respective growers. Presentations have been given at professional meetings in the United States and Chile:
(1) Lalancette, N. 2005. Progress in management of peach rusty spot: basic epidemiology to biorational control. Southern Professional Fruit Workers Conference, Oct 18-20, Decatur, AL.
(2) Lokaj, G.W., Lalancette, N., and Foster, K.A. 2005. Implementation of an integrated peach rusty spot disease management program in commercial orchards. Annual Meeting of The American Phytopathological Society, July 31-Aug 4, Austin, TX
(3) Lalancette, N. and Foster, K.A. 2005. Integration of biofungicides and conventional fungicides for management of rusty spot of peach. Sixth International Peach Symposium, International Society for Horticultural Science, Jan 9-13, Santiago, Chile
Two professional publications are planned for the near future. The first will report on the initial research investigating biorational control agents and integration strategies, while the second publication will present the results of the commercial field validation study.
Stakeholders. Outreach to commercial growers began with active involvement of the three New Jersey growers in the demonstration study. Extension presentations to all growers will be forthcoming during the upcoming Winter, Spring, and Summer of 2006. A presentation titled “Understanding Peach Rusty Spot Epidemics: Optimizing Management and Implementation of Biorational Control” has been scheduled for the National Peach Council program at the Mid-Atlantic Fruit and Vegetable Convention in Hershey, PA, Jan 31–Feb 2, 2006.
Similar presentations will be given at our annual NJ Horticulture Society Fruit Meeting and invited fruit meetings in neighboring states. Other opportunities for outreach will be presentations at local evening and twilight grower meetings and submission of articles in our Plant & Pest Advisory Fruit Newsletter. Eventually, the integrated strategy will be described in a Cooperative Extension Fact Sheet and/or added to the NJ Commercial Tree Fruit Production Guide. Implementation will also begin within our Tree Fruit IPM program.
Areas needing additional study
The newly developed and validated integrated rusty spot control program can be applied to all susceptible cultivars. However, field observations in any given year reveal significant differences in susceptibility of these cultivars to rusty spot infection. Thus, it is possible that the application rates of conventional fungicide and biofungicides used in the program can be optimized for each cultivar, thereby taking advantage of any inherent partial resistance. To accomplish this refinement in the program, further data are needed to quantify the differences in susceptibility among cultivars.