© 2007 Plant Management Network.
Evaluations of New and Current Management Strategies to Control Phomopsis Cane and Leaf Spot of Grape
Mizuho Nita, Kansas State University, Manhattan, KS 66506, Michael A. Ellis, Leslie L. Wilson, and Laurence V. Madden, 1680 Madison Avenue, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691
Nita, M., Ellis, M. A., Wilson, L. L., and Madden, L. V. 2007. Evaluations of new and current management strategies to control Phomopsis cane and leaf spot of grape. Online. Plant Health Progress doi:10.1094/PHP-2007-0726-06-RS.
To extend our understanding of the epidemiology and control of Phomopsis cane and leaf spot of grape, caused by Phomopsis viticola, studies were conducted to: (i) determine efficacy of a dormant fungicide spray program for controlling the disease in its early stage of development (e.g., spring); (ii) evaluate a disease warning system by applying fungicides and fungicide-adjuvant combinations in response to predicted infection periods based on weather conditions; and (iii) correlate commercial control practices and environmental conditions with disease incidence based on a state-wide survey of commercial fields. With the warning system, control was equal to that obtained with a 7-day protectant fungicide program, but with fewer fungicide applications per season. A dormant application provided consistent, yet only moderate, control of the disease. Growers who applied a dormant-season application or early spring protectant fungicides tended to have lower disease incidence than those who did not. There was high variability in disease incidence within vineyards and farms, but no significant difference in incidence among regions of the state. Overall, early season application of protective fungicides was shown to be a key factor for successful management of P. viticola.
Phomopsis cane and leaf spot is a disease of grape caused by the fungus Phomopsis viticola (Sacc.). The disease is common in the US and grape growing regions around the world (5,7,8). This disease, along with Eutypa dieback, caused by Eutypa lata, was formerly known as "dead-arm disease of grape" in the US literature (1,2). Phomopsis viticola can infect many plant parts, including shoots, rachises, leaves, and fruits. Susceptibility of tissues changes over time, but berries and rachises remain susceptible throughout the season (2,9).
Phomopsis viticola survives the winter in grape cane tissues that were infected in previous years (Fig. 1). In spring, the pathogen produces numerous pycnidia on infected canes. Conidia produced within pycnidia are then splash-dispersed by rain onto new plant growth where they germinate and infect if environmental conditions are favorable. The production of conidia is more active in the spring and declines as the growing season progresses (1), and new infections do not sporulate until the following year. Thus, the disease cycle is considered monocyclic. Control of the disease is usually accomplished through selective pruning of diseased canes (thus reducing initial inoculum) and/or the application of protectant fungicides onto new host tissues (12). At least two seasonal applications of fungicide are generally recommended (12).
The damage caused by Phomopsis cane and leaf spot can be direct or indirect. Fruit infection directly reduces yield and indirectly increases labor costs involved in selecting disease-free berries. Poor control could result in loss of the entire yield of a crop due to USDA standards for tolerance of molded berries (3% by weight) (14). Premature dropping of fruits to the ground due to breaking of rachises and shoots also directly reduces potential yield (12). Up to 30% yield loss has been reported due to this disease in southern Ohio (2).
The overall goal of this research was to extend the knowledge and understanding of various aspects of the epidemiology and management of Phomopsis cane and leaf spot of grape, so that producers can employ more effective management tactics (e.g., fewer fungicide applications per season). The specific objectives of this project were to: (i) determine the efficacy of a dormant-season fungicide spray program for controlling the disease in its early stages of development (during spring); (ii) test and evaluate the efficacy of a disease warning system based on weather conditions favorable for disease development; and (iii) conduct a state-wide survey to quantify disease incidence and document common management practices in commercial vineyards.
Effects of a Dormant-Season Fungicide Application
A dormant-season fungicide application may aid in controlling early season infection by reducing primary inoculum originating from infection that occurred in previous seasons. Experiments were conducted in two vineyards in 2003 and 2004 seasons. Calcium polysulfide [Suregard (liquid lime sulfur), 29% a.i., Value Garden Supply LLC, St Joseph, MO] or copper oxychloride sulfate [C-O-C-S WGD (fixed copper), 50% metallic copper equivalent, Clean Crop, Platte Chemical Co. Greeley, CO] were applied to dormant vines in the fall, after leaf drop and periderm formation on the current season canes (mid-November) or at bud swell (mid-April) the following spring. A third group of vines received fungicide applications both in the fall and spring. The control consisted of untreated vines. Calcium polysulfide was applied at 95 liters/ha, and fixed copper was applied at 3.4 kg/ha. Treatments were applied using a handgun sprayer at a pressure of 689 kPa until the point of run-off. Field experiments were conducted in a vineyard of Vitis labrusca 'Concord' vines located at the OARDC in Wooster, OH (10). The experimental design was a randomized complete block with four replications, consisting of three vines per block-treatment combination.
Disease incidence and severity were assessed in early July when disease symptoms were relatively easy to distinguish visually. The center vine of the three in each plot was used for disease assessment. Disease incidence of leaves and internodes and disease severity of internodes were visually estimated, and leaf disease severity was assessed by estimating the number of lesions on each leaf. Ten shoots per vine were randomly selected, and five basal internodes and leaves on a shoot were assessed for the disease (a total of 50 leaves and internodes per treatment per block). For the analysis, an angular transformation (arcsine √ % / 100 ) was used for disease incidence and severity data, except lesion number on leaves for which a square-root transformation was used. Data were analyzed using a linear mixed model analysis of variance (ANOVA) in order to determine differences in effects of spray timing and type of fungicides on disease intensity. Values were then back-transformed after the analysis to obtain treatment means for presentation.
Evaluation of a Disease Warning System
A disease warning system for Phomopsis cane and leaf spot was developed based on the functional relationship between wetness duration (W), air temperature (T), and level of infection (3). Essentially, the warning system involves monitoring leaf wetness and air temperature with electronic sensors to predict disease severity using a previously developed nonlinear model (that assumes the presence of viable inoculum). The purpose of this study was to evaluate the system by testing the efficacy of a fungicide spray program based on the disease warning model to that based on a calendar-based spray schedule. To do so, different fungicides and fungicides mixed with adjuvant were sprayed after predicted infection events. Using the nonlinear model (3) and assuming high inoculum density, predicted infection events were classified into three categories: light (0 < y < 30 lesions per leaf); moderate (30 < y < 90 lesions); and high (y > 90 lesions). These three categories indicate relative expected disease intensity based on a length of wetness duration and average temperature during the wet time period. In this paper, results of treatments based on a predicted moderate infection event are presented.
The weather information was obtained from a datalogger (Model CR23X micrologger, Campbell Scientific, Logan, UT) to which attached were: a thermister (Model 107, Campbell Scientific) located within the grape canopy (~1.7-m above ground) for temperature measurement; two flat circuit-board sensors (Model 237, Campbell Scientific) located inside the grape canopy for leaf wetness measurement; and a tipping bucket rain gauge (Model TR-525M, Campbell Scientific) located next to the datalogger for rainfall measurement.
Field evaluations were conducted in 2001 and, more extensively, in 2002 (11), and results from 2001 and a portion of the 2002 studies were presented in this paper. Experiments were conducted in a vineyard of Vitis labrusca 'Catawba' vines located at the OARDC in Wooster, OH. The study consisted of spraying vines with fungicides benomyl (Benomyl 500W, Dow Agrosciences LLC, Indianapolis, IN), applied at a rate of 0.9 kg/ha, or mancozeb (Dithane M-45, Dow Agrosciences LLC, Indianapolis, IN), applied at a rate of 4.5 kg/ha, in 2001; mancozeb; calcium polysulfide (Suregard [liquid lime sulfur], 29% a.i.), applied at a rate of 4.7 liters/ha, thiophanate-methyl (Topsin-M 70WP, Cerexagri, Inc., King of Prussia, PA), applied at a rate of 1.7 kg/ha, azoxystrobin (Abound Flowable, Syngenta Crop Protection, Inc, Greensboro, NC), applied at a rate of 1.1 liters/ha, and myclobutanil (Nova 40W, Dow Agrosciences LLC), applied at a rate of 0.3 kg/ha, in April 2002. Additional treatments were also evaluated in the latter years of this study, and are described in Nita et al. (11). Treatments were applied according to either a 7-day protectant program or when environmental conditions had been favorable for infection (based on predictions by the model). For warning-system-based treatments, some fungicides were mixed with an adjuvant, in particular, JMS Stylet-Oil (JMS Flower Farms Inc., Vero Beach, FL), applied at a rate of 3.6 liters/ha; or Regulaid (2-butoxyethanol, poloxalene, monopropylene glycol, Kalo Laboratories Inc., Kansas City, MO), applied at a rate of 2.4 liters/ha. Putatively, the adjuvant facilitates intake of the fungicide by plants (13) or improves its efficacy by increasing adhesion (4) or spray coverage (15). Fungicide treatments were applied to vines with an 11.3-L CO2-pressurized hand sprayer operated at 274 kPa pressure. Disease incidence and severity assessments and statistical analyses were described previously.
A State-Wide Survey on Current Management Practices in Commercial Vineyards
Discussions with growers in Ohio indicated variations in both the perceived intensity of the disease and the degree of control provided by standard fungicide applications (Ellis, unpublished), but there were no previous attempts to quantify these factors among grape growing regions. Thus, a state-wide survey to quantify the incidence of Phomopsis cane and leaf spot of grape was conducted in Ohio from 2002-2004 in order to learn more about the management practices employed in commercial vineyards and to determine the magnitude of disease development. The survey pattern depended on the distribution of vineyards in Ohio, and five major areas were selected: (i) Lake Erie east (northeast), (ii) Lake Erie west (north-central to west), (iii) central, (iv) Ohio River Valley (southwest), and (v) Ohio Heartland area (central-east). Within each region, two or three commercial farms were randomly selected from a list of growers provided by the Ohio grape industry.
One to three fields (vineyards) were randomly selected from each farm, and a total of twenty one vineyards were assessed for three seasons. Cultivar was not used in selecting farms and vineyards, but the majority of cultivars examined were of the French-hybrid type (Vitis intraspecific hybrid). Sampling was performed in 8 × 8 (row × vine) grids in each field. Intention of sampling using a grid was for the analyses of the spatial distribution of the disease, but this is not a topic of discussion in this paper. Three shoots per vine were arbitrarily selected and the first five leaves and internodes from the basal part of a shoot were visually assessed for Phomopsis symptoms. Disease incidence of 120 leaves and internodes per row were recorded (there were 8 rows per vineyard). In addition to the disease data, information on disease management practices and the vineyard, such as age of vines, type and timing of fungicide applications, were collected through a return mail survey from each grower. Also, daily and hourly meteorological data from nearby weather stations were collected from on-line databases.
A hierarchical linear mixed model was used to investigate spatial scale factors that might have influenced disease incidence (6). Factors were region, farm within the region, field within farm, and site within each field. All effects in the model were random, except region (which was fixed).
Also, based on the disease incidence data, Kendall's correlation coefficient was calculated and then used to determine which environmental and management variables were most associated with high (or low) disease levels in commercial fields. Observed disease incidence was classified into two categories, 0 = low disease and 1 = high disease. Since economic threshold is not well defined on this disease, two arbitral thresholds of disease incidence (20% and 40%) were selected and named risk variable D20 and D40. Thus, D20 = 1 and D40 = 1 indicated cases where more than 20% and 40%, respectively, of disease incidence was observed. Correlations between these risk categories and influence of rainfall, temperature, relative humidity, fungicide coverage from April to June, dormant-season fungicide application, and age of vines, were examined.
The effect of dormant-season fungicide application on the incidence and severity of leaf infection by P. viticola in 2004 is shown in Table 1. Calcium polysulfide (liquid lime sulfur) application tended to suppress the disease incidence and severity on leaves better than fixed copper applications when applied in spring (Table 1). Spring application of calcium polysulfide provided about 28% and 70% reduction in disease incidence and severity, respectively. Moreover, spring application of fixed copper provided about 22% and 62% reduction in disease incidence and severity. Fall application provided little control regardless of fungicide types, and means for fall application were not significantly different (P ≤ 0.05) from that of untreated control.
Table 1. Effect of dormant fungicide application on the incidence and severity of Phomopsis cane and leaf spot of grape, cultivar ‘Concord’ in 2004 season in Ohio.
z Calcium polysulfide (lime sulfur) was applied at rate of 94.7 liters/ha and fixed copper (COCS) was applied at 3.4 kg/ha.
y Fall and spring application for 2004 season was applied at 6 November 2003 and 20 April 2004, respectively.
x % = Mean disease incidence, 50 observations per plot, four replicate plots per treatment. Disease incidence is a proportion of leaves or internodes showing any visible disease symptoms. Disease incidence was angular transformed (arcsine √ % / 100 ) before analysis with Proc MIXED of SAS (SAS Institute Inc., Cary, NC).
w Mean number of lesions per leaf. Disease severity was estimated by visual assessment of lesion numbers based on a 7-class scale (0 = no lesion, 6 = more than 100 lesions) per leaf, then square-root transformed before analysis with Proc MIXED of SAS (SAS Institute Inc., Cary, NC).
v Means within a column followed by same letter are not significantly different (P ≤ 0.05) based on multiple comparisons of least-squares treatment means determined with a linear mixed model fitted to the data.
One explanation for the reduction in foliar disease provided by a dormant-season application of calcium polysulfide in the spring would be a reduction in the production of viable spores that can potentially infect new growth soon thereafter. Thus, sporulation activities were monitored at two different periods in the year: (i) in the spring with collection of rain-splashed conidia; and (ii) in the winter with observation of pycnidia formation on incubated canes (10). Significantly fewer conidia were collected from, and significantly fewer mature pycnidia were formed, on treated versus untreated control (10). However, there were still a substantial number of pycnidia formed on treated canes and the number of conidia collected under untreated control vines was still high. That was probably why the dormant season application provided only a moderate control.
Disease Warning System
In the preliminary study in 2001, fungicide spray programs were initiated when shoot growth reached 12 cm and continued until 2 weeks after bloom. Three applications of benomyl (plus Sylet Oil) were made in response to predicted moderate infection periods, whereas seven applications of mancozeb were applied according to the 7-day calendar-based protectant program (Table 2). Four applications were made in response to predicted infection periods compared to ten with the protectant calendar-based program in 2002 (11).
Table 2. Evaluation of warning-system-based and 7-day calendar-based protectant fungicide spray programs for control of Phomopsis cane and leaf spot of grape.
z Benomyl, mancozeb, Regulaid, and Stylet-Oil were applied at the rate of 0.84 kg, 3.6 kg, 2.4 liters, and 3.7 liters ai/ha, respectively.
y # = number of applications made.
x % = Mean disease incidence or severity, 50 observations per plot, 4 replicate plots per treatment. Disease severity for internodes was estimated by visual assessment of proportion of five basal internodes covered by lesions. Disease incidence was the proportion of leaves or internodes showing any visible disease symptoms. Disease incidence or severity values were angular transformed (arcsine √ % / 100 ) before analysis with Proc MIXED of SAS (SAS Institute Inc., Cary, NC).
w L = Mean number of lesions per leaf. Disease severity was estimated by visual assessment of lesion numbers based on a 7-class scale (0 = no lesion, 6 = more than 100 lesions) per leaf, then square-root transformed before analysis with Proc MIXED of SAS.
v Means within a column followed by same letter are not significantly different (P ≤ 0.05) based on multiple comparisons of least-squares treatment means determined with a linear mixed model fitted to the data.
Treatments with benomyl plus Stylet Oil sprayed using the warning system in 2001 provided a significant reduction in disease incidence and severity on both leaves and internodes (Table 2). The calendar-based protective schedule resulted in even lower disease incidence and severity. Vines sprayed with mancozeb plus Regulaid using the warning system in 2002 (Table 2) and later years (11) had mean incidence and severity levels that were significantly lower than in the non-sprayed control, but means were similar to those found with the calendar-based fungicide schedule, indicating that disease forecasting may be feasible and practical for this disease.
Efficacy of these and other fungicides, with or without adjuvants, was further examined in a controlled-environment study (9). No curative effects were found in several experiments; therefore, efficacy found in the field study with the warning system (11) (Table 2) likely was due to the protective activity of the fungicides used. That is, use of the warning system facilitated proper timing of fungicides to protect the crop against subsequent infections.
Among twenty-one vineyards assessed over 3 years, Phomopsis cane and leaf spot was found throughout the state, with a high degree of variability (Fig. 2). Results from a hierarchical linear mixed model indicated that disease incidence was significantly affected (P ≤ 0.05) by farm (within regions), field (within farms), and sampling site within fields, with higher variance in disease among sites within the field than among fields or among growers. The disease was found in every vineyard, and there was no difference in disease incidence between regions in any of the 3 years. Thus, there was little evidence that climate or landscape factors at the regional scale were responsible for differences in disease incidence within the state. Rather, disease incidence was more associated with finer scale factors, which would indicate management practices by grower as well as local weather conditions or cultivar differences.
Results from Kendall's correlation coefficient between risk variables (D20 and D40) and environmental and management variables were examined for different time periods from late April to late June. Management variables were developed based on a survey conducted among growers, and results showed that a dormant application of fungicide and early season [late-April (subscript A3 in Table3) to mid-May (M2)] fungicide coverage was associated with reduced disease incidence across the state, i.e., cases with D20 = 0 or D40 = 0 (Table 3). Among environmental variables examined, higher temperature and relative humidity in the mid-May (M2) tended to be associated with higher risk of having disease incidence over 20% (D20 = 1) and 40% (D40 = 1) (Table 3).
Table 3. Kendall correlation coefficients (K) between management variables, weather variables, and indicator variables [D40 (disease incidence > 40%) or D20 (disease incidence > 20%)] for the risk of Phomopsis cane and leaf spot on leaves and shoot internodes in 17 vineyards in Ohio over 3 years (only a subset of variables are shown).
z D40 = 1 if incidence > 40%, and 0 otherwise. D20 = 1 if incidence > 20%, and 0 otherwise.
y T = average temperature, RH = average relative humidity, RH90 = days with average humidity more than 90% (0 to 10 days), C = fungicide coverage in 10-day interval (0 to 10 days, number of days from the day of application, assuming efficacy of one application is effective for 10 days), DOR = dormant application of fungicide (1 or 0 for yes or no). Subscript A3, M1, and M2 represent period between 21 to 30 April, 1 to 10 May, and 11 to 20 May, respectively.
x Numbers in parentheses are significance levels for correlation coefficients.
Phomopsis cane and leaf spot is the most difficult grape disease to control in Ohio (Ellis, unpublished). Even with 10 sprays of mancozeb applied on a regular basis, which is well above the recommended minimum of two applications per season, over 50% of the leaves are infected in a typical year (e.g., see 2002 results in Table 2). Also, state-wide survey results showed median leaf and internode disease incidence were 45% and 50%, respectively. In this series of studies, assessment of the disease was done only on the basal five leaves and internodes, where one would expect to find Phomopsis symptoms, because these leaves and internodes are closest to the cane of the vines (where overwintering of the pathogen occurs). Because the basal leaves and internodes are close to the fruits, disease on this part of the plant provides the most useful information for growers. Disease incidence and severity found would have been lower if all leaves and internodes were assessed.
High disease incidence was consistent with the overwintering of the pathogen in the woody tissue (e.g., canes) of grape vines, and the lack of resistance to the disease. With the high incidence found in the survey study and in field experiments, more awareness of the importance of Phomopsis on grape is needed, and continued effort to improve disease control is imperative.
Some less common disease control tactics evaluated here, such as use of dormant-season fungicide application, showed the potential for improved disease control. Although dormant-season fungicide applications alone did not provide complete control of the disease (10; Table 1), there may be a benefit in applying lime sulfur early in the spring, based on the survey of commercial vineyards and the field experiments (Tables 1 and 3). Because frequent rains during the spring in Ohio can prevent the timely application of protective fungicides, an application of a dormant-season fungicide could provide some level of protection during this critical period when plant tissues would be otherwise unprotected. Also, a dormant-season fungicide application in combination with a standard early-season protective fungicide program may lead to better control of Phomopsis cane and leaf spot, as suggested by the results of the state-wide survey of disease incidence. Since lime sulfur and fixed copper are registered for use with organic production, dormant applications may have a great potential in organic grape production systems. However, because the application of fungicide during the dormant period will not eliminate other early-season protectant fungicide applications, in general, a dormant application will add to the costs of control. Further economic studies are needed to determine if the cost of lime sulfur as a dormant application can be justified based on the level of disease control achieved. It is anticipated that the benefit will depend on the inoculum level in a given vineyard.
Overall, these studies indicated the importance of well-timed fungicide applications for control of this monocyclic disease of grape, either following a calendar-based schedule or a disease-warning system (11) (Table 2). Comparable reduction in disease intensity between the 7-day calendar-based and warning-system-based treatment, and findings from the state-wide survey suggests that fewer applied early season applications of protectant fungicide is probably a key to the successful management of the disease.
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