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Evaluation of a strawberry powdery mildew risk index (Broome-modified Gubler-Thomas powdery mildew index for grape) to time fungicide applications LeBlanc, M.L. 1 , Cuevas, O. 1 , Coons, K. 2 , and Broome, J.C. 2 1 Pacific Ag Research, San Luis Obispo, CA 2 Driscoll’s Strawberry Associates, Watsonville, CA, 95076 Abstract: The Gubler-Thomas powdery mildew risk index, developed for grapes based on temperature-driven fungal growth and reproduction modeling, was modified by Broome based on strawberry work suggesting days with temperatures between 15-25° C and when airborne inoculum is above a 250 conidia/m 3 air/day threshold are favorable for strawberry powdery mildew (Podosphaera aphanis) growth 1, 2 . We evaluate the index for accurately predicting disease incidence for timing fungicide applications on proprietary variety 510Q 89 planted in Santa Maria, CA. Hourly weather station temperature data was used to calculate the risk index and fungicide sprays were timed based on high disease risk (60 or above). A grower conventional fungicide regimen (quinoxyfen and myclobutanil) and organic standard (hydrogen dioxide and Bacillus subtilis QST 713 strain) were applied based on either the risk model, a weekly calendar program, or left untreated (control). The risk model-timed treatments received 11 and calendar-timed received 18 applications. There were no significant differences in marketable yield or powdery mildew AUDPC between the weekly or the risk model timed treatments for either farming system and average mildew percent control for the calendar and model sprayed plots was 35.27% of the control. The Broome-modified Gubler-Thomas powdery mildew index applied to strawberries has the potential for reducing treatment costs and environmental impacts while preserving disease control and yields. Methodology: Proprietary variety 510Q 89 was planted 14 inches apart on 40-inch bed centers (22,400 plants per acre) in Santa Maria, CA. Plots were one row (3.33 feet) by 30 feet, and treatments were replicated four times in a randomized complete block design. Treatments were applied using a backpack sprayer (50 PSI) in 50 – 75 g/a (figure 1d). Hourly regional weather station temperature data was used to calculate the risk index and fungicide sprays were timed based on high disease risk (index 60 or above, see figure 2). A grower conventional fungicide regimen (Quintec® 6 fl oz/a [Dow AgroSciences, quinoxyfen] and Rally® 5 oz/a [Dow AgroSciences, myclobutanil]) and a grower organic standard (Oxidate® 6 fl oz/a [hydrogen dioxide, BioSafe Systems] and Serenade Max® 5 oz/a [Bayer Crop Science, Bacillus subtilis QST 713 strain]) were applied on a rotating basis to 4 replicate plots based on either the risk model or a weekly calendar program. Powdery mildew disease severity was evaluated on a 1 to 5 scale (1 = no disease, 5 = severe) for ten plants per plot (figure 3). Yields of marketable and fungus- infested berries were calculated from 19 harvests (figure 4). All calculations were carried out using ARM9 Software. ANOVA was used to evaluate treatment effect and mean comparison with the LSD test and α=0.05. Gubler-Thomas Modified Powdery Mildew Index for Strawberry • Risk Index (figure 2) based on temperature on 0-100 scale • Based on weather station data, spreadsheet tabulation • Optimum powdery mildew growth 6 hours between 65 to 80°F - add 20 points. • Subtract 10 points for high temperatures above 90° F for 15 minutes or more, or for fewer than 6 hours in optimum range. • High risk is 60 or above. • Shorten or lengthen spray intervals based on risk indicated. • Example: For low risk periods apply sulfur every 14 days, 7 days when high risk. Conclusions: Timing fungicide applications to strawberries based on the Broome-modified Gubler-Thomas powdery mildew index had comparative disease control efficacy (figure 3) and yields (figure 4) as a weekly application schedule for both conventional and organic treatment regimens. By using the risk model, there were seven fewer applications over the season, saving approximately $250/acre in chemical costs alone, yet resulting in similar gross returns (figure 4). Disease control remained high relative to the untreated (figure 3) suggesting that monitoring risk models for fungicide application timing is as effective for maintaining a healthy strawberry crop as standard calendar-based applications. 1. Carisse, O., & Bouchard, J. (2010). Age-related susceptibility of strawberry leaves and berries to infection by podosphaera aphanis. Crop Protection, 29(9):969-978. 2. Miller, T. C., Gubler, W. D., Geng, S., and Rizzo, D. M. 2003. Effects of temperature and water vapor pressure on conidial germination and lesion expansion of Sphaerotheca macularis f. sp.fragariae. Plant Dis. 87:484-492. Figure 2. Application Timing Based on Risk Index 0 10 20 30 40 50 60 70 80 90 100 RISK INDEX Intermediate Risk High Risk Weekly and Risk Model Applications Weekly Applications Only Powdery mildew area under the disease progress curve (AUDPC) was significantly greater in the untreated plots with no significant difference between weekly or risk model fungicide treatment timings, whether standard or biological products were applied. 0 50 100 150 200 250 300 350 400 Untreated Check Quintec + Rally Oxidate + Serenade Max Quintec + Rally Oxidate + Serenade Max POWDERY MILDEW AUDPC Weekly Model Risk Model * Figure 3. Powdery Mildew Severity Highlighted areas of powdery mildew development on strawberry leaves Shielded hand boom with 10 x TG05 full cone nozzles simulates tractor application and ensures total coverage Characteristic leaf curling and red spot development on severely powdery mildew infested leaves a b d Figure 1. Disease Characteristics And Treatment Applications Strawberry susceptibility determined by variety, age (flowers then green fruit) and leaf age (pale green and folded) 1 . Infested green (L) and maturing (R) fruit seen here. c Total yields per acre (calculated) of marketable or powdery mildew infested fruit were not significantly different among weekly- or risk-model application timings for either product rotation. Marketable fruit weight was greater than the untreated and culled fruit weight was less than the untreated for all treated plots. Expected gross returns for 19 weekly harvests from August to November were approximately $11,000/acre in the untreated and greater than $12,500/acre in the treated plots. 0 1000 2000 3000 4000 5000 6000 Untreated Check Quintec + Rally Oxidate + Serenade Max Quintec + Rally Oxidate + Serenade Max YIELD/ACRE (KG) MARKETABLE FRUIT * Weekly Model Risk Model * CULLED FRUIT Figure 4. Berry Yields – kilograms/acre
