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Succinate dehydrogenase inhibitors (SDHIs) are the fungicides most commonly used to control Botrytis fruit rot on commercial strawberry in Florida. The medium-to-high risk of selection of resistance in the causal agent Botrytis cinerea is a threat to the efficacy of this fungicide group. In this study, we characterized the sensitivity of B. cinerea to the SDHI isofetamid, evaluated the SdhB gene mutation associated with resistance, and monitored resistance frequencies to five SDHI fungicides for two consecutive seasons. EC 50 values of 70 isolates were obtained using the spiral gradient dilution (SGD) method and averaged 0.098 µg/ml of isofetamid. EC 50 averages of 3.04 and >500.00 µg/ml were obtained for isolates with the N230I and P225F mutations indicating moderate and high resistance to isofetamid, respectively. A total of 565 B. cinerea isolates collected during 2015-2016 and 2016-2017 seasons from strawberry nurseries and Florida production fields were evaluated using conidial germination assays. Results for the first season showed resistance frequencies of 95, 33, 21, 25, and 0% to boscalid, penthiopyrad, fluopyram, benzovindiflupyr, and isofetamid, respectively. The respective resistance frequencies for the following season were 91, 95, 44, 27, and 1.3%. Only three isolates were found to be moderately resistant to isofetamid during the second season, and the mutation N230I was identified after sequence analysis. These isolates were confirmed to be resistant to isofetamid in fruit assays with disease incidence of 55.6 to 77.0%; however, the conidial production of the isolates was inhibited by an average of 83.9%. In general, isofetamid efficacy was higher than the other evaluated SDHIs, but a slight increase in resistance frequencies was observed in our study.

Succinate dehydrogenase inhibitors (SDHIs) are an essential group of fungicides for managing gray mold, caused by Botrytis cinerea , in numerous crops. Resistance to boscalid, an early-generation SDHI, is widespread worldwide and was linked to mutations in the iron-sulfur protein encoding the SdhB subunit of the SDH complex. Herein, we report on four simultaneous dependent mutations at codons 85 (G85A), 93 (I93V), 158 (M158V), and 168 (V168I) of the membrane-anchored SdhC subunit of B. cinerea . Isolates without and with mutations in SdhC were referred to as C - and C + genotypes, respectively. The C + genotype was found in all the five surveyed hosts from different U.S. regions but its frequency was higher, 25 to 40%, in the tree fruit isolates compared with 12 to 25% in the small fruit populations. The four SdhC mutations were found in isolates without mutations in SdhB or with mutations known to confer resistance to the SDHIs in SdhB. However, the frequency of C + isolates was significantly higher in the SdhB wild-type isolates, which suggests that SDHI sprays may have played a role in selecting for the C - over the C + genotype. Field C + isolates exhibited reduced sensitivity to fluopyram and increased sensitivity to boscalid and penthiopyrad in vitro and on detached fruit. Homology modeling confirmed the positioning of the four mutations in the ubiquinone-binding pocket. The SdhC G85A is found in the proximal ubiquinone binding site and SdhC M158V is positioned in the iron sulfur protein interface next to the [3Fe-4S] cluster, whereas SdhC I93V is positioned next to the heme b with vital functions in the SDH enzyme. Beside the differential sensitivity to the SDHIs, these mutations caused a significant fitness cost in the C + isolates including sporulation and increased sensitivity to reactive oxygen species. The presence of Botrytis populations differentially sensitive to the SDHIs suggests increased risks for resistance development but also opens up new perspective for future gray mold management using different SDHI fungicides.

Botrytis fruit rot (BFR) is a major disease that affects strawberry production in Florida and worldwide. BFR management relies on frequent fungicide applications. A meta-analysis was conducted on the outcomes from nine field trials to evaluate the efficacy and profitability of conventional and biological fungicides compared with a nontreated control (NTC). All trials were conducted in Florida between the 2005/06 and 2016/17 growing seasons. Fungicide treatments were applied weekly, and plots were harvested twice a week for yield and BFR incidence quantification. Treatments were grouped into four categories: NTC, multisite only (Thiram), Standard (captan alternated with fludioxonil + cyprodinil), and Bacillus . Following primary analyses, a random effects network meta-analytical model was fitted to estimate the mean yield and BFR incidence responses for each treatment group and to compare means between pairs of groups. The Thiram and the Standard treatment groups increased yield by 378.8 and 502.2 kg/ha/week, respectively, compared with the NTC. The yield difference between Bacillus and NTC was not statistically significant. Besides increasing yield, Thiram and Standard also reduced BFR incidence by approximately 10% compared with the NTC. The mean yield responses and among-study variability from the meta-analysis were used to estimate the probability of a given yield response in a new future trial. The Standard and Thiram treatment groups showed higher estimated probabilities of increasing yield and resulting in a profitable return on application investments than the Bacillus group of treatments. The results from this study provide growers with information that will aid their decision-making process regarding BFR management.

Botrytis fruit rot (BFR), caused by the necrotrophic fungus Botrytis cinerea, is the most important disease of strawberry and is mainly controlled by applications of fungicides from multiple chemical groups. To develop more effective and sustainable BFR management programs, field trials were conducted to evaluate the efficacy of fluopyram and penthiopyrad, two newly registered succinate dehydrogenase inhibitors (SDHIs), rotated or tank mixed with the multisites thiram and captan or the single-sites fludioxonil and fenhexamid. The treatments were applied at two different strawberry fields during the 2013-14 and 2014-15 seasons. Overall, tank mixtures of fluopyram and penthiopyrad increased yield and reduced BFR better than rotations with the same fungicides. The multisite thiram tank mixed with fluopyram reduced BFR incidence by 63 to 86% versus 56 to 84% when the two fungicides were rotated. Thiram tank mixed with penthiopyrad reduced BFR incidence by 55 to 72% versus 42 to 66% when rotated. Captan rotated or tank mixed with fluopyram had a positive effect on yield and BFR incidence, whereas the combination of captan with penthiopyrad had negative impacts. Similarly, the single-site fenhexamid had significant positive impacts when rotated or tank mixed with fluopyram but resulted in poor BFR control when combined with penthiopyrad. The rotation of fludioxonil with both SDHIs had a significant positive effect, although its combination with fluopyram was more effective. The multirotation consisting of both SDHIs and different multi- and single-site fungicides did not provide a greater efficacy than the dual rotation or tank-mixture programs. Our findings suggest more scrutiny is needed when recommending tank-mixture or rotation partners for new fungicides to ensure compatibility and enhanced BFR management. Future recommendations should emphasize the importance of such selections at an early stage for delaying fungicide resistance development and extending the lifespan of at-risk fungicides.

Botrytis fruit rot, caused by Botrytis cinerea, is one of the most important strawberry diseases worldwide, and fungicide applications are often used to manage the disease in commercial production. Isolates of B. cinerea were collected from conventional and organic strawberry fields in four Brazilian States from 2013 to 2015 and their sensitivity to the main single-site mode-of action fungicides used in Brazil was tested. Resistance to azoxystrobin, iprodione, pyrimethanil, and thiophanate-methyl was found and values for effective concentration that inhibited mycelial growth by 50% were higher than 71.9, 1.2, 5.0, and 688 µg/ml, respectively, regardless the production system. Resistance to these fungicides was observed in 87.5, 76.6, 23.4, and 92.2% of isolates from conventional fields and 31.4, 22.9, 14.3, and 51.4% of isolates from organic fields, respectively. Moreover, frequencies of isolates with multiple fungicide resistance to the four active ingredients were 20.6 and 2.8% whereas 6.3 and 27.8% were sensitive to the four fungicides for conventional and organic areas, respectively. Molecular analyses of the cytochrome b, β-tubulin, and bos1 genes revealed the presence of G143A; E198A; and I365 N/S, Q369P, or N373S mutations, respectively, in resistant isolates of B. cinerea. Field rates of fungicides sprayed preventively to inoculated strawberry fruit failed to control disease caused by the respective resistant isolates.

Botrytis isolates from strawberry transplants originating from Canada and the northern United States as well as isolates collected from strawberry and blueberry commercial fields in the southeastern United States were investigated for the frequency of Botrytis cinerea, other cryptic Botrytis spp. reported recently, and the transposable elements (TE) using six genetic markers. B. cinerea sensu stricto was predominant (94%) in strawberry and blueberry in all surveyed regions. Botrytis group S, a newly reported clade on strawberry from Germany, was found at low frequencies (6%) in strawberry in the United States and Canada and on blueberry isolates from Florida. Neither B. caroliniana nor B. pseudocinerea were detected in the U.S. or Canadian populations. Transposa isolates containing the TE boty and flipper accounted for 74% of 410 isolates studied herein. Isolates containing boty only or flipper only elements were found at 21 and 2%, respectively. However, boty isolates were predominant in the blueberry population with more than 50%. The TE were found in B. cinerea and Botrytis group S at similar frequencies, except that flipper was more frequent (10.7%) in Botrytis group S, compared with 1.6% in B. cinerea. The sensitivity of 256 Botrytis isolates from the different genetic groups described above was evaluated to seven fungicides registered to control gray mold in commercial fields. Results indicate that B. cinerea and transposa isolates have higher resistance frequencies to almost all fungicides tested compared with the other Botrytis genotypes or species, whereas the TE flipper may be related to resistance to fludioxonil. Similarities observed between nursery and commercial field populations and their impact on gray mold development and management are discussed.

Strawberry production in Florida and South Carolina is affected by two major diseases, anthracnose fruit rot (AFR) and Botrytis fruit rot (BFR), caused by Colletotrichum acutatum and Botrytis cinerea, respectively. The effective management of both diseases traditionally relied on weekly fungicide applications. However, to improve timing and reduce the number of fungicide sprays, many growers follow the Strawberry Advisory System (StAS), a decision support system for forecasting fungicide applications based on environmental conditions and previously developed models. The objective of this study was to perform a meta-analysis to determine the effectiveness of the StAS for AFR and BFR management compared with a calendar-based spray program. Thirty-nine trials were conducted from 2009 to 2014 in Florida and South Carolina commercial strawberry fields. Meta-analysis was conducted to quantify the treatment effects on four effect sizes, all based on the difference in response variables for StAS and the calendar-based treatments in each trial. The mean difference in BFR incidence, AFR incidence, yield, and number of marketable fruit between the two treatments was not significantly different from 0 (P < 0.05). However, the number of fungicide applications per season was reduced by a median of seven when using the StAS, a 50% reduction in sprays compared with the calendar-based approach. Effect sizes were not influenced by location or the favorability of the environment for disease development. These findings indicate that use of StAS in commercial fields is effective in controlling fruit rot diseases with no reduction in yield while substantially reducing fungicide applications.

Strawberry transplants produced in nurseries across Canada, northern United States, and California are shipped annually to other strawberry-growing regions, including Florida. Botrytis cinerea, the causal agent of gray mold, causes latent infections on transplants which are suggested as a potential source of primary inoculum in strawberry fields. In this study, we investigated the survival of B. cinerea isolates over the summer in Florida, the presence of B. cinerea in transplants from 14 nurseries from Canada and the United States in 2011, 2012, and 2013, and the sensitivity of nursery population to several botryticides. Botrytis cinerea was detected on dead strawberry plants sampled from commercial strawberry fields between March and June but not in July and August, suggesting that the fungus does not over-summer in strawberry fields in Florida. Nursery transplants surveyed in 2011, 2012, and 2013 showed B. cinerea incidences of 20 to 37, 20 to 83, and 2.5 to 92.5%, respectively. In total, 409 isolates were tested for sensitivity to pyraclostrobin, boscalid, pyrimethanil, fenhexamid, iprodione, penthiopyrad, fluopyram, and fludioxonil. Overall, respective resistance frequencies were 91.7, 79.3, 33.2, 20.7, 2.4, 0.2, 0.2, and 0.0%. A majority of isolates tested were resistant to either 3 or 4 fungicides simultaneously. These findings reinforce the need for an integrated approach between strawberry nurseries and production fields to improve gray mold management and mitigate future risks of resistance development in B. cinerea.

Florida strawberry farmers usually rely on weekly calendar fungicide applications to prevent yield loss from Botrytis fruit rot (BFR). The Strawberry Advisory System (SAS), which helps optimize the timing of fungicide applications, is an additional tool in the farmers' arsenal of disease control options. This study uses data from two-year production trials in Florida combined with historical strawberry prices to measure the economic performance of the Calendar and SAS-based BFR management practices under output price risk and producers' risk preferences. We use Monte Carlo simulation to model the effect of the variability in weekly output prices on farm returns. Results reveal that the traditional calendar-based system is not the least risky BFR management option as growers expected to be. SAS-based options such as SAS-Switch and SAS-Luna would be preferred by both risk-neutral and risk-averse producers and are recommended under high disease pressure conditions. [ABSTRACT FROM AUTHOR]