The cranberry fruit rot (CFR) a term used to describe general softening and deterioration of cranberries, affects the fruits both during preharvest and postharvest stages, significantly impacting the quality of cranberries intended for fresh-market consumption and processing (Oudemans et al., 1998). Research on fruit rot, dating back to the early 1900s, has highlighted the involvement of numerous taxonomically diverse fungal species, emphasizing the complexity of the disease (Bain, 1926; Stevens, 1924, 1932; Caruso et al., 2000; Conti et al., 2021; McManus et al., 2003; Olatinwo et al., 2004; Vilka and Bankina, 2013; Zuckerman, 1958).

The distribution of CFR fungal species, along with the incidence and severity of the resulting fruit rot, varies across cultivars, geographical locations, growing seasons, and phenological stages (Stevens, 1924; Bergman and Wilcox, 1936; Zuckerman, 1958; McManus et al., 2003; Oudemans et al., 1998; Polashock et al., 2009; Wells-Hansen and McManus, 2017). Literature suggests that 10–15 fungal species are most prevalent across growing regions. These include Colletotrichum fioriniae (Marcelino & Gouli ex R.G. Shivas & Y.P. Tan; teleomorph Glomerella acutata var. fiorinae), Colletotrichum fructivorum (V.P. Doyle, P.V. Oudem. & S.A. Rehner; teleomorph Glomerella cingulata), Coleophoma empetri Rostr., Phyllosticta vaccinii (Earle), Physalospora vaccinii Shear (Arx & E. Müll.), Allantophomopsis lycopodina (Höhn) Carris, Allantophomopsis cytisporea (Fr.) Petr., Strasseria geniculata (Berk. & Broome) Höhn, Phyllosticta elongata Weid. (teleomorph Botryosphaeria vaccinii), Fusicoccum putrefaciens Shear (teleomorph Godronia cassandrae), Monilinia oxycocci (Woronin) Honey, and Phomopsis vaccinii Shear (teleomorph Diaporthe vaccinii). Each species is associated with specific fruit rot diseases—bitter rot, ripe rot, early rot, blotch rot, black rot, berry speckle, end rot, cotton ball, and viscid rot—collectively known as the CFR disease complex (McManus et al., 2003; Oudemans et al., 1998; Polashock et al., 2009; Raj, 1983; Weir et al., 2012; McManus, 2001). Our evaluations during the 2021 and 2022 growing seasons identified C. empetri, A. cytisporea, B. vaccinii, P. vaccinii, and various Colletotrichum species as the most prevalent fungal species in Massachusetts (unpublished data).

Cranberry handlers (the processing industry) often heavily discount deliveries with over 12% fruit rot, whereas those exceeding 20% are typically rejected outright. Consequently, potential yield losses due to CFR vary widely, spanning from 20% to 100% if not managed strategically (McManus et al., 2003; Olatinwo et al., 2004; Oudemans et al., 1998; Stiles and Oudemans, 1999; Wells et al., 2014; Wells-Hansen and McManus, 2017).

Currently, cranberry growers in Massachusetts and New Jersey rely on “in bloom” (during flowering) application of 3-5 fungicides for CFR management (Caruso et al., 2000; Oudemans et al., 1998). Despite fungicide use, fruit rot in these two states ranges from less than 1% up to 15% (Oudemans et al., 1998). Along with decreasing the fungal pathogen inoculum in the field (Averill et al., 2008), fungicide applications significantly increase the production costs and the risk of fungicide resistance development (Hahn, 2014; Lucas et al., 2015).

Further complicating this situation, in the past six years, there are increasing restrictions imposed by key export markets [(European Union (EU)], on the use of broad-spectrum fungicides (mancozeb and chlorothalonils) owing to perceived environmental and health risks. These fungicides have been the backbone of CFR management and fungicide resistance management for the past 50 years (Caruso et al., 2000). In 2019, the European Union (EU) published their decision to lower the Maximum Residue Limit (MRL) for chlorothalonil to 0.01 ppm (parts per million) from 5 ppm for cranberry fruit/processed products destined for the EU market. Additionally, the EU announced a non-renewal of mancozeb beginning in 2021, with a risk based MRL review pending. These regulatory constraints caused a significant shift in fungicide use patterns in Massachusetts. Commercial fungicides are categorized by the Fungicide Resistance Action Committee (FRAC, 2020) based on their target site, chemical class, and risk of resistance development (Brent and Hollomon, 1998). Most growers are now relying on other classes of fungicides (single-site mode of action) from FRAC (Fungicide Resistance Action Committee) Group 3 (systemic demethylation inhibitors- DMIs) with active ingredients (a.i) prothioconazole (Proline^®), fenbuconazole (Indar^®); and Group 11 (strobilurins- a group of QoI -Quinone Outside Inhibitor fungicides) with a.i. azoxystrobin (Abound^®). Although these newer classes of fungicides are highly effective and less harmful to human and environmental health, they pose a much higher risk of selecting for fungicide resistance compared to broad-spectrum fungicides. Resistance to these groups of fungicides has been well documented across several cropping systems (Ding et al., 2019; Hunter et al., 1984; Kikuhara et al., 2019; Rosenzweig et al., 2019; Schnabel et al., 2004; Trueman et al., 2017; Wolfe, 1985; Yamaguchi et al., 2000).

For the long-term sustainability of the cranberry industry, it is essential to diversify the fruit rot management tool kit. In our search for novel fungicide options, we identified several promising novel groups of fungicides that were reported to have effectiveness against a broad spectrum of fungal pathogens across various crops. These include succinate dehydrogenase inhibitors (SDHIs), such as benzovindiflupyr and pydiflumetofen, cyprodinil fungicides from the anilino-pyrimidine group (FRAC Group 9), and fludioxonil from the phenylpyrrole group (FRAC Group 12) (Ayer et al., 2021; Ishii et al., 2016; Wedge et al., 2007; Thomidis and Prodromou, 2018; Xiao and Boal, 2009; Mouden et al., 2016; Forster et al., 2007). Each fungicide exhibits a distinct mode of action. For instance, SDHIs target succinate dehydrogenase (SDH), a critical mitochondrial enzyme essential for energy production in eukaryotic organisms (Hospital et al., 2023). Similarly, anilino-pyrimidine fungicides, such as cyprodinil, inhibit methionine biosynthesis and the secretion of hydrolytic enzymes, thereby disrupting fungal development (Mosbach et al., 2017). These diverse mechanisms help reduce the risk of fungicide resistance. Thus the current study was initiated with an objective to evaluate the efficacy of selected fungicides from FRAC Groups 7 (containing benzovindiflupyr and pydiflumetofen), 9 (containing cyprodinil), 12 (containing fludioxonil) on fruit rot incidence and fruit yield.

Massachusetts growers have primarily relied on cultural control methods, such as late water flooding, pruning, and sanding, as well as chemical control options like chlorothalonils, mancozebs, copper-based fungicides, Quinone outside Inhibitors (FRAC group 11), and Demethylation Inhibitor (DMI) fungicides (FRAC group 3) to manage fruit rot and improve fruit quality. In a typical commercial setting, up to five fungicide applications are made per season, significantly raising cultivation costs. Besides the economic burden, growers have faced additional challenges in the past five years due to increased restrictions on broad-spectrum, multi-site fungicides such as mancozebs and chlorothalonils. These two groups of fungicides have been the backbone of cranberry fruit rot (CFR) and resistance management in Massachusetts (Caruso et al., 2000). Additionally, resistance to DMI and QoI fungicides has been well-documented across several cropping systems (Ding et al., 2019; Hunter et al., 1984; Kikuhara et al., 2019; Rosenzweig et al., 2019; Schnabel et al., 2004; Trueman et al., 2017; Wolfe, 1985; Yamaguchi et al., 2000).

Due to the complexity of the cranberry fruit rot, which involves multiple fungal pathogens, screening new fungicides and securing registrations requires significant time and resources. This has created an urgent need to diversify the existing CFR toolkit to reduce overreliance on current fungicides.

To address this, we evaluated the effectiveness of several novel broad-spectrum fungicide groups (FRAC 7, 9, 12), individually and in combination with azoxystrobin (FRAC 11). The active ingredients assessed included benzovindiflupyr and pydiflumetofen (FRAC 7), which are succinate dehydrogenase inhibitors that target the enzyme SDH in complex II of the mitochondrial respiration chain, disrupting the tricarboxylic acid cycle and electron transport chain (Keon et al., 1991). Cyprodinil (FRAC 9), an anilino-pyrimidine fungicide, inhibits methionine biosynthesis, germ tube elongation and hyphal growth (National Center for Biotechnology Information, 2025), whereas fludioxonil (FRAC 12), a phenylpyrrole fungicide, causes several physiological changes in pathogens, such as membrane hyperpolarization, changes in carbon metabolism, and metabolite accumulation, ultimately leading to hyphal swelling and bursting (Kilani and Fillinger, 2016). Azoxystrobin (FRAC 11), a quinone outside inhibitor, arrests fungal growth by inhibiting mitochondrial respiration at the Qo-site in complex III, thereby blocking electron transport (Becker et al., 1981).

Significant differences in fruit rot incidence and yield were observed among treatments and between growing seasons. Fungicides containing, pydiflumetofen, pydiflumetofen & fludioxonil + azoxystrobin, and benzovindiflupyr + azoxystrobin consistently resulted in lower rot incidence and higher yields. Cyprodinil & fludioxonil combined with azoxystrobin was evaluated only during the 2021 growing season, where it demonstrated significant efficacy in reducing fruit rot and enhancing yield. Wood et al. (2023) highlighted the role of benzovindiflupyr, cyprodinil, and fludioxonil in inhibiting the mycelial growth of CFR fungi in vitro. Similarly Wisconsin field trials (McManus and Perry, 2018, 2019) reported the efficacy of cyprodinil & fludioxonil, pydiflumetofen, and benzovindiflupyr in managing fruit rot through field trials. In contrast to our study, which evaluated three spray regimes (early, mid, and late bloom) of these compounds individually and in combination with azoxystrobin, McManus and Perry (2018, 2019) tested them in two spray regimes (full bloom and early fruit set). In our trials, only treatments tank-mixed with azoxystrobin (except pydiflumetofen) reduced rot levels effectively. Conversely, McManus and Perry (2018, 2019) observed significantly lower rot levels when these products were applied individually. In the 2022 growing season, azoxystrobin applied alone in three spray regimes did not reduce rot compared to the non-sprayed control. However, azoxystrobin co-applied with pydiflumetofen, benzovindiflupyr, and pydiflumetofen & fludioxonil significantly lowered fruit rot, suggesting a synergistic effect.

Fruit rot levels across treatments ranged from 23% to 48% in 2021; 14% to 29% in 2022; 8 to 31% in 2023; and 12 to 55% in 2024. To ensure high rot levels for evaluating treatment efficacy, we tested three spray regimes, whereas growers in Massachusetts typically use 4-5 fungicide applications for fruit rot management. Rot levels ranging from 3 to 95 were reported in three spray regime fungicide trials on ‘Early Black’ cultivar over the years (Caruso, 1999; 2000; 2002a; 2002b; 2003; 2004; 2005; 2010; 2011), and 4 to 90% on ‘Ben Lear’ cultivar (Caruso, 2008, 2009) from MA before. No literature was found on rot levels for the ‘Demoranville’ cultivar. The differences between years likely stem from variations in weather. The current study did not investigate the effects of cranberry cultivar on rot levels comprehensively, as treatments were evaluated on ‘Demoranville’ in 2021 and 2023, while on ‘Ben Lear’ and ‘Stevens’ were evaluated for only one year each. In 2021, fruit rot levels were notably higher. Even the best treatment resulted in 23% rot. Many growers also reported increased fruit rot that year, which was likely due to the exceptionally wet conditions in early September. September 2021 experienced 279 mm of total precipitation, far exceeding the average of 117 mm (Visual Crossing). In contrast, September of 2022, 2023 and 2024 recorded 83, 159, 110 mm of total precipitation.). Cranberry fruit rot is positively correlated with increased rainfall, particularly late in the season when the fruits are mature. Excessive rainfall stresses the fruit, making it softer and more susceptible to disease spread (Shear et al., 1931; Wells and McManus, 2013).

These results highlight the potential role of these novel active ingredients, which were previously unregistered for cranberry, in managing CFR. They also emphasize the enhanced efficacy of these fungicides when tank-mixed with azoxystrobin, leading to better fruit rot control and enhanced yields. The synergistic effects of azoxystrobin with other fungicides have been noted in previous studies, such as its combination with difenoconazole (Bhuvaneswari and Krishnam Raju, 2012), propiconazole (Uppala and Zhou, 2018) against rice sheath blight, and with difenoconazole and tebuconazole against squash powdery mildew (Noshy et al., 2019). Given the complex nature of fruit rot and the wide spectrum of fungi involved, combining fungicides with different modes of action helps reduce selection pressure on pathogens and thus the development of resistance.

In conclusion, the findings from the current study demonstrate the potential of novel fungicides from FRAC groups 7, 9, and 12 to diversify the CFR management toolkit and their synergistic effects when combined with azoxystrobin.