Reproduction of Didymella bryoniae on Nine Species of Cucurbits Under Field Conditions

Agro Active Potential of Bacillus subtilis PE7 against Didymella bryoniae (Auersw.), the Causal Agent of Gummy Stem Blight of Cucumis melo

Microbial agents such as the Bacillus species are recognized for their role as biocontrol agents against various phytopathogens through the production of diverse bioactive compounds. This study evaluates the effectiveness of Bacillus subtilis PE7 in inhibiting the growth of Didymella bryoniae, the pathogen responsible for gummy stem blight (GSB) in cucurbits. Dual culture assays demonstrate significant antifungal activity of strain PE7 against D. bryoniae. Volatile organic compounds (VOCs) produced by strain PE7 effectively impede mycelial formation in D. bryoniae, resulting in a high inhibition rate. Light microscopy revealed that D. bryoniae hyphae exposed to VOCs exhibited abnormal morphology, including swelling and excessive branching. Supplementing a potato dextrose agar (PDA) medium with a 30% B. subtilis PE7 culture filtrate significantly decreased mycelial growth. Moreover, combining a 30% culture filtrate with half the recommended concentration of a chemical fungicide yielded a more potent antifungal effect than using the full fungicide concentration alone, inducing dense mycelial formation and irregular hyphal morphology in D. bryoniae. Strain PE7 was highly resilient and was able to survive in fungicide solutions. Additionally, B. subtilis PE7 enhanced the nutrient content, growth, and development of melon plants while mitigating the severity of GSB compared to fungicide and fertilizer treatments. These findings highlight B. subtilis PE7 as a promising biocontrol candidate for integrated disease management in crop production.

Widespread Resistance to Tebuconazole and Cross-Resistance to Other DMI Fungicides in Stagonosporopsis citrulli Isolated from Watermelon in South Carolina

Tebuconazole, a demethylation-inhibitor (DMI) fungicide, is widely used on watermelon and muskmelon because it is inexpensive and has been effective against Stagonosporopsis citrulli, the primary causal agent of gummy stem blight in the southeastern United States. Most isolates (94% of 251) collected from watermelon in South Carolina in 2019 and 2021 were moderately resistant to tebuconazole at 3.0 mg/liter in vitro. Ninety isolates were identified as S. citrulli, and no isolates of S. caricae were found in this study. On watermelon and muskmelon seedlings treated with the field rate of tebuconazole, sensitive, moderately resistant, and highly resistant isolates were controlled 99, 74, and 45%, respectively. In vitro, tebuconazole-sensitive isolates were moderately resistant to tetraconazole and flutriafol but sensitive to difenoconazole and prothioconazole, while highly resistant isolates were highly resistant to tetraconazole and flutriafol and moderately resistant to difenoconazole and prothioconazole. On watermelon seedlings treated with field rates of five DMI fungicides in the greenhouse, severity of gummy stem blight did not differ significantly from the nontreated control when seedlings were inoculated with a highly resistant isolate, while severity was lower with all DMIs on seedlings inoculated with a sensitive isolate, although severity was greater with tetraconazole than with the other four DMIs. In the field, tetraconazole rotated with mancozeb did not reduce severity of gummy stem blight caused by a tebuconazole-sensitive isolate when compared to the nontreated control, while the other four DMIs did. With a highly resistant isolate, all DMIs rotated with mancozeb reduced severity of gummy stem blight compared to the nontreated control, but severity with tetraconazole and tebuconazole was greater than with mancozeb alone, and severity with flutriafol, difenoconazole, prothioconazole, and difenoconazole plus cyprodinil did not differ from mancozeb applied alone. Results from in vitro, greenhouse, and field experiments with the five DMI fungicides were highly correlated with each other. Thus, determining relative colony diameters with a discriminatory dose of 3 mg/liter of tebuconazole is an effective way to identify isolates of S. citrulli highly resistant to tebuconazole.

Premix Fungicides That Reduce Development of Fruiting Bodies But Not Leaf Lesions by Stagonosporopsis citrulli on Watermelon Leaves in the Field

Fungicide applications are the main method to manage gummy stem blight on watermelon (Citrullus lanatus) and other cucurbits, but it is unknown whether fungicides affect development of leaf lesions or fruiting bodies by Stagonosporopsis citrulli. Cyprodinil plus fludioxonil (Switch), cyprodinil plus difenoconazole (Inspire Super), cyprodinil (Vangard), fludioxonil (Cannonball), and difenoconazole (Inspire) were applied to watermelon in rotation with chlorothalonil (Bravo) in fall 2017, 2018, and 2019. Water and chlorothalonil applied weekly served as control treatments. All fungicides reduced disease severity (percentage of leaf area diseased) and area under the disease progress curve (AUDPC) in field plots compared with water. Cyprodinil plus fludioxonil and cyprodinil plus difenoconazole reduced disease severity and AUDPC more than chlorothalonil. Fungicides did not affect the number, diameter, expansion, or area of lesions. All fungicides reduced the number of lesions with fruiting bodies of S. citrulli compared with water (P < 0.05). Cyprodinil plus fludioxonil and cyprodinil plus difenoconazole reduced the percentage of leaf lesions with fruiting bodies, and the diameter and area of the portions of leaf lesions covered with fruiting bodies, compared with water and chlorothalonil. Premix fungicides containing cyprodinil reduced fruiting body formation by S. citrulli, which may partially explain their efficacy in managing gummy stem blight.

Reducing Chlorothalonil Use in Fungicide Spray Programs for Powdery Mildew, Anthracnose, and Gummy Stem Blight in Melons

Fungicides are applied to nearly 80% of U.S. melon acreage to manage the numerous foliar and fruit diseases that threaten yield. Chlorothalonil is the most widely used fungicide but has been associated with negative effects on human and bee health. We designed alternative fungicide programs to examine the impact of reducing chlorothalonil use (Bravo Weather Stik) on watermelon, cantaloupe, and honeydew melon in 2016, 2017, and 2018 in Maryland. Chlorothalonil was replaced in the tank mix of weekly sprays of targeted fungicides with either polyoxin D zinc salt (Oso) or an extract of Reynoutria sachalinensis (Regalia). Powdery mildew (PM; Podosphaera xanthii), gummy stem blight (GSB; Stagonosporopsis spp.), and anthracnose (Colletotrichum orbiculare) were the most prevalent diseases to occur in the 3 years. Replacing chlorothalonil with the biopesticides as the tank-mix component of the fungicide spray program was successful in reducing GSB and PM severity in cantaloupe, honeydew melon, and watermelon compared with the untreated control, with the exception of GSB in 2017 in cantaloupe, and similar to the program including chlorothalonil in all cases, except anthracnose in watermelon. Anthracnose disease severity was not significantly reduced compared with the untreated control when chlorothalonil was replaced with the biopesticides and yields were not improved over the chlorothalonil-alone treatment in any of the trials. Therefore, replacement of chlorothalonil may not fully address its loss as a fungicide resistance management tool but efficacy can be maintained when polyoxin D is alternated with R. sachalinensis as a tank mix with targeted fungicides to manage PM and GSB.

Occurrence of Foliar Pathogens of Watermelon on Commercial Farms in South Carolina Estimated with Stratified Cluster Sampling

A survey of foliar pathogens of watermelon based on two-stage cluster sampling was conducted on commercial farms in South Carolina in spring 2015, spring and fall 2016, and fall 2017. In total, 60 fields from 27 different growers in seven counties representing the main watermelon-producing areas in the state were sampled, using a stratified two-stage cluster sampling approach. In the sampling design, counties corresponded to strata, growers to first-stage clusters, and fields to second-stage clusters. In each field, 100 symptomatic leaves were collected at five equidistant sampling points along four transects encompassing a square shape of 2,500 m². After collection, pathogens were identified based on reproductive structures formed on leaves during >12 h incubation. Estimates were obtained for the statewide probability of pathogen occurrence and associations between pathogen pairs. Six fungal pathogens, Stagonosporopsis spp., Podosphaera xanthii, Cercospora citrullina, Colletotrichum orbiculare, Myrothecium sensu lato (s.l.), and Corynespora cassiicola; the oomycete Pseudoperonospora cubensis; and three viral pathogens were identified on the examined leaves. With the exception of fall 2017, Stagonosporopsis spp. was the most prevalent pathogen in every season, followed by P. xanthii. The highest occurrence of P. cubensis was in spring 2015; it did not occur in 2016. The highest occurrence of C. orbiculare was in spring 2016; it did not occur in spring 2015. Myrothecium s.l. was the most common pathogen in fall 2017 and the second most common pathogen occurring by itself in fall 2016. The third most common pathogen in fall 2017, Corynespora cassiicola, was not observed in any other season. Eight of the 80 isolates of Stagonosporopsis spp. collected were identified as S. caricae, the rest as S. citrulli. All isolates of S. caricae were found in spring 2015 and originated from two fields in different counties. A total of three positive and five negative associations were found between pathogen pairs co-occurring on the same leaf. A positive association between Stagonosporopsis spp. and C. citrullina was the only significant association between pathogens found in two seasons, spring 2015 and spring 2016. Based on estimates of probability of pathogen occurrence across seasons, Stagonosporopsis spp. and P. xanthii are the most common pathogens on watermelons in South Carolina. This is the first report of C. cassiicola, S. caricae, and Myrothecium s.l. on watermelon in South Carolina.

Susceptibility of Fourteen New Cucurbit Species to Gummy Stem Blight Caused by Stagonosporopsis citrulli Under Field Conditions

At least 24 species of cucurbits from 13 genera are known to be susceptible to gummy stem blight, caused by three species of Stagonosporopsis. Cankers that are formed on crowns and stems play an important role in the disease cycle and the survival of the pathogen. Fourteen cucurbit species of unknown susceptibility representing 12 genera, four taxonomic tribes, and four geographic origins were inoculated with Stagonosporopsis citrulli in Charleston, SC, in spring 2015, spring 2016, and fall 2016 to evaluate their level of susceptibility to gummy stem blight and the ability of the pathogen to reproduce on crown cankers. An additional species, Cucumis melo, was included as a reference due to its known high susceptibility. Data sets of area under the disease progress curve (AUDPC) for foliar severity and crown cankers, final percentage of diseased leaf area, final percentage of plants with cankers, final percentage of plants with fruiting bodies, and rates of increase in canker incidence were analyzed to evaluate susceptibility. Results were similar for datasets of AUDPC and final ratings but there were more differences for AUDPC. In all experiments, Apodanthera sagittifolia, Ecballium elaterium, and Kedrostis leloja were at least as susceptible to foliar blight as the reference C. melo. K. leloja was as susceptible to crown cankers as C. melo in all experiments and A. sagittifolia and E. elaterium were among the species most susceptible to crown cankers in two experiments. Coccinia grandis was highly resistant to gummy stem blight and had a few cankers only in fall 2016. Sicana odorifera and Zehneria pallidinervia also consistently grouped with the most resistant species. Incidence of crown cankers on Cucumis melo and K. leloja increased at the fastest rate of all species in all experiments and had, along with E. elaterium, the highest incidence of crowns with fruiting bodies. In general, the most susceptible species also were most suitable for reproduction of the pathogen and had the fastest disease progression. The tribes Benincaseae and Cucurbiteae had consistently lower levels of foliar blight than Bryonieae and Coniandreae. The tribe Benincaseae had a consistently lower AUDPC for canker incidence than Bryonieae and Coniandreae. The species originating from Europe (E. elaterium) was consistently most susceptible to both symptoms, while African species grouped with the least susceptible species in all experiments. To the best of our knowledge, this is the first report of susceptibility to gummy stem blight of 14 species and the first report of susceptibility of the cucurbit tribes Coniandreae and Gomphogyneae. This expands the host range of Stagonosporopsis citrulli to 37 species representing 21 genera and seven tribes in the family Cucurbitaceae. This study demonstrates the importance of crown cankers as reproductive sites for S. citrulli.

Baseline Sensitivity of Didymella bryoniae to Cyprodinil and Fludioxonil and Field Efficacy of these Fungicides Against Isolates Resistant to Pyraclostrobin and Boscalid

To prevent yield reductions from gummy stem blight, fungicides often must be applied to watermelon (Citrullus lanatus) and muskmelon (Cucumis melo). Didymella bryoniae, the ascomycete fungus that causes gummy stem blight, is resistant to thiophanate-methyl, quinone-outside inhibitors (QoI), boscalid, and penthiopyrad. In place of these fungicides, premixtures of cyprodinil and fludioxonil (Switch 62.5WG) or cyprodinil and difenoconazole (Inspire Super 2.82SC) are used. The objectives of this study were to examine baseline isolates of D. bryoniae for sensitivity to cyprodinil and fludioxonil and to determine the efficacy of cyprodinil-fludioxonil and cyprodinil-difenoconazole against isolates resistant to QoI fungicides and boscalid. Colony diameters of 146 isolates of D. bryoniae collected in South Carolina and other U.S. states prior to 2008 were measured on glucose minimal medium amended with cyprodinil or fludioxonil. Mean effective concentration values that reduced relative colony diameter by 50% were 0.052 and 0.099 mg/liter cyprodinil and fludioxonil, respectively. In autumn 2008, 2009, and 2011, field-grown watermelon inoculated with isolates resistant to QoI fungicides and boscalid was treated with boscalid-pyraclostrobin alternated with chlorothalonil, cyprodinil-fludioxonil alternated with chlorothalonil, cyprodinil-difenoconazole alternated with chlorothalonil, tebuconazole alternated with chlorothalonil, chlorothalonil, or water. In 2008 and 2011, both cyprodinil treatments reduced disease severity compared with the water control treatment and chlorothalonil alone. In 2008 and 2009, cyprodinil-fludioxonil reduced severity compared with boscalid-pyraclostrobin and, in 2008, cyprodinil-difenoconazole and tebuconazole also did. Use of cyprodinil-fludioxonil should control gummy stem blight effectively and may delay development of resistance to cyprodinil and fludioxonil in D. bryoniae. However, because Botrytis cinerea became resistant to both cyprodinil and fludioxonil after multiple applications of cyprodinil-fludioxonil per season, prudent fungicide rotations should be followed when using cyprodinil-containing fungicides against D. bryoniae.