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Fusarium graminearum is an important fungus causing a variety of maize diseases, including stalk rot, ear rot, and sheath rot. However, conidia of F. graminearum are not easily obtained under normal culture conditions, which seriously affects the identification and pathogenicity assessment of the isolates and screening of resistance sources. This study was undertaken to develop and utilize a rapid sporulation technique of F. graminearum using liquid cultivation, which could meet the needs of various tests. The results show that the optimum conditions for sporulation of F. graminearum were as follows: culture medium, 0.154 mol/liter of saline; temperature, 28 to 30°C; incubation time, 96 h; initial pH, 9 to 10; illumination, continuous ultraviolet light; and shaking speed, 150 rpm. Using this culture method, conidial concentration of tested F. graminearum strains can reach >1.5 × 105 conidia/ml. Compared with the existing methods using mung bean and carboxylmethyl cellulose as matrix, saline is relatively inexpensive, and the culture process, relatively quick. Overall, this study provided a systematic, rapid, and simple method to obtain a large number of conidia of F. graminearum.

Phytophthora blight, caused by Phytophthora capsici, is one of the most destructive diseases in the production of solanaceous and cucurbitaceous vegetable crops. Fluopicolide has been used to control the disease; however, reduced efficacy of the fungicide was observed in Georgia. P. capsici isolates were collected from commercial vegetable fields in Georgia in 2018 and 2019 to determine sensitivity to fluopicolide, which were phenotyped to have 43.1% of the isolates as resistant. The fitness of resistant (R) and sensitive (S) isolates was assessed through mycelial growth and sporulation assays exposed to the fungicide (0 or 50 µg/ml). Fluopicolide did not reduce mycelial growth, sporangial production, and zoospore germination of the R isolates. In the absence of fluopicolide, there was no significant difference between the R and S isolates in sporangial production but mycelial growth and zoospore germination of the R isolates was greater than the S isolates (P = 0.01 and 0.001, respectively). The R isolates had an ability similar to that of the S isolates to induce disease on Aristotle bell pepper, and most of the R and S isolates caused the same level of disease on Paladin. Inoculating squash fruit using different R:S ratios and recovering R and S isolates after five cycles of inoculation resulted in similar trends in changes of R versus S isolate ratios. Overall, it appeared that fitness and competitive ability of the R isolates were not reduced compared with the S isolates. This is the first report of the occurrence of field isolates of P. capsici resistant to fluopicolide in the world. The results have significant implications in providing guidance for growers to avoid or limit use of this fungicide in vegetable production.

Diseases caused by oomycete plant pathogens result in devastating losses to agriculture and native forests, despite the significant research efforts that have advanced our understanding of these organisms. Limiting these pathogens has been challenging to plant pathologists and plant health practitioners. In this first focus issue , titled Managing Stubborn Oomycete Plant Pathogens, Plant Health Progress has assembled an array of manuscripts on the biology and management of Phytophthora, Pythium, Pseudoperonospora, Peronospora, and Aphanomyces spp. This focus issue has 28 peer-reviewed papers including three diagnostic guides, three mini-reviews, three briefs, two surveys, and 17 research papers. Of the 28 papers, 20 are on diseases caused by Phytophthora, four on Pythium, three on downy mildews, and one on Aphanomyces. All advance our understanding of these stubborn oomycete pathogens.

Phytophthora capsici, the causal agent of Phytophthora blight, is a prominent and economically damaging oomycete pathogen in South Georgia. P. capsici causes crown, root, leaf, stem, and fruit infections on a wide range of vegetable crops. Oomycete pathogens such as P. capsici are dispersed in water, as their zoospores are flagellated and can move through runoff. Irrigation ponds are often reservoirs for different pathogens, and reusing the captured runoff is increasing in popularity to decrease irrigation costs. This combination allows for unintended outbreaks of diseases by pumping the contaminated runoff onto susceptible crops. Detection and identification of these pathogens are crucial steps in disease management, and rapid detection can ensure timely application of disease control measures. In this study, 42 irrigation ponds in nine counties from South Georgia were surveyed for the presence of P. capsici using a novel filtration method in conjunction with a LAMP assay specific for P. capsici. Ten ponds in five counties were found to have P. capsici as detected from the assay, suggesting that testing of irrigation ponds for P. capsici and other pathogens should be conducted to assist in preventing disease outbreaks.

Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum is a serious, widespread disease of watermelon throughout the southern United States. To investigate whether soil temperature affects disease development, three cultivars of triploid watermelon were transplanted March 17 to 21, April 7 to 11, and April 26 to May 2 in 2015 and 2016 at Charleston, SC, and Tifton, GA into fields naturally infested with F. oxysporum f. sp. niveum. Incidence of Fusarium wilt was lower with late-season than with early and midseason transplanting in all four experiments (P ≤ 0.01). Cultivar Citation had more wilted plants than the cultivars Fascination and Melody in three of four experiments (P ≤ 0.05). In South Carolina, planting date did not affect weight and number of marketable fruit ≥4.5 kg apiece. In Georgia in 2016, weight and number of marketable fruit were greater with late transplanting than with early and midseason transplanting. In both states, yield and value for Fascination and Melody were higher than for Citation. Soil temperature averaged over the 4-week period after transplanting was negatively correlated with disease incidence for all four experiments (r = –0.737, P = 0.006). Transplanting after mid-April and choosing a cultivar with resistance to F. oxysporum f. sp. niveum race 1, like Fascination, or tolerance, like Melody, can help manage Fusarium wilt of watermelon and increase marketable yields in the southern United States.

Fusarium wilt, incited by the fungus Fusarium oxysporum f. sp. niveum, is a soilborne disease that affects watermelon production worldwide. Approaches for effective management of Fusarium wilt in watermelon are limited. Studies conducted in recent years indicated that prothioconazole and thiophanate-methyl reduced the disease significantly under field conditions. However, effects of the fungicides on different life stages of F. oxysporum f. sp. niveum and potential existence of fungicide resistance in F. oxysporum f. sp. niveum populations are unknown. In the present study, effects of prothioconazole and thiophanate-methyl on mycelium growth and spore germination of F. oxysporum f. sp. niveum isolates collected in watermelon fields in Georgia were determined. In vitro mycelium growth studies indicated that all 100 isolates evaluated were sensitive to prothioconazole; the effective concentration that suppressed mycelium growth by 50% ranged from 0.75 to 5.69 μg/ml (averaged 1.62 μg/ml). In contrast, 33 and 4% of the isolates were resistant to thiophanate-methyl at 10 and 100 μg/ml, respectively. Microconidial germination assays showed that 36 and 64% of the isolates tested were sensitive or intermediately sensitive to prothioconazole at 100 μg/ml but the fungicide did not inhibit spore germination at 10 μg/ml. Sequencing a portion of the β-tubulin gene of eight isolates resistant or sensitive to thiophanate-methyl indicated that fungicide resistance was associated with a point mutation at nucleotide position 200, resulting in a substitution of phenylalanine by tyrosine. This is the first report of isolates of F. oxysporum resistant to thiophanate-methyl. Results of the research suggest that prothioconazole may be a viable option for management of Fusarium wilt of watermelon whereas thiophanate-methyl should be used judiciously due to the existence of isolates resistant to the fungicide.

About 50% of the watermelons in the United States are produced in the southeastern states, where optimal conditions for development of Phytophthora fruit rot prevail. Phytophthora fruit rot significantly limits watermelon production by causing serious yield losses before and after fruit harvest. Efficacy of fungicide rotation programs and Melcast-scheduled sprays for managing Phytophthora fruit rot was determined by conducting experiments in Phytophthora capsici-infested fields at three locations in southeastern United States (North Carolina, South Carolina, and Georgia). The mini seedless cultivar Wonder and seeded cultivar Mickey Lee (pollenizer) were used. Five weekly applications of fungicides were made at all locations. Significant fruit rot (53 to 91%, mean 68%) was observed in the nontreated control plots in all three years (2013 to 2015) and across locations. All fungicide rotation programs significantly reduced Phytophthora fruit rot compared with nontreated controls. Overall, the rotation of Zampro alternated with Orondis was highly effective across three locations and two years. Rotations of Actigard followed by Ranman+Ridomil Gold, Presidio, V-10208, and Orondis, or rotation of Revus alternated with Presidio were similarly effective. Use of Melcast, a melon disease-forecasting tool, may occasionally enable savings of one spray application without significantly impacting control. Although many fungicides are available for use in rotations, under very heavy rain and pathogen pressure, the fungicides alone may not offer adequate protection; therefore, an integrated approach should be used with other management options including well-drained fields.

Black shank incited by Phytophthora nicotianae is a devastating disease in the production of tobacco. Fungicides have been commonly used for managing the disease; however, there is only a narrow pool of effective fungicides. A few new fungicides became available in recent years, including fluopicolide, mandipropamid, and oxathiapiprolin, which reduced diseases incited by oomycetes under field conditions. Limited information is available regarding sensitivity of P. nicotianae isolates to these new fungicides. Research was conducted to determine effects of the three new fungicides on P. nicotianae isolates from tobacco in Georgia. Studies with 106 isolates indicated that they did not grow when agar medium was amended with the fungicides at the rate of 1 μg/ml. Twenty isolates were used for in vitro studies to determine sensitivity to the fungicides. Fluopicolide, mandipropamid, and oxathiapiprolin inhibited mycelial growth of the isolates with mean EC50 values (effective concentrations that provide 50% growth reduction) of 0.09, 0.04, and 0.001 μg/ml, respectively. EC50 values of fluopicolide, mandipropamid, and oxathiapiprolin for inhibiting sporangial formation were 0.15, 0.03, and 0.0002 μg/ml, respectively. EC50 values for suppressing zoospore germination averaged 0.16, 0.04, and 0.002 μg/ml for fluopicolide, mandipropamid, and oxathiapiprolin, respectively. Results from the study indicated that P. nicotianae isolates from tobacco in Georgia were sensitive to the fungicides, with lower EC50 for oxathiapiprolin than for fluopicolide and mandipropamid. The information on effectiveness and baseline sensitivity of fungicides on P. nicotianae will facilitate monitoring of resistance development in the pathogen population.

Gummy stem blight (GSB), caused by the fungus Didymella bryoniae, is considered the most widespread and destructive disease of watermelon in the southeastern United States. The quinone outside-inhibiting (QoI) fungicide azoxystrobin (AZO), which inhibits mitochondrial respiration by binding to the outer, quinone-oxidizing pocket of the cytochrome bc1 (cyt b) enzyme complex, was initially very effective in controlling GSB. However, resistance to AZO has been observed in D. bryoniae in many watermelon-producing regions. In this study, the DNA sequences of partial cyt b genes of four AZO-resistant (AZO-R) and four AZO-sensitive (AZO-S) isolates of D. bryoniae confirmed the amino acid substitution of glycine by alanine at the 143 codon (G143A) in the AZO-R isolates tested. Allele-specific primers were designed to detect the resistant or sensitive allele at codon 143 of the cyt b gene, which amplified a 165-bp polymerase chain reaction (PCR) product from genomic DNA of nine AZO-R and nine AZO-S isolates of D. bryoniae, respectively. The primer pairs did not amplify DNA from other pathogens tested in the study. The results indicated that the PCR assays developed in the study were specific in differentiating AZO-R and AZO-S isolates and could facilitate AZO resistance detection in D. bryoniae.

Black shank, caused by Phytophthora nicotianae, is responsible for serious yield and quality reduction in tobacco production. Application of effective fungicides continues to be a viable component in developing integrated disease management programs. Experiments were conducted in 2011 to 2013 to determine the efficacy and application methods of a new fungicide, Zorvec (a.i. oxathiapiprolin), for management of black shank under field conditions. Oxathiapiprolin is the first member of a new class of isoxazoline fungicide. Application of Zorvec (0.35 liter/ha) onto tobacco seedlings 1 week prior to transplanting in conjunction with directed applications of the product at 0.7 liter/ha at first cultivation and lay-by (last cultivation) reduced black shank significantly compared to the nontreated control in the experiments conducted in 2011 and 2012. Application of Zorvec at 1.4 liter/ha through transplant water followed by directed sprays at first cultivation and lay-by at 0.7 liter/ha reduced black shank significantly compared with the nontreated control in 2012 and 2013 studies. These treatments were not significantly different (P = 0.05) in disease reduction compared to mefenoxam. All treatments involving Zorvec increased tobacco yield significantly (P = 0.05) or showed a tendency to increase tobacco yield over the nontreated control in all experiments conducted in 2011 to 2013. The results indicated that the new fungicide oxathiapiprolin was effective in reduction of P. nicotianae on tobacco.

Root rot was observed on cowpea [Vigna unguiculata (Linn.) Walp] in Tift County, Georgia, in May of 2015. The disease occurred on approximately 10% of cowpea plants in 2 fields (2 ha), a distance of about 5 miles apart. Symptoms appeared as yellowing lower leaves, dwarfed growth, sunken reddish brown lesions on roots and stems under the soil line, secondary roots became dark brown and rotted, and infected plants eventually wilted and died. Infected plants were collected from each field and roots were surface disinfested in 0.5% NaOCl for 2 min, rinsed in sterile distilled water, and cultured on potato dextrose agar (Full-strength PDA) at 26°C. A fungus was isolated on all plates, which produced whitish aerial mycelium with pale brown pigments in the agar. Macroconidia were produced on PDA at 26°C after 7 days. Macroconidia were produced from monophialides on branched conidiophores in sporadochia and had pronounced dorsiventral curvature, with a tapered and elongated apical cell and prominent foot-shaped b...

The first reported occurrence of onion downy mildew (ODM) on Vidalia sweet onions in Georgia was in 1999. Although the disease is not common every year, two epidemics have occurred since 1999. The authors report here on the impact of a recent outbreak of ODM on the $100-million Vidalia onion crop in 2012. Accepted for publication 29 January 2013. Published 28 March 2013.