First Report of Resistance to Pyraclostrobin, Boscalid, and Thiophanate-methyl in Colletotrichum gloeosporioides from Blueberry in Georgia

Fungicide resistance profiles of Alternaria spp. associated with fruit rot of blueberry in Georgia, USA

Georgia blueberry growers experience significant losses annually due to fruit rots including Alternaria rot caused by Alternaria spp. Fungicide applications from bloom through harvest are typically recommended for management of fruit rots, however fungicide resistance development has the potential to complicate management activities by reducing fungicide efficacy. To evaluate fungicide resistance issues in Georgia, 46 isolates of Alternaria spp. from ripe blueberry fruit from four major blueberry-producing counties were collected and identified by morphological and molecular features. The majority of the isolates were Alternaria alternata (n=43) but also included Alternaria tenuissima (n=1), Alternaria dumosa (n=1), and Alternaria limoniasperae (n=1). All isolates were assessed for resistance to fungicides which included fludioxonil, fluazinam, metconazole, cyprodinil, pydiflumetofen, boscalid, and pyraclostrobin. For all tested fungicides, with the exception of pyraclostrobin, a mycelial growth inhibition assay was used to determine the EC50 values. For pyraclostrobin, a spore germination assay was used. EC50 value ranges of A. alternata for fludioxonil, fluazinam, cyprodinil, metconazole, pydiflumetofen, boscalid, and pyraclostrobin were 0.037 to 0.234 µg/mL, 0.025 to 0.125 µg/mL, 0.015 to 0.404 µg/mL, 0.125 to 5.729 µg/mL, 0.008 to 1.114 µg/mL, 0.551 to >100 µg/mL, and 0.04 to >100 µg/mL, respectively. These EC50 values suggest that all tested Alternaria spp. isolates were sensitive to fludioxonil, fluazinam, metconazole, and cyprodinil. However, 12 Alternaria spp. isolates showed reduced sensitivity to pydiflumetofen, 21 were resistant to boscalid and 10 were resistant to pyraclostrobin. Among these resistant isolates, 6 were resistant to both of the two latter fungicides. Sequencing portions of the sdhB, sdhC and sdhD genes from boscalid-resistant isolates and the cytochrome b gene from pyraclostrobin-resistant isolates revealed the presence of known resistance mutations in resistant isolates - including H134Q or G79R mutations in the sdhC gene or H134R mutations in the sdhD gene of some, but not all, boscalid-resistant isolates, and the presence of the G143A mutation in pyraclostrobin-resistant isolates. Our findings indicate that resistance to boscalid and pyraclostrobin is present in Alternaria spp. from Georgia blueberries and suggest that growers utilizing these fungicides in some Georgia locations may experience Alternaria fruit rot control failures.

Managing fruit rot diseases of Vaccinium corymbosum

Blueberry is an important perennial fruit crop with expanding consumption and production worldwide. Consumer demand for blueberries has grown due to the desirable flavor and numerous health benefits, and fresh market production in the U.S. has risen in turn. U.S. imports have also increased to satisfy year-round consumer demand for fresh blueberries. Pre- and post-harvest fruit diseases such as anthracnose (caused by Colletotrichum spp.) and botrytis fruit rot (caused by Botrytis spp.) have a significant impact on fruit quality and consumer acceptance. These are also among the most difficult diseases to control in the blueberry cropping system. These latent pathogens can cause significant losses both in the field, and especially during transport and marketplace storage. Although both diseases result in rotted fruit, the biology and infection strategies of the causal pathogens are very different, and the management strategies differ. Innovations for management, such as improved molecular detection assays for fungicide resistance, postharvest imaging, breeding resistant cultivars, and biopesticides have been developed for improved fruit quality. Development and integration of new strategies is critical for the long-term success of the blueberry industry.

Fungicide resistance in Colletotrichum fructicola and Colletotrichum siamense causing peach anthracnose in China

Peach is one of the popular and economically important fruit crops in China. Peach cultivation is hampered due to attacks of anthracnose disease, causing significant economic losses. Colletotrichum fructicola and Colletotrichum siamense belong to the Colletotrichum gloeosporioides species complex and are considered major pathogens of peach anthracnose. Application of different groups of fungicides is a routine approach for controlling this disease. However, fungicide resistance is a significant drawback in managing peach anthracnose nowadays. In this study, 39 isolates of C. fructicola and 41 isolates of C. siamense were collected from different locations in various provinces in China. The sensitivity of C. fructicola and C. siamense to some commonly used fungicides, i.e., carbendazim, iprodione, fluopyram, and propiconazole, was determined. All the isolates of C. fructicola collected from Guangdong province showed high resistance to carbendazim, whereas isolates collected from Guizhou province were sensitive. In C. siamense, isolates collected from Hebei province showed moderate resistance, while those from Shandong province were sensitive to carbendazim. On the other hand, all the isolates of C. fructicola and C. siamense showed high resistance to the dicarboximide (DCF) fungicide iprodione and succinate dehydrogenase inhibitor (SDHI) fungicide fluopyram. However, they are all sensitive to the demethylation inhibitor (DMI) fungicide propiconazole. Positive cross-resistance was observed between carbendazim and benomyl as they are members of the same methyl benzimidazole carbamate (MBC) group. While no correlation of sensitivity was observed between different groups of fungicides. No significant differences were found in each fitness parameter between carbendazim-resistant and sensitive isolates in both species. Molecular characterization of the β-tubulin 2 (TUB2) gene revealed that in C. fructicola, the E198A point mutation was the determinant for the high resistance to carbendazim, while the F200Y point mutation was linked with the moderate resistance to carbendazim in C. siamense. Based on the results of this study, DMI fungicides, e.g., propiconazole or prochloraz could be used to control peach anthracnose, especially at locations where the pathogens have already developed the resistance to carbendazim and other fungicides.

Fungicide Sensitivity of Colletotrichum Species Causing Bitter Rot of Apple in the Mid-Atlantic U.S.A

Apple growers in the Mid-Atlantic region of the U.S.A. have reported increased losses to bitter rot of apple. We tested the hypothesis that this increase is because the Colletotrichum population has developed resistance to commonly used single-mode-of-action (single-MoA) fungicides. We screened 220 Colletotrichum isolates obtained from 38 apple orchards in the Mid-Atlantic region for resistance to 11 fungicides in Fungicide Resistance Action Committee (FRAC) groups 1, 7, 9, 11, 12, and 29. Eleven (5%) of these isolates were resistant to FRAC group 1 with confirmed β-tubulin E198A mutations, and two (<1%) were also resistant to FRAC group 11 with confirmed cytochrome-b G143A mutations. Such low frequencies of resistant isolates indicate that fungicide resistance is unlikely to be the cause of any regional increase in bitter rot. A subsample of isolates was subsequently tested in vitro for sensitivity to every single-MoA fungicide registered for apple in the Mid-Atlantic U.S.A. (22 fungicides; FRAC groups 1, 3, 7, 9, 11, 12, and 29), and 13 fungicides were tested in field trials. These fungicides varied widely in efficacy both within and between FRAC groups. Comparisons of results from our in vitro tests with results from our field trials and other field trials conducted across the eastern U.S.A. suggested that EC₂₅ values (concentrations that reduce growth by 25%) are better predictors of fungicide efficacy in normal field conditions than EC₅₀ values. We present these results as a guideline for choosing single-MoA fungicides for bitter rot control in the Mid-Atlantic U.S.A.

Sensitivity of Colletotrichum nymphaeae to Six Fungicides and Characterization of Fludioxonil-Resistant Isolates in China

Colletotrichum nymphaeae is the dominant species causing anthracnose disease of peach in China. In this study, 140 isolates of C. nymphaeae were assessed for their sensitivity to six fungicides. It was found that C. nymphaeae was highly resistant to carbendazim, procymidone, and boscalid but sensitive to pyraclostrobin and prochloraz. For fludioxonil, the fungus exhibited differential sensitivities (i.e., approximately 14% of isolates were resistant to fludioxonil and the resistance was stable). Fludioxonil-resistant isolates had a mean EC₅₀ value of 2.2380 µg/ml, whereas the mean EC₅₀ value was 0.0194 µg/ml in fludioxonil-sensitive isolates. The mean EC₅₀ values of C. nymphaeae for pyraclostrobin and prochloraz were 0.0083 µg/ml and 0.002 µg/ml, respectively. No cross-resistance was observed between fungicides from different groups. Mycelial growth rate, control efficacy, and osmotic stress responses were significantly different (P < 0.05) between fludioxonil-sensitive (Flu^S) and -resistant (Flu^R) isolates, but no significant difference was observed (P > 0.05) in virulence and sporulation between Flu^S and Flu^R isolates. No mutation was detected in coding regions of the CnOs-1, Cal, Hk1, Hog1, TPI, and Mrr1 genes. Interestingly, with fludioxonil treatment, the expression of ABC transporter gene atrB was significantly overexpressed in some resistant isolates. However, overexpression of the atrB gene was not detected in one moderately and one highly resistant isolate, indicating that other unknown mechanisms may be involved. Current findings uncovered several effective chemicals and provided the foundation for designing management strategies to practically control peach anthracnose with the most effective demethylation inhibitor fungicides and quinone outside inhibitor fungicides.