A Conservative Mutant Version of the Mrr1 Transcription Factor Correlates with Reduced Sensitivity to Fludioxonil in Botrytis cinerea.

Fludioxonil is a highly effective phenylpyrrole fungicide for controlling Botrytis cinerea. Although the field efficacy of fludioxonil remains high, Botrytis cinerea isolates with reduced sensitivity have been reported globally. The molecular target of fludioxonil still remains unknown; however, a mechanism of reduced sensitivity to fludioxonil underlies the overexpression of the ATP binding cassette (ABC) transporter AtrB in a dependent pathway of the Mrr1 transcription factor. Fludioxonil is a key player in controlling B. cinerea infection in table grapes in Chile. However, some isolates with a reduced sensitivity to fludioxonil were detected. This study observed endogenous atrB overexpression in Chilean isolates with reduced sensitivity to fludioxonil (n = 22) compared to the sensitive isolates (n = 10). All isolates increased the expression of atrB in a growth medium supplemented with fludioxonil (0.05 µg/mL). However, sensitive isolates showed lower atrB expression than those with reduced fludioxonil sensitivity. Remarkably, a mutant version of the transcription factor Mrr1 carrying 21 amino acid modifications was identified in all isolates with reduced sensitivity to fludioxonil. These changes alter the protein's transcription factor domain and the C-terminal portion of the protein but not the Zn (2)-C6 fungal-type DNA-binding domain. These results suggest a direct relationship between the conserved and divergent mutant version of mrr1 and sensitivity to fludioxonil. This study provides a new target for developing molecular diagnostic strategies to monitor B. cinerea's sensitivity to fludioxonil in the field.

Chilean Botrytis cinerea Isolates with Reduced Sensitivity to Fludioxonil Exhibit Low to Null Fitness Penalties.

The main phytosanitary problem for table grape production in Chile is gray mold caused by the fungus Botrytis cinerea. To manage this issue, the primary method utilized is chemical control. Fludioxonil, a phenylpyrrole, is highly effective in controlling B. cinerea and other plant pathogens. Consistently, there have been no field reports of reduced efficacy of fludioxonil; however, subpopulations with reduced sensitivity to fludioxonil are on the rise globally, as per increasing reports. Our study involved a large-scale evaluation of B. cinerea's sensitivity to fludioxonil in the Central Valley of Chile's primary table grape production area during the growing seasons from 2015 to 2018. Out of 2,207 isolates, only 1.04% of the isolates (n = 23) exceeded the sensitivity threshold value of 1 µg/ml. Remarkably, 95.7% are concentrated in a geographic region (Valparaíso Region). Isolates with reduced sensitivity to fludioxonil showed growth comparable with sensitive isolates and even more robust growth under nutritional deficit, temperature, or osmotic stress, suggesting greater environmental adaptation. When table grape detached berries were stored at 0°C, isolates less sensitive to fludioxonil caused larger lesions than sensitive isolates (2.82 mm compared with 1.48 mm). However, the lesions generated by both types of isolates were equivalent at room temperature. This study found no cross-resistance between fludioxonil and fenhexamid, an essential fungicide integrated with fludioxonil in Chilean B. cinerea control programs. All the Chilean isolates with reduced sensitivity to fludioxonil were controlled by the fludioxonil/cyprodinil mixture, a commonly employed form of fludioxonil. The cyprodinil sensitivity in the isolates with reduced sensitivity to fludioxonil explains their low field frequency despite their null fitness penalties. However, the emergence of fludioxonil-resistant isolates inside the Chilean B. cinerea population demands a comprehensive analysis of their genetic bases, accompanied by monitoring tools that allow the permanence of field fludioxonil efficacy.

Botrytis prunorum Associated to Vitis vinifera Blossom Blight in Chile.

Table grapes are highly susceptible to Botrytis cinerea infections during the bloom period. After reaching the flower development stage, B. cinerea remains quiescent until berry ripening or gives rise to blossom blight under specific climate conditions. A research study was conducted on the Chilean Central Valley during the 2018-2019 growing season. Flowers of Vitis vinifera cv. Thompson Seedless were collected and B. cinerea was isolated together to a second and morphologically different species, characterized by white mycelium and low to no sporulation (11.4% of total isolates). Three randomly selected isolates within this population were genetically examined and identified as Botrytis prunorum based on a phylogenetic multilocus approach using partial regions of genes RPB2, HSP60, and G3PDH or NEP1 and NEP2. Pathogenicity tests showed that B. prunorum infects and causes wilting in healthy table grape flowers. B. prunorum isolates were able to infect Thompson Seedless berries, inducing lesions between 13.11 and 41.53% with respect to the lesion diameter generated by B. cinerea B05.10. The fungicide sensitivity was evaluated. The three genetically characterized isolates were sensitive to boscalid and to cyprodinil/fludioxonil mixture with a mean EC50 value of 5.5 µg/ml and 0.065 µg/ml, respectively. However, loss of sensitivity to fenhexamid was determined, with a mean EC50 value of 5.13 µg/ml. Our understanding about blossom blight in V. vinifera has been limited to B. cinerea. Here we associated B. prunorum as a second causal agent of this disease in Chile. This data represents a first approach to the epidemiological characteristics of B. prunorum associated with blossom blight in table grapes.